Technique of High Vacuum¹

SOME of the equations from the kinetic theory are important in the design, construction, and operation of vacuum apparatus. Accordingly, we will begin our treatment of the technique of high vacuum with a discussion of them. The derivations of these equations are omitted, since we are interested only in their applications.

The laws of ideal gases. The laws of ideal gases are represented, mathematically, by Eqs. 1 and 2.

The total pressure, given by Eq. 2, is the sum of these partial pressures. The value of the constant R, the so-called universal gas constant, is independent of the molecular weight of the gas, but its value does depend on the units in which the pressure and volume are expressed. In vacuum work the pressure is usually expressed in millimeters of mercury² and the volume in cubic centimeters, in which case R has the value of 62,370.

Eqs. 1 and 2 are based on the assumptions, first, that the molecules are infinitely small and, second, that no intermolecular forces exist. Neither assumption is valid for real gases. Nevertheless, the equations describe the behavior of real gases, especially hydrogen and helium, with sufficient accuracy for our purposes here. Although the equations break down at elevated pressures (pressures greater than 1 atmosphere), they become increasingly precise if the pressure is reduced. And, at pressures encountered in vacuum work, Eqs. 1 and 2 not only apply to the description of the behavior of gases but describe the behavior of many unsaturated vapors as well.

The mean free path. The mean free path is the average distance traversed by molecules between successive intermolecular collisions. The magnitude of this quantity is determined by the size of the molecules and is given by the formula

Eq. 5 predicts that the heat conductivity is also independent of the pressure. This was established experimentally by Stefan.⁴

Eqs. 4 and 5 are derived from the assumption that the mean free path is small in comparison with the size of the apparatus. Table I shows the pressures at which this assumption becomes invalid.

If Meyer and Maxwell had reduced the pressure in their bell jar below about 10^-1 mm, they would have observed a decrease in the damping effect on the torsion pendulum. Likewise, if Stefan had extended his observations, he would have found a decrease in the heat conductivity towards 10^-1 mm and its complete disappearance below about mm.

Pumping speeds. Consider that a vessel contains a gas at pressure P and opens through an aperture to a region where a high vacuum is maintained. Further assume that this high vacuum is to be maintained at a pressure so much lower than P that it is essentially a perfect vacuum. The volume of gas escaping through the aperture per unit time, dV/dt, measured at pressure P, is given by the formula

A hypothetical aperture of unit area communicating with an essentially perfect vacuum may be regarded as a pump with a speed of 11.7 liters/sec. Oil and mercury diffusion pumps have two characteristics in common with such an aperture. They have pumping speeds of the same order of magnitude as the aperture, and their observed pumping speeds are roughly constant over a considerable pressure range.

The speed of a diffusion pump is, accordingly, expressed as the volume of gas passing through the throat of the pump measured at the pressure which obtains at the throat. The speed factor of a pump is the ratio of its speed per unit area of the throat to the value 11.7 liters/sec. A good oil diffusion pump has a speed factor of about 0.5 or 0.6. The speed factor for mercury diffusion pumps⁵ varies from 0.1 to 0.3.

The pumping speed of diffusion pumps can be measured by means of a leak like the one shown in Fig. 1. Gas at atmospheric pressure is allowed to leak into the pumping line. The rate at which the gas is introduced is measured by the motion of a mercury pellet in the calibrated capillary tube. At the same time the pressure at the throat of the pump is determined with a vacuum manometer. The rate dV/dt at which gas passes through the pump is obtained by multiplying the volume which the mercury pellet sweeps through per unit time by the ratio of the pressure in the capillary (that is, the barometric pressure) to the pressure which obtains at the pump throat.

Conductance of vacuum pumping lines. Ordinarily, a pump is connected to an apparatus by a tube or system of tubes which constitute the vacuum pumping line. The measured speed of the pump, which we will designate , at one end of the vacuum line is greater than the effective pumping speed, S, at the other end of the line. Naturally, the difference between and S is small if the pumping tubes are short and have a large diameter. The difference between and determines the capacity of a vacuum line. The capacity is the reciprocal of W, theresistance of the vacuum line to the flow of gas. The relationship of the quantities , S, and W is given by the formula

The coefficient of Eq. 9 becomes unity if 29, the molecular weight of air, is substituted for M, room temperature of 300K. is substituted for T, and is substituted for , where r is the radius of the tube. It is further required that and r be expressed in millimeters and that W be expressed in sec./liter instead of sec./cm3. After making these substitutions and neglecting the second term in the parentheses, Eq. 9 reduces to

Evacuation. The factors determining the rate at which an apparatus is evacuated are the volume of the apparatus, V, the effective speed of the system of pumps, S., and the limiting pressure which the pumps are capable of attaining, . The method of evaluating the first factor, V, is obvious. The value of S may be calculated from the values of and W by Eqs. 8 and 10, or it may be measured by connecting the leak and gauge to the apparatus.

The value of is not easy to estimate, so it is necessary to measure it with a gauge. does not depend on the pumping speed of the pumps on tight systems which are outgassed. When the system is leaking or giving off gas, depends on the rate of leaking as well as the speed of the pumps. On a tight outgassed system the limiting pressure for mercury diffusion pumps equipped with a liquid air trap is 10 mm or less. For oil diffusion pumps without traps the limiting pressure varies from 10 to 10 mm, although lower values are occasionally reported. The vacuum attainable with mechanical pumps is usually 10^-2 to 10 mm. The water aspirator is restricted to work at pressures above the vapor pressure of water, about 25 mm of mercury at room temperature.

The effect of outgassing on is illustrated by an experiment described by Dushman.⁸ He found a limiting pressure of 0.033 bar for a Gaede rotary pump connected to a vacuum gauge when the connecting glass tube was giving off gas. When the glass tubing, however, was baked out until its surface was free of absorbed moisture and other gases, the limiting pressure was reduced to 0.0007 bar.

The rate at which the pressure is reduced in an apparatus, as determined by the pumping speed S, the volume V, and the limiting pressure , is given by the equation

If and are much larger than , then may be neglected, and Eq. 12 can be simplified to the form

Outgassing of glass and metals. Outgassing removes gases adsorbed to the surface of glass and metal. It is necessary to outgas exposed glass and metal in order to obtain the highest degree of vacuum. Prolonged heating of glass at 150° to 200°C. in vacuum removes most of the gases adsorbed on the surface, while further heating to 300°C. removes the final monomolecular film of water and adsorbed gases. Gases liberated when the heating is carried above this temperature originate from the decomposition of the glass.¹¹

In practice, lead-glass apparatus is outgassed by heating it in an oven or with a soft flame to a maximum temperature of 360°C. for a time varying from 10 minutes to an hour or more. Lime glass and hard glass are heated to 400° and 500°C. respectively. Higher temperatures are to be avoided, since the annealing or softening point of soft glass is only 425°C. and of hard glass 550°C.

Before a glass apparatus is sealed off from the pumps, the seal-off constriction is heated for a minute or two at a temperature just below the softening point of the glass.

