Deadband (Hysteresis): In a digital controller, there may be one switching point at which the signal increases and another switching point at which the signal decreases. The difference between the two switching points is called hysteresis or deadband.

Degree: An incremental value in the temperature scale, i.e. there are 100 degrees between the ice and the boiling point of water in the Celsius scale and 180°F between the same two points in the Fahrenheit scale.

Deviation: The difference between the value of the controlled variable and the value at which it is being controlled.

Differential: For an on/off controller, it refers to the temperature difference between the temperature at which the controller turns heat off and the temperature at which the heat is turned back on. It is expressed in degrees.

Differential Input: A signal-input circuit where SIG LO and SIG HI are electrically floating with respect to ANALOG GND (METER GND which is normally tied to DIG GND). This allows the measurement of the voltage difference between two signals tied to the same ground and provides superior common-mode noise rejection.

Digit: A measure of the display span of a panel meter. By convention a full digit can assume any value from 0 through 9, a 1/2 digit will display a 1 and overload at 2, a 3/4 digit will display digits up to 3 and overload at 4, etc. For example, a meter with a display span of +3999 counts is said to be a 3 3/4 digit meter.

DIN (Deutsche Industries Norm): A set of German standards recognized throughout the world. The 1/8 DIN standard for panel meters specifies an outer bezel dimension of 96 x 48 mm and a panel cutout of 92 x 45 mm.

DIN 43760: The standard that defines the characteristics of a 100 ohm platinum RTD having a resistance vs. temperature curve specified by = 0.00385 ohms per ohms per degree C.

Dissipation Constant: The ratio for a thermistor which relates a change in internal power dissipation to a resultant change of body temperature.

Drift: A change of a reading or a setpoint value over long periods due to several factors including change in ambient temperature, time, and line voltage.

Droop: A common occurrence in time-proportional controllers. It refers to the difference in temperature between the setpoint and where the system temperature actually stabilizes due to the time-proportioning action of the controller.

Dual Element Sensor: A sensor assembly with two independent sensing elements.

Dualslope A/D Converter: An analog-to-digital converter which integrates the signal for a specific time, then counts time intervals for a reference voltage to bring the integrated signal back to zero. Such converters provide high resolution at low cost, excellent normal-mode noise rejection, and minimal dependence on circuit elements.

Duplex Wire: A pair of wires insulated from each other and with an outer jacket of insulation around the inner insulated pair.

Duty Cycle: The total time to complete one on/off cycle usually refers to the on/off cycle time of a temperature controller.

Ground: 1. The electrical neutral line having the same potential as the surrounding earth. 2. The negative side of do power supply. 3. Reference point for an electrical system.

Grounded Junction: A form of construction of a thermocouple probe where the hot or measuring junction is in electrical contact with the sheath material so that the sheath and thermocouple will have the same electrical potential.

Hysteresis: (see Deadband)

Hold: Meter HOLD is an external input which is used to freeze the display BCD HOLD is an external input used to freeze the BCD output while allowing the A/D process to continue operation.

Input Impedance: The resistance of a panel meter as seen from the source in the case of a voltmeter. This resistance has to be taken into account when the source impedance is high; in the case of an ammeter when the source impedance is low.

Insulated Junction: See ungrounded junction.

Insulation Resistance: The resistance value of the cover material of an electrical conductor, expressed in ohms.

Integral: A form of temperature control. See automatic reset, #2.

Interchangeability Error: A measurement error that can occur if two or more probes are used to make the same measurement. It is caused by a slight variation in characteristics of different probes.

Isothermal: A process or area that is at a constant temperature.

Limits of Error: A tolerance band for the thermal electric response of thermocouple wire expressed in degrees or percentage defined by ANSI specification MC96.1 (1975).

Load: The electrical demand of a process expressed as power (watts), current (amps) or resistance (ohms).

Loop Resistance: The total resistance of a thermocouple circuit caused by the resistance of the thermocouple wire. Usually used in reference to analog pyrometers which have typical loop resistance requirements of 10 ohms.

LSD (Least-Significant Digit): The rightmost active (non dummy) digit of the display.

Manual Reset (Adjustment): The adjustment of a proportioning controller which shifts the proportioning band in relationship to the setpoint to eliminate droop or offset errors.

Manual Reset (Switch): The switch in a limit controller that manually resets the controller after the limit has been exceeded.

Maximum Operating Temperature: The maximum temperature at which an instrument or sensor can be safely operated.

Measuring Junction: The thermocouple junction referred to as the hot junction that is used to measure an unknown temperature.

Microvolt: One millionth of a volt, 10^-6 volts.

Mil: One thousandth of an inch (0.001),10^-3.

Milliamp: One thousandth of an amp, 10^-3 amps, symbol mA.

