Measuring static charge with an electrometer?

[SArepairman]

I accidentally posted this in test equipment. Damn all the oscilloscope junkies.

Can an electrometer be used to measure static charge (since it measures coulombs). Like in the EEVblog video where Dave Jones has some sort of meter that measures surface charge (it has a round plate iirc).

Is there some kind of attachment for common tri-axial connection electrometers?
Since the input impedance is gargantuan (10^14 ohms, or 100 teraohms), could I just use regular voltage measurement mode and then a high voltage divider probe (1000:1), designed so that meter can read 35kV apparently, and touch stuff with a little disk (so it has greater surface area) to get a good charge reading? The idea behind the disk is so you are not measuring a single point but a wide area.

OR will static charges build up to way more then 35kV and potentially damage my meter?

[SArepairman]

bump

[Odysseus]

As you've seen, the electrometer in Dave's video uses a capacitance divider principle to determine the voltage of the surface under test.  The external plate forms a simple tiny plate capacitor with the surface and inside the device is another, larger capacitor.  The same charge is accumulated in each cap because they are connected in series, but because of the different capacitances, the internal sensed voltage on the larger cap is much more manageable by ordinary circuits.

If what you're interested in is Q, instead of V, then all you need to know is the capacitance of the plate capacitor and use Q=CV to get your answer.  Notice in the video that the device is designed to be held a specific distance away from the surface, about one inch or a couple of centimeters.  This is so that the capacitance of the plate cap is known to a rough degree of accuracy.

Alternatively, you can use Q=CV with the internal capacitor as well, which will probably be much simpler and more accurate.  But you'll have to do a bit of surgery on it first   

[SArepairman]

does anyone have more information on designing such a thing? it sounds like a fine project.

[SArepairman]

to be clear, I want to turn my keithley electrometer into a http://en.wikipedia.org/wiki/Electrostatic_voltmeter

[Vgkid]

Wouldn't you have to worry about accidently zapping the input of your electrometer, esd.

[SArepairman]

only if it touches the surface right?

[T3sl4co1l]

You definitely can't use a voltage divider probe, because that consumes current, not charge.  A charge will drain away quickly.  You need a capacitor divider, which does consume charge (instead of current).

If the probe setup (guard wire and all) has a known and calibrated capacitance, you can measure charge directly as Q = V*C.  A "10x" probe would be a capacitor of 9 times the value in parallel, and so on.  For measuring the voltage (such as a static charge) on a known capacitor, use the charge conservation equations to determine the charge given to your probe capacitance (obviously, such a method necessarily disturbs the original charge).

To measure static charge or electric field, you need a calibrated capacitance plate of some sort to do the analogous thing.  Because the capacitance varies (you're bringing a plate near another), the voltage (but not charge) on the unknown part is only momentarily disturbed.

Tim

[LaserSteve]

Its fairly simple to place a grounded, spinning, metal butterfly plate in front of the FET sensor plate.  Now you have a AC signal representing charge on the object.  The sensor sees alternatively ground and the charged object.

Google "Field Mill".

 The best tiny one I ever had a used a 25 mm metal drum with a 5 millimeter slot in it. A small 6 mm curved plate was periodically masked by the drum, which was driven by a tiny  DC motor. An opamp with one input switched  to ground with a FET synced by the motor commutation served as a crude lock-in amplifier.  Worked a charm with just a simple FET as the pre-amp. It actually used a current sensing resistor in the grounded end of the motor with a comparator to drive the FET sync.



Here is a refined example: http://www.precisionstrobe.com/jc/fieldmill/fieldmill.html

Steve

[SArepairman]

I browsed through the website and I found it interesting, however I was looking for more of a small instrument suitable for a lab. The 25mm probe seems reasonable to have on the bench.

I assume that precision work was done it could be made even smaller right? Like into a "pen" form (5-15x smaller then what you have used)?

I was thinking to have the the motor located on top and to use a long shaft supported by two ball bearings (top and bottom

motor -> ball bearing -> 6-8 inch shaft like a pen -> ball bearing -> stator (attached to pen shaft)

What voltage resolution did you end up getting ?