When metals are strongly heated in a vacuum, they give off adsorbed gases as well as absorbed gases and gas arising from the decomposition of oxides near the surface. Gases under the surface layer of the metal, both dissolved gases and those held in chemical combination, are difficult to remove, even at elevated temperatures, unless the metal is fused. The metal oxides, with the exception of chromium oxide, are readily dissociated in vacuum at elevated temperatures. Metals which have been fused in vacuum are now available commercially.¹²

Surface gas on tungsten wire is liberated by a temperature of 1500°C. From 70 to 80 per cent of this gas is carbon monoxide, and the remainder is hydrogen and carbon dioxide.¹³ The volume of surface gas evolved, measured at standard conditions, amounts to three or four times the volume of the tungsten wire. Sweetser studied the gas liberated by copper, nickel, Monel, and copper-coated nickel-iron alloy (Dumet). He found that these metals rarely gave off a volume of gas greater than the volume of the wire.¹⁴

Marshall and Norton have studied the gases given off by tungsten, molybdenum, and graphite.¹⁵ After these materials have been outgassed by prolonged heating in vacuum at temperatures above 1800°C., they may be exposed to atmospheric pressure, and the gases which they then take up are readily removed by subsequent reheating to a moderate temperature in vacuum. However, they should not be touched with the fingers.

Many metals may be heated in hydrogen to remove surface contamination. At the same time dissolved gases near the surface of the metal are, in part, replaced by the hydrogen. This substitution is desirable, since hydrogen comes off readily when the metal is subsequently heated in vacuum either in a bake-out oven or by high-frequency induction.

Vapor pressure of waxes. Table III gives the results of Zabel's measurements of the relative vapor pressures of waxes used in vacuum work. The numbers given there represent the results of measurements taken with an ionization gauge.

The wax compounded from shellac and butyl phthalate (see Notes on the Materials of Research) should exhibit a low vapor pressure, judging from Table III.

Getters may be grouped into three classes, depending on the manner in which they remove residual gases. Some depend on the physical adsorption of the residual gases on the refrigerated surface of a porous substance like charcoal or silica gel; others absorb the gas in the manner that hydrogen is absorbed by palladium black or tantalum; and still others combine with the residual gas chemically.

The high absorbing capacity of charcoal and silica gel is due in part to their large surfaces. The surface of charcoal, for example, is estimated to be as great as 2500 square meters per gram. Absorbent charcoal to be used for removing residual gas is itself first outgassed by heating it in the vacuum produced by the roughing pumps. It should not be heated above the softening temperature of Pyrex, because it will lose some of its absorption capacity owing to "crystallization" of the charcoal and attendant loss of surface area. After this outgassing the pumps are turned off to isolate the vacuum system, and the charcoal is cooled (preferably with liquid air) to develop its absorbing capacity. The absorbing power of charcoal for various gases at 0°C. and-185°C (liquid air temperature) is given in Table IV.

Of the metal getters, tantalum is of special interest. It absorbs hydrogen in large volumes-it may absorb as much as 740 times its own volume of gas at temperatures around 600°C. This absorbed gas is given off when the metal is heated in vacuum at temperatures greater than 800°C. At high temperatures, tantalum is one of the metals most easily outgassed. At elevated temperatures the residual gases, oxygen and nitrogen, are also removed by chemical combination with tantalum. Because of these properties, it is frequently used for radio-tube anodes. The metals columbium and zirconium behave in much the same way as tantalum.

Tungsten and molybdenum, at temperatures above 1000°C., are effective getters.¹⁷ Oxygen is removed by these metals by the formation of oxides which are volatile at temperatures above 1000°C. Hydrogen is dissociated by the high temperature and condenses as atomic hydrogen on the container walls, especially if they are cooled with liquid air.

The alkali metals react with nitrogen, oxygen, hydrogen, and mercury vapor. The absorption of nitrogen, oxygen, and hydrogen is especially strong when the alkali metal is the cathode of a glow discharge.

Barium, calcium, and magnesium are extensively used as getters, since they combine chemically with all residual gases (noble gases excepted). Barium is more active chemically than calcium. These metals are introduced by various ways into the vacuum tubes in which they are to serve as getters. Calcium may be introduced in the form of fresh filings. Barium may be introduced in the form of copper or nickel-covered wire. Either metal may be formed directly in vacuum by reducing it at elevated temperatures from one of its compounds. Usually the introduced metal is vaporized and condensed on the walls of the sealed-off vacuum system, where it forms a mirror. The getter action of the metal is greater in the vapor phase, although the condensed mirror film, especially a film of barium, will react chemically with residual gases which may subsequently appear in the apparatus.

A metal film exhibits, in addition to the chemical action, a physical action which may be of considerable significance. This physical action, the adsorption of gases, is strong because the metal surface is clean. Dushman gives an elementary calculation illustrating this action.¹⁸ A spherical bulb 5 cm in radius containing residual gas at a pressure of about 1/10 mm of mercury will be completely evacuated when sufficient gas is adsorbed on the inside surface of the bulb or on a clean metal film to form a monomolecular layer.

Water and many vapors may be effectively removed by a trap cooled in liquid air. The density of water vapor in a gas, after it is passed through a liquid air trap, is 10 mg/liter. The relative effectiveness of some of the more commonly used drying agents is shown in Table V.¹⁹ Of these, phosphorus pentoxide is the one most frequently used in vacuum work. It should be fused to reduce its vapor pressure and to prevent it from flying about when the system is evacuated.

Fig. 3 illustrates a typical static vacuum system. It represents an X-ray tube being evacuated with a mercury diffusion pump of moderate speed. Pressures as low as 10 mm (or even 10 mm) are obtained in some static vacuum systems. Such extremely high vacuum is required for investigating the photoelectric effect, thermionic emission, and other physical phenomena for which the slightest contamination of a surface is to be avoided. Static vacuum systems are not treated extensively here. The reader who is especially interested in them is referred to the literature.

Kinetic vacuum systems are characterized by a limiting pressure of 10to 10 mm obtained by the use of extremely fast pumps. These pumps, as well as the apparatus which they exhaust, are usually made in the machine shop from ordinary brass and steel. The metal is not outgassed as in static vacuum systems.

Kinetic vacuum systems are inferior to static systems, where surface contamination must be scrupulously avoided. They are, however, satisfactory for applications where the function of the vacuum is to allow the unhindered motion of molecular rays, electrons, ions, and light quanta. For example, kinetic vacuum systems have been applied with success to the vacuum evaporation process for metalizing large telescope mirrors, to the maintenance of vacuum in high-voltage X-ray tubes, metal rectifier tubes, and oscillator tubes, and to the evacuation of spectrographs.

Fig. 4 shows a kinetic vacuum system for the metalization of glass mirrors. There are two obstacles in the way of getting a high vacuum in such a system. First, outgassing by heating is precluded on account of the use of wax seals and on account of the fact that the system may contain thick glass mirrors which cannot be safely heated. Second, there is more chance of small leaks appearing than in a static vacuum system, since the system shown in Fig. 4 must be repeatedly opened. The recent development of fast oil diffusion pumps, which give the degree of vacuum required in spite of these obstacles, has been mainly responsible for the modern extensive use of this type of flexible vacuum system.

Diffusion pumps. Diffusion pumps will operate only if the pressure is less than a few tenths of a millimeter of mercury, and they operate best with a "backing pressure" of a few hundredths of a millimeter of mercury. The necessary "backing pressure" is obtained by mechanical pumps. The operation of a mercury diffusion pump is illustrated in Fig. 5. The pump shown here illustrates Langmuir's practical adaptation of Gaede's discovery of the principle of diffusion pumping.²⁰ The following explanation of its action applies as well to the action of oil diffusion pumps.