Millimeter: One thousandth of a meter,10^-3 meters, symbol mm.

Millivolt: One thousandth of a volt, 10^-3 volts symbol mV.

Mineral-insulated Thermocouple: A type of thermocouple cable which has an outer metal sheath and mineral (magnesium oxide) insulation inside separating a pair of thermocouple wires from themselves and from the outer sheath. This cable is usually drawn down to compact the mineral insulation and is available in diameters from .375 to .010 inches. It is ideally suited for high-temperature and severe-duty applications.

Multiplex: A technique which allows different input (or output) signals to use the same lines at different times, controlled by an external signal. Multiplexing is used to save on wiring and I/O ports.

NEMA 1/ UL Type 1: Intended for indoor use, to provide a degree of protection against contact with the enclosed equipment and against limited amount of falling dirt.

NEMA 4/ UL Type 4: A standard from the National Electrical Manufacturers Association which defines enclosures intended for indoor or outdoor use primarily to provide a degree of protection against windblown dust and rain, splashing water, and hose-directed water.
Testing - Hosedown: Enclosure is subjected to a stream of water from a hose which has a 1 inch diameter nozzle and which delivers at least 65 gallons per minute from a distance of 10 to 12 feet for a minimum of 5 minutes.

NEMA 7/ UL Type 7: A standard from the National Electrical Manufacturers Association, which defines explosion-proof enclosures for use in locations classified as Class 1, Groups A, B. C or D, as specified in the National Electrical Code.

NEMA 12/ UL Type 12: A standard from the National Electrical Manufacturers Association, which defines enclosures with protection against falling dirt, circulating dust, and dripping non-corrosive liquids.
Testing - Drip: Enclosure is subjected to a drip pan which produces both splashing and driupping.
Testing - Atomized Water: Enclosure is subjected to a spray of atomized water by using a nozzle that produces a 3" round diameter spray, 12" from closure. The air pressure is at 30psi.

NEMA-size Case: An older US case standard for panel meters, which requires a panel cutout of 3.93 x 1.69 inches.

NMR (Normal-Mode Rejection): The ability of a panel meter to filter out noise superimposed on the signal and applied across the SIG HI to SIG LO input terminals. Normally expressed in dB at 50/60 Hz.

Nicrosil/nisil: A nickel chrome/nickel silicone thermal alloy used to measure high temperatures. Inconsistencies in thermoelectric voltages exist in these alloys with respect to the wire gage.

Noise: An unwanted electrical interference on the signal wires.

NPT: National Pipe Thread.

Ofhc: Oxygen-free high-conductivity copper. The industrial designation of the pure copper used in a Type T thermocouple.

Ohmmeter: An instrument used to measure electrical resistance.

OMEGALLOY†: A nickel chrome/nickel silicone thermal alloy used to measure high temperatures. Inconsistencies in thermoelectric voltages exist in these alloys with respect to the wire gage.
†Registered Trademarks of OMEGA ENGINEERING, INC.

On/off Controller: A controller whose action is fully on or fully off.

ORP: Oxidation Reduction Potential

Overshoot: The number of degrees that a process exceeds the setpoint temperature when coming up to the setpoint temperature.

PPM: Abbreviation for “parts per million”, sometimes used to express temperature coefficients. For instance, 100 ppm is identical to 0.01%.

Platinel: A non-standard, high temperature platinum thermocouple alloy whose thermoelectric voltage nearly matches a Type K thermocouple (trademark of Englehard Industries).

Platinum: A noble metal which in its pure form is the negative wire of Type R and Type S thermocouples.

Platinum 6% Rhodium: The platinum-rhodium alloy used as the negative wire in conjunction with platinum 30% rhodium to form a Type B thermocouple.

Platinum 10% Rhodium: The platinum-rhodium alloy used as the positive wire in conjunction with pure platinum to form a Type S thermocouple.

Platinum 13% Rhodium: The platinum-rhodium alloy used as the positive wire in conjunction with pure platinum to form a Type R thermocouple.

Platinum 30% Rhodium: The platinum-rhodium alloy used as the positive wire in conjunction with platinum 6% rhodium to form a Type B thermocouple.

Platinum 67: To develop thermal emf tables for thermocouples the National Bureau of Standards paired each thermocouple alloy against a pure platinum wire (designated Platinum 27 prior to 1973, and currently Platinum 67). The thermal emf’s of any alloy combination can be determined by summing the “vs. Pt-67” emf’s of the alloys, i e. the emf table for a Type K thermocouple is derived from the Chromel vs. Pt-67 and the Alumel vs Pt-67 values.

Primary Standard (NBS): The standard reference units and physical constants maintained by the National Bureau of Standards upon which all measurement units in the United States are based.

