Dear All,
I am trying to measure surface charge of an electret by electrometer (keithley 6514)set up but I am getting a hard time. I have keithley 6514 meter with a triax cable. I have an aluminium probe which I keep finite distance away (10-15mm) away from the surface and try to measure the charge by the electrometer. Every time I am getting either non reproducible or erratic values. I am not sure whether this process of measurement is correct or I should take special precuation to measure it. I did with Faraday cage also using a metal shielding but still there was not believable data. If any body has tried this kind of measurement by electrometer please let me know or discuss.
Thanks
Ankur
You have a proper device for measuring surface charge. However, your aluminum probe cannot work for measurement of surface potential. 10-15 mm from the surface produce a very high resistance even for your Keithley. Even if you bring you probe very close to the surface of your electret, the air gap will be of very high resistance.
There is a commercial device for measuring a surface potential and charge. They are quite expensive http://www.biolinscientific.com/ksvnima/products/?card=KP7
You need to make the air gap between the aluminum probe and the surface conductive. The easiest way to achieve this is to ionize the air with a radioactive source like Americium-241. This how ionized smoke detector works. I do not suggest that you took apart a smoke detector for the Americium-241 source. There is more elegant solution. The company Detector Technology http://www.dtechnology.com.hk/smoke-americium_241_alpha_foil
offers Americium-242 alpha foil. If you attached a small strip of the foul to the tip of your probe with conductive glue, you will be measure the surface potential. You should attach a measuring ground to your electret, firmly position you probe about 2-3 mm above the surface, and measure the surface potential. Knowing the potential and capacitance you could calculate the surface charge. Make sure that you know how handle radioactive materials. Take a permission from your administration to do this work in your laboratory.
Thanks Dr. Vitaly Vodyanoy for your insightful comments. One query I have, is there any standard electret available or any standard charged surface available which I can measure and calibrate so that I have a confidence that I am measuring correct charge.
I am not aware of any standard electret. But instead of electret you can use a clean unoxidized metal surface (for example gold) and apply to this plate a small voltage through a known resistor. Position your probe with the Am-241 strip 2-3 mm above the surface and measure the voltage of the capacitor produced by the gold plate and the probe with the electrometer. Because you know the applied voltage and can calculate (or measure) the capacitance, you can compare the voltage that applied to the plate and that of measured with an electrometer. Naturally, you could calculate the capacitor charge (CV). This way you could calibrate your system. You need to choose the resistor to be able to neglect the resistance of the air gap. Make sure that you have a complete electrical circle and ONE ground. You also need to be careful not burn your electrometer – use a proper voltage range.
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