A stream of mercury vapor is obtained by heating liquid mercury in boiler B to a temperature of about 110°C. The vapor stream which effuses from the attached chimney is indicated by arrows. This stream forms a partition between chamber N and chamber M. The vapor finally condenses on the water-cooled walls of chamber N and returns under the influence of gravity to the boiler as liquid. Gas molecules in chamber N which diffuse into the vapor partition have a small chance of penetrating it and entering chamber M. Rather, it is more probable that they will be carried by the stream back into chamber N. However, gas molecules in M which diffuse into the vapor partition are carried along by molecular bombardment into N, where they are removed by the mechanical pump.

The pressure in N must exceed that in M by a factor of the order of 100 if the rate of diffusion is to be the same in both directions across the vapor partition. Where N is evacuated by an auxiliary diffusion pump instead of the mechanical pump, pressures of 10 mm of mercury or lower can be obtained in a tight glass apparatus connected to M (provided mercury vapor is removed with a liquid air trap).

Mercury pumps have been studied by many investigators ²¹ Figs. 6 to 12 are representative of the designs which have evolved as a result of these studies. We will not discuss these pumps in detail, as we are mainly interested in this chapter in kinetic vacuum systems and oil diffusion pumps. With oil pumps it is not uncommon to have pumping speeds of some tens or hundreds of liters per second, whereas with mercury diffusion pumps the speeds are ordinarily only a fraction of a liter per second up to a few liters per second.

The use of oils as diffusion pump liquids. There have been many attempts to find a substitute for mercury as a pumping medium, for the use of mercury has one considerable disadvantage, namely, its vapor pressure is so high that traps are required to prevent it from diffusing into the vacuum system and destroying the vacuum. These traps, having a high resistance to the flow of gas, choke the pump. The only widely used substitutes for mercury are oils. The oils used for this purpose are either especially refined petroleum oils of the naphthene type as developed by C. R. Burch,²² or they are organic compounds such as butyl phthalate as developed by Hickman and Sanford²³ of the Eastman Kodak Laboratories. Recently, Hickman has recommended a new synthetic organic oil called Octoil, which is claimed to be superior to butyl phthalate.²⁴ Oils of the type developed by Burch are manufactured under Metropolitan Vickers' patents under the trade name of Apiezon oil.²⁵ Similar oils are now available in this country which yield pressures below 10-6 mm of mercury.²⁶

Oil pumps have the advantage over mercury pumps that they do not require traps except in certain applications. Another advantage is that oil pumps may be fabricated either from steel or from brass and copper, whereas metal mercury pumps must be constructed of steel with welded joints. Brass and copper pumps can be assembled with soft solder, except for the boiler and chimney, where it is advisable to use silver solder. Aside from the questions of traps and construction, the contrast between oil and mercury pumps is less distinct. Oil pumps without traps do not give quite as low a limiting pressure as trapped mercury pumps, although their speed may be many times greater. If traps are used, there is probably little difference between the limiting pressures attainable. Oi1 pumps have the advantage that a baked-out total obstruction charcoal tube at room temperature is as effective as a liquid air trap. However, the use of a total obstruction charcoal trap sacrifices the higher pumping speed of the oil pump.

It is not advisable to use a single oil pump. One should use at least two oil pumps in series. The second pump serves to keep the oil in the first purified. The limiting pressure is about tenfold lower when a second pump is used. Because mercury pumps will operate against a slightly higher back pressure than oil pumps, there are many cases in which a single mercury diffusion pump is adequate.

Oil diffusion pumps. Oil diffusion pumps are like mercury diffusion pumps in several respects. They have the same functional elements-a boiler to vaporize the oil and a chimney for conducting the vapor to the jet. The two types of pumps are also similar in the way in which they function. The oil vapor is protected from the ~et across the throat of the pump and condenses on the cooled walls which form the outer boundary of the throat; and the condensed oil drains from the condensing surface back into the boiler by gravity. The vapor jet may be arranged in several ways: It may be directed upward as in the upjet mercury pump shown in Fig. 5, it may be directed downward as in the umbrella down-jet mercury pump shown in Fig. 6, or it may project laterally as shown in Fig. 7.

Although oil and mercury diffusion pumps have the same functional elements, they differ in the details of construction. The construction of oil diffusion pumps can be carried out in an ordinary machine shop. The important considerations for proper construction are outlined below:

1. The oil is decomposed slightly at the working temperatures of the boiler. This decomposition is accelerated by the higher temperature necessary when the cross section of the boiler is not large enough to afford an adequate surface from which to create vapor, or when the chimney and jet are not ample to deliver the required amount of vapor without an excessively high pressure drop.

2. Since oil has a low latent heat, the pump should be designed so that the heat required to maintain the working temperature of the chimney and jet is supplied by conduction from the heater rather than by condensation of oil vapor. Naturally, copper is the best material for constructing the chimney on account of its large heat conductivity.

3. The decomposition of the oil is catalyzed by copper and brass and not by nickel. Accordingly, all parts of the pump exposed to the hot oil should be nickel-plated.²⁷

4. The amount of oil decomposed in a given time is proportional to the amount of oil present in the boiler. It is, therefore, advisable to have only a shallow layer of oil in the boiler.

5. At least two single - jet pumps in series should be used. Multiple- jet pumps are not recommended because of the difficulty of regulating the flow of vapor to the various jets and of supplying the necessary amount of vapor required by them without an excessive boiler temperature.

6. Throat clearances narrower than 1/8 inch are practical only for up-jet pumps. Condensed oil will bridge gaps of this narrowness in pumps of the down-jet type.

7. Backward evaporation of the oil into the pumping line should be restrained by the use of baffles.

8. Cold oil is a better solvent for many gases and vapors than hot oil. Accordingly, the condensed oil should be returned to the boiler at the maximum temperature possible. Otherwise, a certain amount of the exhaust gases and vapors dissolve in the condensed oil and contaminate it.

9. The use of electric heat for the boiler is advisable, since it is subject to more delicate control than gas heat. A Carload heater unit, such as used in electric stoves, can be re-coiled into a helix of 2 inches in outside diameter or as a flat spiral of smaller cumenslons.

Figs. 13 to 18 illustrate several oil pumps which are currently popular.²⁸ The pump shown in Fig. 13, designed by Sloan, Thornton, and Jenkins, satisfies the requirements for good design outlined above and at the same time combines these features together with simplicity of construction. The following description of this pump is a quotation from a paper of Sloan, Thornton, and Jenkins.²⁹

The Apiezon oil diffusion pump was originally developed by the Metropolitan Vickers Company in England for this very purpose of continuously exhausting radio tubes. The oil is sold commercially in this country.

