Design and Evaluation of an Aerosol Electrometer with Low Noise and a Wide Dynamic Range

Abstract

A low-noise aerosol electrometer with a wide dynamic range has been designed for measuring the total net charge on high concentration aerosol particles within the range of −500 pA to +500 pA. The performance of the aerosol electrometer was evaluated by a series of experiments to determine linearity, sensitivity and noise. The relative errors were controlled within 5.0%, 1.0% and 0.3% at the range of −40 pA to +40 pA, ±40 pA to ±100 pA, and ±100 pA to ±500 pA respectively. The stability of the designed aerosol electrometer was found to be highly sensitive to temperature variations, but under temperature control, the root mean square noise and the peak-to-peak noise were 1.040 fA and 5.2 fA respectively, which are very close to the calculated theoretical limit of the current noise. The excellent correlation and the advantage of a wide dynamic range have been demonstrated by comparing with the designed aerosol electrometer to a commercial aerosol electrometer. A 99.7% (R2) statistical correlation was obtained; meanwhile, the designed aerosol electrometer operated well even when an overrange phenomenon appeared in the commercial aerosol electrometer.

On the Performance of an Aerosol Electrometer with Enhanced Detection Limit

“…Moreover, a theoretical model have been proposed to analyze the effect factors of electric current measurement noise. The electrometer circuit used in the AE, which is similar with our previous work [ 16 ], has been shown in Figure 3 . It consists of a current-voltage converter, a level converter and a high resolution analog-digital converter (ADC).…”

“…Therefore, the detection limit of particle number concentration is mainly subject to the particle detection efficiency and the electric current measurement noise. Compared with our previous work [ 16 ], the FC and electrometer have been optimized to increase the collection efficiency and decrease the electric current measurement noise. In the FC, the surface area of the filter has been increased by using a cup-shaped metal filter instead of disc-shaped metal filter for sake of improving the collecting efficiency.…”

“…In the FC, the surface area of the filter has been increased by using a cup-shaped metal filter instead of disc-shaped metal filter for sake of improving the collecting efficiency. As stated in work [ 16 ], the metal filter is sintered copper powder filter element. In order to increase the collection efficiency by increasing particle interception efficiency, 300-mesh spherical copper powder has been used to produce the metal filter.…”

“…In order to decrease the electric current measurement noise, zero offset should be controlled. Apart from the strategies mentioned in [ 16 ], zero offset can also be decreased by increase the value of feedback resistor. However, it will decrease the dynamic range, which is defined as a decibel logarithmic value of the ratio of the largest and smallest signal values.…”

“…Second, although the Faraday cup was connected to the Keithley 6430 by a low noise cable, vibration noise can’t be completely eliminated. In our previous work [ 16 ], an electrometer was designed for measuring electrical current from high number concentration aerosol particles ranging from −500 pA to +500 pA. The long-term output drift was within 5.2 fA over a 20-h period, and the root mean square (RMS) noise was 1.04 fA, however, the designed AE in that study was focus more on wide dynamic range, and was lack of testing for the particle detection efficiency.…”