Probe: A generic term that is used to describe many types of temperature sensors.

Process Meter: A panel meter with sizeable zero and span adjustment capabilities, which can be scaled for readout in engineering units for signals such as 420 mA, 1050 mA and 1-5 V.

Process Variable (PV): The parameter that is controlled or measured. Typical examples are temperature, relative humidity, pressure flow, level, etc. The high process variable is the highest value of the process range, expressed in engineering units. The low process variable is the lowest value of the process range.

Proportioning Band: A temperature band expressed in degrees within which a temperature controller’s time proportioning function is active.

Proportioning Control Mode: A time-proportioning controller where the amount of time that the relay is energized is dependent upon the system’s temperature.

Proportioning Control Plus Derivative Function: A time proportioning controller with a derivative function The derivative function senses the rate at which a system’s temperature is either increasing or decreasing and adjusts the cycle time of the controller to minimize overshoot or undershoot.

Proportioning Control Plus Integral: A two-mode controller with time proportioning and integral (auto reset) action. The integral function automatically adjusts the temperature at which a system has stabilized back to the setpoint temperature, thereby eliminating droop in the system.

Proportioning Control with Integral and Derivative Functions: Three-mode PID controller. A time-proportioning controller with integral and derivative functions. The integral function automatically adjusts the system temperature to the setpoint temperature to eliminate droop due to the time-proportioning function. The derivative function senses the rate of rise or fall of the system temperature and automatically adjusts the cycle time of the controller to minimize overshoot or undershoot.

Protection Head: An enclosure usually made out of metal at the end of a probe where connections to the probe are made.

Protection Tube: A metal or ceramic tube closed at one end into which a temperature sensor is inserted. The tube protects the sensor from the medium into which it is inserted.

Rankine (°R): An absolute temperature scale based upon the Fahrenheit scale with 180° between the ice point and boiling point of water. 459.67°R = 0°F.

Rate Action: The derivative function of a temperature controller.

Rate Time: The time interval over which the system temperature is sampled for the derivative function.

Ratiometric Measurement: A measurement technique where an external signal is used to provide the voltage reference for the dual-slope A/D converter. The external signal can be derived from the voltage excitation applied to a bridge circuit or pick-off supply, thereby eliminating errors due to power supply fluctuations.

Reference Junction: The cold junction in a thermocouple circuit which is held at a stable known temperature. The standard reference temperature is 0°C (32°F); however, other temperatures can be used.

Relay (Mechanical): An electromechanical device that completes or interrupts a circuit by physically moving electrical contacts into contact with each other.
Form “A”–A single-pole, single-throw (SPST) relay with normally open (NO) and common contacts. These contacts close when the relay coil is energized. They open when power is removed from the coil.
Form “B”–A single-pole, single throw (SPST) relay with normally closed (NC) and common contacts. These contacts open when the relay coil is energized. They close when power is removed from the coil.
Form “C”–A single-pole, double-throw relay with normally open (NO), normally closed (NC) and common contacts.

Relay (Solid State): A solid state switching device which completes or interrupts a circuit electrically with no moving pans.

Repeatability: The ability of a probe or instrument to give the same output or reading under repeated identical conditions.

Resistance: The resistance to the flow of electric current measured in ohms. For a conductor resistance is a function of diameter, resistivity (an intrinsic property of the material) and length.

Resistance Ratio Characteristic: For thermistors, the ratio of the resistance of the thermistor at 25°C to the resistance at 125°C.

Resistance Temperature Characteristic: A relationship between a thermistor’s resistance and the temperature.

Resolution: The input corresponding to a one-unit change in the least significant digit. (Good resolution is not necessarily equal to good accuracy.)

Response Time (Time Constant): The time required by a sensor to reach 63.2% of a step change in temperature under a specified set of conditions. Five time constants are required for the sensor to stabilize at 100% of the step change value.

RTD: Resistance Temperature Detector.

Seebeck Effect: When a circuit is formed by a junction of two dissimilar metals and the junctions are held at different temperatures, a current will flow in the circuit caused by the difference in temperature between the two junctions.

Seebeck EMF: The open circuit voltage caused by the difference in temperature between the hot and cold junctions of a circuit made from two dissimilar metals.

Secondary Standard: A standard of unit measurement derived from a primary standard.

Sensitivity: The minimum change in input signal to which an instrument can respond.

Setpoint: The temperature at which a controller is set to control a system.

Settling Time: The time taken for the display to settle within one digit of the final value when a step is applied to the meter input.

Sheath Thermocouple: A thermocouple made out of mineral-insulated thermocouple cable which has an outer metal sheath.