Fig. 13 is typical of the simplified designs which have been widely adopted in this country. The outer shell 2" in diameter consists of a water-jacketed brass cylinder with a copper plate silver-soldered into its bottom. In the cavity beneath the bottom plate is placed an electric heater which boils the Apiezon "B" oil at less than 200°C in the chamber above. The oil vapor rises through the copper chimney and is deflected downward by a spun copper umbrella. The 5/16" clearance between the edge of the umbrella and the condensing wall is not critical, although an optimum exists for any specified set of pressures. Around the chimney is a glass heat shield, and a metal baffle plate to retard the rise of oil vapor from the roof of the boiler, but these can be omitted without serious consequences. The two baffles above the umbrella prevent the escape of oil vapor directly into the region being evacuated. The convenient baffle system shown here reduces the speed of the pump to less than half, so that its overall speed is only thirty liters per second. This is more than sufficient for these oscillator tubes, since the connecting system reduces the speed to less than ten liters per second. A pressure in the oscillators of 10^-5 mm is sufficient.

Incidentally, the same general design is also well suited to larger pumps of 4" and 6" diameter, for use with larger tubes. The speed of an oil pump can be greatly increased by enlarging the diameter of the overhead region which contains the baffles necessary to guard against escaping oil vapor.

A 2-inch pump of such construction will have a pumping speed of about 30 liters/sec., or a speed factor slightly greater than 50 per cent.

If such a high speed is not needed, an upjet pump may serve. Fig. 14 shows Hickman and Sanford's all-glass design of an up-jet pump.

Fig. 15 shows an all-metal up-jet pump designed by Edwin McMillan.³⁰ With the boiler temperature adjusted to give maximum pumping speed, this pump will work at a rate of 4 liters/sec. against a backing pressure of 1/2 mm of mercury. If the boiler temperature is too high, the action of the pump will be erratic, since returning condensed oil interferes with the vapor jet.

A design combining glass and metal construction, developed by Joseph E. Henderson,³¹ is shown in Fig. 16. He reports this pump to be capable of working against a backing pressure of a few tenths of a millimeter pressure in contrast to the pressure of about mm required for oil pumps with a throat opening of 1/8 inch or more. Pressures as low as 10 mm of mercury were obtained with it when it was operated with a charcoal trap.

A pump designed by Zabel with a novel oil heater added by James A. Bearden³² is shown in Fig. 17. The advantage of a pump of this design is that it quickly starts working after the heater is turned on.

More recently, K. C. D. Hickman and others have experimented with pumps in which the oil is continually purified.³³ Pumps of this type are particularly suitable for work with gases and vapors which dissolve in the oil or decompose it. Fig. 18 shows a pump which incorporates some of the results of Hickman's investigations.

Mercury traps. Mercury vapor diffuses from a mercury diffusion pump into the exhausted vessel unless it is removed in a trap by condensation on a cold surface. Besides the inconvenience and expensive necessity of requiring a refrigerant, the use of traps has the more serious result of choking the pump. This is especially true for big mercury pumps of high speed. For example, a mercury pump with a speed of several hundred liters per second at its throat may have an effective speed beyond the trap of only several tens of liters per second.

The common trap designs for condensing mercury and water vapors are illustrated in Fig. 19. Type A, the simplest, is frequently used for trapping the vapors from a McLeod gauge. It is also useful in conjunction with an ionization or Pirani gauge for hunting leaks. Type B, the most common type, may be conveniently constructed from metal and a simple glass tube as shown at B', or it may be constructed as shown at B" with a separator or baffle to cause the gas to circulate against the cold walls of the glass tube. Both types A and B are immersed in the refrigerant liquid. Types C, C', and C" contain their own refrigerant, but because of inferior heat insulation these traps are less economical to keep cold.

As refrigerant liquids for trapping mercury-and water vapor, either liquid air or dry ice in acetone may be used. The temperature of the former varies from -190°C. to -183°C., depending on the extent to which the nitrogen has been boiled out of the liquid air, leaving liquid oxygen. The temperature of dry ice-acetone mixture is about-78°C. At the temperature of liquid air the vapor pressure of mercury is 1.7 X 10 mm, while at-78°C. it is 3.2 X 10 mm. For trapping water, liquid air temperatures are sufficiently low. However, since the vapor pressure of ice is about 10mm at-78°, the dry ice-acetone mixture is not sufficiently cold to trap water vapor effectively. Accordingly, when this refrigerant is used for mercury, it is necessary at the same time to expose anhydrous phosphorus pentoxide in the vacuum in order to remove the water vapor.

The vapor pressure of the vacuum pump oils used in roughing pumps, according to Dushman, is 10 to 10^-4 mm at ordinary temperatures, 1/5 of this value at 0°C., and negligibly small at the temperature of dry ice or liquid air.

Carbon dioxide is adequately trapped by traps cooled by liquid air, since its vapor pressure, at liquid air temperature, varies from 10 mm to 10 mm. Carbon monoxide, methane, ethane, and ethylene, having considerably higher vapor pressures, are not effectively trapped even by a liquid air trap.

Virtual leaks. Gases will condense when their partial pressure is above the vapor pressure corresponding to the trap temperature. (However, they will re-evaporate later when the pumps reduce the pressure to a sufficiently low value.) This condensation may give rise to a virtual leak if the trap is cooled too soon after the evacuation of a system is started. We use the term verbal leak because the system appears to have a leak, when it is, in fact, quite tight. As an example, consider a system with traps cooled with a dry ice-acetone mixture but with phosphorus pentoxide omitted. Some of the water vapor originally in the system, both in the air and from the walls where it is held adsorbed, will be condensed in the trap. As the evacuation of the system proceeds, the pressure will approach a limit of 10 mm, this being the pressure of the water vapor in the trap, and the system will exhibit all the "symptoms" of a leak. The same effect is encountered if liquid air is put on the system too soon. Some of the water vapor will condense on the upper regions of the trap walls, and as the liquid air level around the trap falls, owing to evaporation, the temperature of the water condensed as ice will rise until it begins to sublime, producing a virtual leak. On the one hand, these ice crystals are too cold to evaporate rapidly and be evacuated by the system (or colder regions of the trap), while, on the other hand, they are warm enough to degrade the vacuum. Likewise, gases like ethylene may condense in a trap cooled by liquid air and degrade the vacuum.

To avoid virtual leaks, the proper procedure is to keep the traps warm until a vacuum is obtained at which mercury begins to diffuse into the evacuated apparatus, that is, until a pressure of about 10^-2 mm is obtained. Then the tip of the trap is cooled until the vacuum reaches its limit, , and finally the trap is immersed in the liquid air to the full depth.

"Oil" traps. The vapor pressures of vacuum-pumping oils, such as Apiezon "B" oil, are very low, but gases produced by thermal decomposition of the oil may give rise to some deterioration of the vacuum and necessitate the use of traps. For example, when Bearden evacuated an X-ray tube with the diffusion pump shown in Fig. 17, he found that a carbon deposit formed on the target of the tube.³⁴ He found, also, that the filaments of the tube deteriorated at an excessive rate. However, the-use of a refrigerated trap greatly reduced these effects. The trap he used was cooled with dry ice in alcohol.

The trap shown in Fig. 20 was designed by Hickman for diffusion pumps which use Octoil.³⁵ According to him, it is sufficient to cool the trap with running water. Electric refrigerator units are sometimes used to trap vapors from oil pumps. These are, naturally, justified only in large and permanent installations.

