Abstract:
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This paper presents a method created to accurately generate and measure a standard electronic current in the range of 100fA to 500fA (10x10-15 Amps). The measurement system consists of instruments such as a voltage source, 100G? (1011 Ohms) resistor, multimeter, and Max4000 digital electrometer. All of these instruments are traceable to the National Institute of Standards and Technology (NIST). A custom-made Faraday cage is also used to improve the signal to noise ratio. Based on Ohm's law, the voltage source and resistor create a current used to calibrate the Max4000 electrometer. Uncertainties are estimated to be 5% from 100fA to 300fA, and 1.5% from 300fA to 500fA. The immediate application of this method is the development of a NIST traceable standard to calibrate small current instruments that are used to measure sub-micrometer aerosol concentration. The aerosol concentration measurement is accomplished by using the electrometer to measure the current produced by an aerosol flow containing particles with a known charge distribution. By measuring the aerosol volumetric flow rate and number of charges over time, the particle concentration can be calculated. Particle counters used to verify the performance of gas mask and respirator testers are calibrated by this standard, which in turn has a direct effect on the health of both military and civilian personnel.
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