Signal Conditioner: A circuit module which offsets attenuates, amplifies linearizes and/or filters the signal for input to the A/D converter. The typical output signal conditioner is +2 Vdc.

Single-ended Input: A signal-input circuit where SIG LO (or sometimes SIG HI is tied to METER GND. Ground loops are normally not a problem in AC-powered meters, since METER GND is transformer-isolated from AC GND.

Span: The difference between the upper and lower limits of a range expressed in the same units as the range.

Span Adjustment: The ability to adjust the gain of a process or strain meter so that a specified display span in engineering units corresponds to a specified signal span. For instance, a display span of 200°F may correspond to the 16 mA span of a 420 mA transmitter signal.

Spare: A connector point reserved for options, specials, or other configurations. The point is identified by an IE#) for location on the electrical schematic.

Specific Heat: The ratio of thermal energy required to raise the temperature of a body f ° to the thermal energy required to raise an equal mass of water 1°.

SSR: Solid State Relay: (see Relay, Solid State).

Stability: The quality of an instrument or sensor to maintain a consistent output when a constant input is applied.

Super Cooling: The cooling of a liquid below its freezing temperature without the formation of the solid phase.

Super Heating: 1. The heating of a liquid above its boiling temperature without the formation of the gaseous phase. 2. The heating of the gaseous phase considerably above the boiling-point temperature to improve the thermodynamic efficiency of a system.

TEMPCO: Abbreviation for “temperature coefficient”: the error introduced by a change in temperature. Normally expressed in %/°C or ppm/°C.

Thermal Conductivity: The property of a material to conduct heat in the form of thermal energy.

Thermal EMF: See Seebeck EMF.

Thermal Expansion: An increase in size due to an increase in temperature expressed in units of an increase in length or increase in size per degree, i.e. inches/inch/degree C.

Thermal Gradient: The distribution of a differential temperature through a body or across a surface.

Thermistor: A temperature-sensing element composed of sintered semiconductor material which exhibits a large change in resistance proportional to a small change in temperature. Thermistors usually have negative temperature coefficients.

Thermocouple: The junction of two dissimilar metals which has a voltage output proportional to the difference in temperature between the hot junction and the lead wires (cold junction) (refer to Seebeck EMF).

Standard thermocouple types are:

Thermowell: A closed-end tube designed to protect temperature sensors from harsh environments high pressure and flows. They can be installed into a system by pipe thread or welded flange and are usually made of corrosion-resistant metal or ceramic material depending upon the application.

Transmitter (Two-wire): A device which is used to transmit temperature data from either a thermocouple or RTD via a two-wire current loop. The loop has an external power supply and the transmitter acts as a variable resistor with respect to its input signal.

True RMS: The true root-mean-square value of an AC or AC-plus-DC signal, often used to determine power of a signal. For a perfect sine wave the RMS value is 1.11072 times the rectified average value which is utilized for low-cost metering. For significantly non-sinusoidal signals, a true RMS converter is required.

TTL-compatible: For digital input circuits, a logic 1 is obtained for inputs of 2.0 to 5.5 V which can source 40 µA and a logic 0 is obtained for inputs of 0 to 0.8 V which can sink 1.6 mA. For digital output signals, a logic 1 is represented by 2.4 to 5.5 V with a current source capability of at least 400 µA; and a logic 0 is represented by 0 to 0.6 V with a current sink capability of at least 16 mA.

LS-TTL Compatible: For digital input circuits, a logic 1 is obtained for inputs of 2.0 to 5.5 V which can source 20 pA, and a logic 0 is obtained for inputs of 0 to 0.8 V which can sink 400 uA. For digital output signals, a logic 1 is represented by 2.4 to 5.5 V with a current source capability of at least 400 uA; and a logic 0 is represented by 0 to 0.8 V with a current sink capability of at least 16 mA. “LS” stands for low-power Schottky.

TTL Unit Load: A load with TTL voltage levels, which will draw 40 µA for a logic 1 and -1.6 mA for a logic 0.

LS-TTL Unit Load: A load with LS-TTL voltage levels, which will draw 20uA for a logic 1 and -400 uA for a logic 0.

Ungrounded Junction: A form of construction of a thermocouple probe where the hot or measuring junction is fully enclosed by and insulated from the sheath material.

Union: A form of pipe lining where two extension pipes are joined at a separable coupling.

Zero Offset: 1. The difference expressed in degrees between true zero and an indication given by a measuring instrument. 2. See Zero Suppression.

Zero Power Resistance: The resistance of a thermistor or RTD element with no power being dissipated.

Zero Suppression: The span of an indicator or chart recorder may be offset from zero (zero suppressed) such that neither limit of the span win be zero. (Example: a temperature recorder which records a 100° span from 400° to 500° is said to have 400% zero suppression.