In ordinary experimental work, charcoal traps are satisfactory for use with oil diffusion pumps. Several charcoal trap designs are shown in Fig. 21. Of these, the total obstruction trap, A, is the most effective, although it has the highest resistance, W, for the gases passing through it. Becker and Jaycox suggested a trap of type A. They found that a charcoal trap removed oil and condensable vapors to such a degree that an ionization gauge indicated a "pressure" as low as 10 mm of mercury.36³⁶ This has been confirmed by Joseph E. Henderson.³⁷

When charcoal traps become charged with oil and vapors, it is necessary to bake them out. Becker and Jaycox observed that condensed pump oils are decomposed by baking them in contact with charcoal, and that the decomposition products are gases.

Construction of kinetic vacuum systems. Glass was formerly used extensively for the construction of vacuum apparatus, but now metal has replaced it for many uses. Glass as a construction material is characterized by its transparency, high electrical insulating quality, and by the fact that it is easily cleaned and may be baked out and sealed off to give a more or less permanent vacuum. Also, auxiliary parts can be welded to an apparatus without the use of any gaskets or sealing wax. These welds are easily tested for leaks with a spark.

Unfortunately, large and complicated apparatus is difficult to construct from glass. On the other hand, large vacuum systems made of metal are not fragile, and repairs and alterations on them can be easily made in the machine shop.

The metal most frequently used is yellow brass. A vacuum-tight apparatus can be made from plates and cylinders of this metal, screwed together and "painted" on the outside with beeswax and rosin mixture; or the plates, cylinders, and so forth, may be fitted together with rubber or lead fuse-wire gaskets. The brass parts may also be soft-soldered or silver-soldered, depending on the temperature resistance and strength required.

Steel apparatus may be soft-soldered, silver-soldered, brazed, or welded. Electric welding is quite satisfactory for vacuum work if it is done in two or three "passes" with shielded electrodes. It is generally less subject to leaks than gas welding, and it does not warp the work as much. Steel vacuum tanks, especially if they are rusty, are sometimes coated on the inside with Apiezon wax "W" to stop leaks as well as to offer a surface which does not give off gas.

Since metal vacuum walls outgas more than glass, small leaks are more difficult to find. It is a common procedure to coat the outside of metal apparatus with lacquer, which seals small leaks and at the same time gives a workmanlike appearance to the apparatus. Glyptal is heat resistant. For example, it may even be used for coating the outside surfaces of diffusion-pump boilers.

Many things are exposed in kinetic vacuum systems which one would not expose in static vacuum systems. Chief among them are rubber (especially as used for gaskets), waxed packing, beeswax and rosin mixture, Apiezon wax, and ordinary machined metal parts which are not outgassed.

Wood, paints and varnishes, porous cements, and rust should not be exposed even in a kinetic vacuum system.

Rubber hose may be used for connections, and with a pinch clamp it serves as a venting device. Rubber should not be exposed to high vacuum if pressures of the order of 10 or less are desired.

Joints. Two tubes of glass or metal may be butt-joined by slipping a wide rubber band over them. The rubber surface, including the junctions of the rubber to the tubes, is painted with several coats of shellac as shown in Fig. 22. This type of joint is easily disconnected. For small tubes, a short length of rubber hose makes a convenient connection. Rubber tape or strips of raw rubber may also be used. Inasmuch as rubber is somewhat permeable to some gases and gives off hydrogen sulphide and other vapors in vacuum, the connected tubes should always fit together neatly to decrease the area of rubber exposed. The joint may be first wrapped with sheet aluminum and then withrubber.³⁸ This procedure decreases the area of rubber exposed. If any considerable area of rubber is exposed, it is advisable to boil it in a 15 per cent caustic solution (potassium hydroxide or sodium hydroxide) to dissolve free sulphur aild remove talc from its surface. It is then washed with water and dried, either with alcohol or by a vacuum pump If rubber tubing becomes porous and checked with age, it should be painted on the outside with castor oil.

Two metal tubes may be joined with flanges which are sealed with a tongue and groove joint fitted with a rubber gasket as shown in Fig, 23. This construction is recommended where mechanical strength is desired and also where the joint must withstand moderate internal pressure. The tongue should have the same thickness as the groove to within a few thousandths of an inch, so that the rubber I gasket will not extrude as the pressure for fitting the joint is applied. The gasket is cut from a sheet of packing with a cutter like the one shown. The rubber gasket is used dry, and if the tongue and groove have bright smooth surfaces, the joint is sure to be free from leaks. Furthermore, the joint exposes very little rubber surface to the vacuum system.

In another type of joint, shown in Fig. 24, a lead fuse wire can be used as a gasket instead of rubber. The gasket in this case is a loop of 20-ampere fuse wire, butt-welded by means of the heat from a match and a little soldering flux. The circumference of this loop is made slightly shorter than required and is stretched into the groove to make a snug fit. The pressure applied in the flange flows the lead into intimate contact with the two elements of the joint. Lead-wire joints can be used on systems to operate at elevated temperatures, since they will hold to higher temperatures than tongue and groove joints sealed with rubber. A lead gasket of this type is used on the 40-inch bell jar for aluminizing astronomical mirrors as shown in Fig. 13 of Coating of Surfaces: Evaporation and Sputtering. This particular joint has been made more than a hundred times, and it has been consistently vacuum-tight. Aluminum wire holds to even higher temperatures.

Seals. It is frequently necessary to make a vacuum-tight seal between a glass bell jar and a metal base plate. Formerly, stopcock grease was used, applied to the foot of the bell jar This type of seal was not always tight, and the grease frequently entered the apparatus and contaminated exposed surfaces. A better procedure is to use wax instead of stopcock grease. The bell jar is set on the base plate, both the foot of the bell jar and the base plate being clean and dry. Beeswax and rosin mixture, smoking hot, is then applied with a medicine dropper to the outer edge of the bell-jar flange to effect the seal, as illustrated by Fig. 25. The bell jar can be removed from the base plate in the following manner: After-scraping away the wax with a putty knife, loosen the jar by striking a sharp blow at the top with the palm of the hand or by driving a razor blade gently under the edge of the jar. If a metal bell jar is used, a recess may be provided so that the seal can be cracked by prying with a screw driver after as much of the wax as possible has been scraped away.

Windows may be sealed over observation ports in a similar manner. The wax is applied with the medicine dropper, and the seal is effected without sensibly heating either the port or the window.

Windows may be sealed with hard wax. It is necessary to heat both the port and the window to temperatures above 100°C. when hard waxes such as Apiezon "W," Picein, shellac, or DeKhotinsky wax are used. First the window and port are carefully cleaned, and then the window is clamped in the desired position. After being heated to the required temperature, the wax is applied to the outside edge of the window, from where it will be drawn between the window and the port by capillary force. The wax drawn under the window forms a thin bonding layer of large area, which exposes a minimum surface of wax to the vacuum. (See Fig. 26.)

Fig. 27 shows the procedure for sealing two glass tubes together with Picein wax to form a butt joint or telescope joint. The procedure here is to wrap a soft strip of Picein around the warmed glass tubes. This strip is molded from a stick of wax after it is thoroughly softened. The stick of wax is softened by alternately heating it in a Bunsen flame until its surface is liquid and withdrawing it to cool until its surface solidifies. When the strip is ready and while it is still soft it is wrapped around the warmed joint and molded as shown in Fig. 27. The wax will not stick to the fingers if they are damp. After the glass and wax are cool, a flame is applied to fuse the wax superficially and insure tangential contact to the tubing.

