Charge Sensor Operation
The PS-2132 and CI-6555 charge sensors measure the charge across a 10 nF internal capacitor that is placed in series with the external capacitor. Capacitors in series must have the same charge, so by measuring the voltage across the known internal capacitor, the charge on the external capacitor can be calculated by Qexternal = Qinternal = Cinternal*V. (The charge sensors can measure the charge on capacitors much smaller than 10 nF, provided that the voltage used to charge the capacitances is proportionately larger; however, higher voltages at significant capacitance are not recommended due to safety considerations.)
Proper Measurement Procedure with the PS-2132 or CI-6555 Charge Sensors
Insulators with Trapped Surface Charges (Proof Planes, Fur, Acetate Films etc.)
Low-Leakage Capacitors with Capacitances of 0.1-1000 nF
If the charge did not change significantly before the leads were connected to the capacitor, then the stabilized value of the charge should be close to the actual charge on the capacitor; otherwise, a large amount of charge was transferred by the operator to the capacitor. Avoid getting wool or synthetic fabrics close to the leads of the charge sensor, discharge your hands to the ground plane, and repeat the measurements.
Objects with pico-Coulomb (pC) Charges (ES-9043 parallel plate capacitor, etc)
The PASPort and ScienceWorkshop Charge Sensors cannot give accurate results for these devices because the value of the voltage on the internal capacitor is too low and because the leakage time is too short (<< 100 s); these measurements require an electrometer-Faraday cage system, such as the ES-9078 Basic Electrometer and ES-9042A Faraday Ice Pail system. The charge Q would be determined as Csystem * Velectrometer, where Csystem ~ 50 pF.
The exact value of Csystem is determined calibrating the system with a low-leakage capacitor (examples: Teflon, polypropylene 10 pF) of known capacitance as described in the ES-9078A manual.
Objects with femto-Coulomb Charges
Charges in the range of less than 1 fC can be measured with the electrometer connected to a ScienceWorkshop analog input port ; The voltage output by the electrometer is 1/20th of the input voltage and the capacitance of the system is ~50 pF, so the minimum charge Q that can be resolved is given by Q [nC] = (Vres [V]), where Vres is the voltage resolution of the analog input channel of the interface.
Refer to the chart below for the resolution of our signal interfaces:
G ≡ User-Selected Gain Setting
When performing sensitive measurements with the electrometer, one will need to do the following:
Creation Date: 07/16/2004
Last Modified: 01/29/2015