Electrodes. In the chapter on glass blowing, we discussed the construction details for leading electrical conductors into glass apparatus. In a kinetic vacuum system, electrodes are usually fastened through holes in a metal wall. Construction details are shown in Fig. 28 for high-current conductors and in Fig. 29 for high-potential conductors. The high-current conductor or electrode consists of a brass screw bolted into the vacuum wall, the head and body of the screw being insulated from the metal vacuum wall with mica. After the insulation has been tested with a lamp, the whole assembly is made vacuum-tight by coating the screwhead, insulation, and the local area of the outside surface of the vacuum wall with beeswax and rosin mixture or with glyptal lacquer. Beeswax and rosin mixture is used if the operation temperature is about room temperature. Glyptal, after baking to polymerize it, is used for operation temperatures up to about 100°.

Valves. Valves are used on the low-vacuum side of diffusion pumps to prevent oil in the mechanical pumps from flowing into the other parts of the apparatus. Between the diffusion pumps and the apparatus, large valves are useful to allow by-passing the diffusion pumps. For example, in the vacuum system shown in Fig. 4, a large 4-inch valve makes it possible to open up the mail vacuum chamber and re-evacuate it without destroying the vacuum in the diffusion pumps. Valves between various parts of a large vacuum system facilitate narrowing the search for leaks, since one part after another can be isolated.

The simplest valve for venting a vacuum system is a short length of rubber hose and a pinch clamp. Rubber vacuum hose is now available in sizes up to 1 inch in diameter.³⁹ This large hose may be used in short lengths on the high vacuum side of the diffusion pump when the pumps have a high capacity and when a vacuum of only 10^-4 is desired. Usually, however, it is advisable to confine the use of rubber hose to the low-vacuum side of the diffusion pumps.

Ordinary plumbing valves can be modified for use in high-vacuum work. The glands are repacked with twine soaked in Apiezon compound beeswax, stopcock grease, or universal wax. Since the rubber gaskets supplied in these valves are often too hard for vacuum work, it is necessary to replace them with softer rubber. It is advisable to make a new end for the valve so that the new gasket rubber can be retained in a groove. The outside of the valve may be painted with shellac or glyptal lacquer as insurance against leaks, it may be coated with Apiezon wax "W," or it may be tinned. DuMond and Rose have described valves equipped with a sylphon bellows as a substitute for a packing gland.⁴⁰ This is illustrated in Fig. 30. A packless valve of this type manufactured by the Hoffman Company can be readily adapted to vacuum work as shown in Fig. 31.⁴¹

Ordinary stopcocks can be sealed with stopcock grease for use in a high-vacuum system. Stopcock grease is made by digesting 1 part pale crepe rubber cut in small pieces with 1 part Apiezon compound "M." This digestion is carried out in a balloon flask with prolonged mechanical stirring at an elevated temperature obtained by means of a water or steam bath.

When it is necessary to avoid grease on a stopcock, bankers' sealing wax, Apiezon wax "W," or Picein can be used.⁴² Of these waxes, Picein exhibits the best body. With any one of them the valve is warmed until the wax becomes plastic each time that it is turned. (See Fig. 32.) Stopcocks may be lubricated with dry graphite and sealed with mercury.

Mechanical motion. Mechanical motion can be introduced into a vacuum system through nonferrous vacuum walls with a magnet. An armature or bar magnet is fastened to the moving part inside the system and actuated by an electromagnet outside. The armature can be hermetically sealed in a glass tube to avoid outgassing.

A metal bellows can be used to introduce the reciprocating or oscillating motion of a lever.⁴³ When the end of the lever executes a circular motion, this motion can be transformed into rotation inside the vacuum.

Van de Graaf has developed the high-speed sealed shaft shown in Fig. 33. The packing used is Apiezon grease "M" charged with graphite and the pumping action of the right- and left-handed screws, cut on the shaft, prevents the extrusion of the packing compound.

Mechanical motion can be introduced through an ordinary packing gland packed with cotton twine soaked in Apiezon compound "Q" as shown in Fig. 4.

Leaks. In planning a metal vacuum system, a part of the construction cost should be set aside to provide suitable fittings, plugs, plates, and tie bolts. The use of these makes it possible to pump air or hydrogen into separate compartments of the apparatus until the pressure is 50 or 100 lbs./square inch. For detecting leaks the pumped-up compartment is submerged in water or painted with liquid soap solution. Hydrogen, which may be used instead of air to pump up the apparatus, has the advantage over air that it diffuses through small holes approximately four times faster. When leaks are found, they may be repaired by welding or soldering or by merely peening the surface. After the whole apparatus is put together, the outside of the system is coated with several layers of glyptal varnish, alternating the color of the varnish coats, say blue and red, to facilitate complete coverage with each one of them. If possible, the coating is baked at a temperature of about 120°C.

Leaks are usually found in a glass apparatus by passing the ungrounded high-potential electrode of a spark coil or high-frequency coil over the surface of the glass. When the electrode comes near the leaking channel, a spark jumps to it and causes residual gas inside the apparatus to become luminous. As a safety precaution, a spark gap of 4 to 2 inch should be connected in parallel with the electrode and the ground to prevent an excessive potential which might puncture the glass.

Leaks in metal apparatus which are not detected by immersing the apparatus in water or painting it with soap solution are more difficult to locate. In general, the procedure for finding them involves covering the walls of the apparatus with a liquid which solidifies, with water, or with a gas. In any case, while the search is in progress, the apparatus is maintained at the lowest pressure possible.

If a liquid covering is used, it is applied to local areas in progression until the offending region is located. As covering one may use a molten mixture of beeswax and resin, or it may be a thick solution of either shellac in alcohol or glyptal lacquer brushed on the walls, or it may be cellulose acetate solution sprayed on the walls. When a solution of shellac (or lacquer) is applied to the outside of a leaking channel, the solution is drawn into the channel by the vacuum. As the solvent evaporates from this solution into the vacuum chamber, the liquid in the channel congeals. Thus, the leaking channel is, in effect, filled with a solid shellac core. The amount of solvent passing into the vacuum through this core is negligible in cases where the procedure is suitable.

When the leak is covered with the solution, the vacuum usually improves at once. This improvement may be indicated by the disappearance of luminosity in a connected discharge tube and finally by sparking across an alternate gap. If an ionization or Pirani vacuum gauge is used, covering of the leak is indicated by motion of the spot of light on the scale of the instrument.

The general region in which leaks are located may be determined by temporarily covering the region with water. As the vapor pressure of water is only about 1/30 of an atmosphere, the leak may be expected to be attenuated 30-fold when it is covered.

The third procedure for finding leaks involves covering general regions of the apparatus with gas, carbon dioxide for the top parts, since it is heavier than air, and illuminating gas for the bottom. Webster has described the use of a rubber "coffer dam" to facilitate the management of the gas.⁴⁴ Illuminating gas may be blown on various parts of the apparatus from a hose, or the surface may be gone over with a wad of cotton wet with ether. Evidence that the leak is admitting gas instead of air is a change in character of the luminescence in a discharge tube connected to the apparatus or a change in reading of a vacuum gauge separated from the apparatus by a liquid air trap.

There are two procedures for using a discharge tube with illuminating gas, carbon dioxide, or ether. By the first, the obtainable vacuum is necessarily so poor, on account of the leak, that a distinct discharge is obtained. When the leak is covered, the luminosity in the positive column changes from the brownish-red color characteristic of air to the bluish-green of carbon dioxide or to the white of gas and ether. By the second procedure, used when the leak is small and a lower pressure is attainable in the system, the luminosity in the discharge is feeble. Webster suggests connecting the discharge tube behind one of the diffusion pumps as shown in Fig. 4. The backing pumps are then shut off, preferably just behind the discharge tube connection. The diffusion pump compresses the gas which the leak may be admitting, resulting in a more brilliant luminescence in the discharge tube.

A liquid air trap may be connected between the apparatus and a vacuum when carbon dioxide or other condensable gases are used. With this arrangement, when the leak is admitting carbon dioxide, the trap condenses this gas, thus preventing it from entering the gauge. At the same time air and other gases which do not condense in the trap are removed by the pumps. As a result, even though the pressure in the system may have increased, an improvement of the vacuum is indicated.

Obviously, a gauge which reads continuously (Knudsen, Pirani, or ionization gauge) is preferred to a McLeod gauge for hunting leaks. Relative rather than absolute readings of the pressure are sufficient for locating leaks. Thus, the Pirani and ionization gauges are satisfactory, although they do not give absolute pressure determinations.

Vacuum gauges. A vacuum gauge determines the pressure in an evacuated apparatus by a measurement of some physical property of the residual gases, such as viscosity, heat conductivity, and so forth. The measurement of the response of a gauge to the residual gas naturally becomes more delicate as the gas becomes more and more tenuous. Finally, below a certain pressure limit (which is characteristic of a given gauge) the gauge does not behave measurably different from what it would if the vacuum were perfect. For example, a discharge tube will give qualitative indications of pressure down to about 10 mm of mercury. Below this pressure the tube becomes nonluminous and nonconducting. The characteristic limits for some of the other Eauzes are as follows:

The ionization gauge measures with a galvanometer the positive ions that are formed in an electric field when the residual gas is bombarded with electrons. The Langmuir gauge depends on the measurement of viscosity, and the Pirani gauge on the measurement of heat conduction of the residual gas. The Knudsen absolute manometer measures the momentum transferred from a hot to a cold surface by the gas molecules.

Of the above gauges, only the McLeod and Knudsen are absolute manometers in the sense that their geometry and other measurable characteristics of construction and operation determine their response at a given pressure. The McLeod gauge is the simplest and most reliable for permanent gases, but it has the disadvantage of giving erratic response or no response at all to water vapor, carbon dioxide, ammonia, and pump oil vapors which adsorb on the walls of the gauge or condense to a liquid. This disadvantage is serious, inasmuch as water vapor, carbon dioxide, and so forth are often of importance in the last stages of obtaining a high vacuum. The Knudsen gauge responds to gases and vapors alike.

The response of an ionization gauge is difficult to predict from its construction details, and it must be calibrated with a McLeod gauge using permanent gases. Furthermore, before the pressure can be inferred, it is necessary to make corrections for the molecular weight of the gas and also for the possibility that the gas may be dissociated by the electron bombardment. Quantitative application of the gauge is unreliable to the degree to which these corrections are uncertain. Likewise, the response of the Pirani gauge depends on the molecular weight of the residual gas, and it must be calibrated with a McLeod gauge that uses permanent gases. The same is true for the viscosity gauge.

The McLeod gauge.⁴⁵ Although many improvements have been made in the McLeod gauge, they have seldom been applied The gauge as ordinarily used today is essentially the same as it was originally. We will discuss here the simple form of the gauge illustrated in Fig. 34. It is made of glass as shown and is mounted on a vertical board. The difference in the heights of the mercury levels in the gauge and in the reservoir is approximately equal to the barometric pressure B. As the reservoir is raised, the mercury level in the gauge comes above the Y-branch, thus isolating a definite volume V₁ of the residual gas. This is isolated at the unknown pressure , the pressure of the residual gas in the apparatus to which the gauge is connected. As the mercury reservoir is further raised, the isolated residual gas is compressed, and when its volume has been reduced to a volume , the pressure is great enough to produce a sensible difference in the height of the mercury meniscus in the two capillaries, A and B. At the left, in Fig. 34, the mercury levels are shown at the beginning of a measurement, and at the right they are shown in two different positions corresponding to two methods of making readings. In one, if the meniscus in B is adjusted to the same height as the top of capillary A, the final volume, , is equal to, when is the cross-section area of the capillary. The decrease in volume from V₁ to is ordinarily of the order of onehundred-thousandfold, with a corresponding increase of pressure in the capillary over that which obtained originally. The construction of the gauge with the comparison capillary B of identical bore with A eliminates the necessity of making corrections for surface tension. Referring to Eq. 1, we see that the product V₁1 is, in this case, a constant. The original product, V₁, is equal to the final product, . From this we get the expression connecting the unknown pressure with the observed manometer difference, :

The second procedure of making the observations on and is illustrated at the right in Fig. 34. The gas is compressed to a definite mark on capillary A at a distance Aho from the top, so that the final volume, , is the same for every measurement. The final pressure necessary to compress volume V₁ to is , and the pressure in the system is determined by these quantities, according to the following equation:

The McLeod gauge is thoroughly reliable for the permanent gases from 10 mm to 10^-4 mm of mercury. It is less reliable to 10. Below this the indications are only qualitative, and at 10 the mercury often sticks in the top of capillary A.

The gauge is most reliable after it has been outgassed by gently warming it with a soft flame. Three gauges with different values of V₁ are necessary to cover adequately the complete pressure range from 10 to 10 mm. Many of the designs of McLeod gauges are more elaborate than the one shown in Fig. 34. For example, three bulbs may be mounted together with one reservoir, one for low pressures, one for intermediate pressures, and one for high pressures.

The McLeod gauge is fragile. If it breaks, not only is the gauge lost but what is often more serious, mercury may get into the vacuum system. In glass vacuum systems using mercury pumps this is not as serious as it may be in kinetic vacuum systems. These systems, fabricated of brass with soft-soldered joints, are attacked by mercury and the joints are destroyed.

Accidents with this gauge are usually caused by bringing the reservoir up too quickly. Then mercury in V₁ acquires enough momentum to shatter the bulb when the metal surface arrives at the opening of the capillary tube with no cushion of air to soften the shock.

Admitting air into the vacuum system is to be avoided when the mercury is not completely out of V₁. The admission of air will have the same result as carelessness in raising the reservoir.

Sometimes a mercury pellet will remain in capillary A when the reservoir is lowered. It can usually be brought down by tapping the capillary (after the mercury is all out of V₁). If this treatment fails, the capillary should be heated with a soft gas flame. In the latter case, a sheet of asbestos is placed behind the capillary to protect the calibration scale from the flame.

The capillary tubes used for the construction of McLeod gauges are seldom larger than 2 or 3 mm or smaller than 1/2 mm bore. The volume of the bulb, V₁, ordinarily varies from 50 to 500 cc. Only pure distilled mercury should be used. Mercury is attacked by the sulphur present in rubber hose, so that dross is produced which adheres to the inside of the gauge and may become very annoying. A gauge contaminated with this sulphide may be cleaned out by the combined action of zinc dust and nitric acid. Rubber hose for use on a gauge should be cleaned before it is used by passing hot caustic potash solution back and forth through it for a quarter of an hour or so. The tubing should be thoroughly washed free of caustic and dried before use.

In cases where it is necessary to avoid contamination of the vacuum system with mercury vapor, a liquid air trap should be connected between the vacuum system and the gauge. For kinetic vacuum systems this precaution is often omitted. A stopcock between the gauge and the system which is kept closed when the gauge is not in use minimizes contamination.

The ionization gauge.⁴⁶ Ionization gauges are triodes mounted in a glass bulb connected to the apparatus in which the pressure is to be measured. They are electrically connected as shown in Fig. 35.

Electrons emitted from the filament are accelerated to the grid, and their momentum would carry them to the plate if an inverse field more than sufficient to prevent this were not impressed between the grid and the plate. They therefore return to the grid and are finally collected on it. However, while they are between the grid and the plate, they bombard and ionize some of the molecules of the residual gas present there. These ions are collected on the plate and measured with a sensitive galvanometer. The ratio o ~of this ion current to the current of bombarding electrons or grid current is proportional to the pressure at pressures below about 10^-4 mm.

An ionization gauge may be made from an ordinary three-element radio tube equipped with a glass connection to the vacuum system. Such gauges are useful for the pressure range from 10^-3 mm to 10^-6 mm of mercury.

Fig. 36 shows the construction details of a gauge designed to have higher insulation of the-plate than an ordinary radio tube. Measurements with it are possible to a pressure of 10^-9 mm of mercury. The upper end of a glass bulb supports the plate assembly, while the lower end supports the combined grid and filament assembly. The grid is made from a piece of nickel screen rolled to form a cylinder. This is bound mechanically to the central glass tube through the bottom by wrapping it with wire, and it is connected electrically to the grid electrode with one loose end of the wrapping wire. There are two filaments, but only one is used. The other is held in reserve to be used if the first is accidentally burned out. The filaments may be replaced by cutting the central tube at S.

Expensive auxiliary electrical instruments are required for this gauge. They should be protected with Littelfuses as shown in the wiring diagram (Fig. 35).

The plate may be outgassed with high-frequency currents or by electron bombardment. In the latter case, an alternating potential of 500 volts is applied between the filaments and the plate. The amount of heat developed depends on the emission from the filament, and this is controlled by the filament current. Outgassing of the plate and glass walls of the gauge is necessary if quantitative measurements are to be made. However, for hunting leaks it is necessary only to outgas the plate once.

Dunnington has made a gauge using 30-mil helices of tungsten wire for both plate and grid. These helices are outgassed simply by passing a current through them for a few seconds. He found that such a gauge did not have a linear relationship between pressure and ratio of plate to grid currents. Once calibrated, however, it was found to be very reliable.

At a given pressure, the ratio of plate to grid current is different for different values of the grid current. For this reason, it is necessary to adjust the grid current to some definite value, usually in the range of 10 to 50 milliamperes.

Ordinarily, the filament is mounted in a bulb fitted with a connecting tube and is balanced with an identical compensating filament mounted in an adjacent arm of the bridge. This auxiliary bulb is evacuated and sealed off at a very low pressure. The use of an auxiliary bulb serves to make the gauge insensitive to variations in room temperature. Changes in the over-all temperature of one bulb are the same as changes in the other, so that the galvanometer does not respond to these changes but only to the changes produced by the residual gas in the one bulb.

Fig. 38 shows a calibration curve of a Pirani gauge manufactured by E. Leybold Nachfolger. The pressure range over which it is useful extends from 1/10 mm to 10^-4 mm.

The construction of the Pirani gauge, together with the theory of its use, is treated in detail by several authors, who should be consulted by anyone planning to use the gauge for quantitative measurement. A gauge useful for qualitative work, as for hunting leaks, can be improvised from two ordinary 20- to 40-watt vacuum tungsten lamps, one of which is fitted with a connecting tube. Fig. 39 shows the construction details for this gauge. The bridge galvanometer should have a sensitivity of about 10^-8 ampere division. Sometimes uncertain contact to the supporting wires may cause variable heat loss from the filament, and this should be suspected if the gauge is erratic. Tapping will often define the contact.

The Langmuir gauge.⁴⁸ Langmuir's viscosity gauge is made with a flattened quartz fiber about 50 thick and from five to ten times as wide. This quartz ribbon is about 5 cm long and is mounted in one end of a glass tube about 25 mm in diameter, as shown in Fig. 40. When this ribbon is set vibrating in a high vacuum, the amplitude changes very slowly because the damping by the residual gas is almost negligible, and, owing to the low internal viscosity of fused quartz, the loss of vibrational energy from this source is also low. From atmospheric pressure down to a few millimeters of mercury, the damping produced by the molecules of the residual gas is nearly independent of pressure. Over the transition range of pressure, where the damping varies from this constant value to zero, the time required for the amplitude of vibration to decrease to half is an index of the pressure. Within this range the relation between the time, t, the pressure, P, and the molecular weight of the residual gas is given by the following formula:

A feature of this gauge is its small volume. Because there are no metal parts exposed, the gauge is suitable for measuring the pressure of corrosive gases like the halogens. This gauge, in conjunction with a McLeod gauge, may be used for measuring the molecular weight of an unknown gas at low pressures.

The flat quartz fibers may be obtained by drawing them out of the side rather than the end of a quartz tube or by following the technique given in Chapter V.

Figs. 40 and 41 show construction details and the method of mounting the fiber together with a pivoted glass tube, which contains an iron armature operated by an external electromagnet, to start the fiber vibrating. An optical arrangement for observing the amplitude of vibration is also shown. An image of the quartz fiber is projected on a scale with a simple lens.

The Knudsen gauge.⁴⁹ Fig. 42 shows the Knudsen gauge as designed by DuMond. When this gauge is constructed according to the specifications outlined by him, it is claimed to have a definite sensitivity, so that no preliminary McLeod calibration for it is needed. The gauge shown here differs slightly from DuMond's design in that it is equipped with a permanent (Alnico) magnet for damping.

Also, it has a special liquid air trap for determining what fraction of the pressure indication is produced by condensable vapors.

The Knudsen gauge is to be preferred to the McLeod gauge where it is important to avoid contaminating a vacuum system with mercury. No expensive auxiliary instruments are required with the Knudsen gauge, as with the ionization gauge. Furthermore, the filaments will not burn out and the suspension is not delicate.

It is advisable to modify DuMond's design so that all connections and supports fasten to one end plate. This facilitates making repairs. The metal case thus becomes, in effect, a water-cooled covering "bell jar" fitted with a window.

Notes:

¹ This chapter is intended primarily to supplement the works on vacuum technique as listed: Dunyoer, L., Vacuum Practice. New York: D. Van Nostrand and Company, 1926. Dushman, S., Frank. Inst., J., 211, 689 (1931).Dushman, S., High Vacuum. Schenectady: General Electric Company, 1922. Goetz, A., Physik und Technik des Hochvakuums. Aktges. Braunchweig: Friedrich Vieweg und Sohn, 1926. Kaye, G. W. C., High Vacua. New York: Longmans, Green and Company, 1927. Newman, F. H., The Production and Measurement of Low Pressures. New York: D. Van Nostrand and Company, 1925.