[volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?

[Jerry Hancock]

The part I struggle with is “make one” as T ohm meters are ridiculously sensitive to things like fingerprints, for example.  I had a hard time just putting cables together that had the particular… I was going to say insulation, but it was more than that, basically everything is important and specific when you are dealing with T ohm meters.  Setting aside the stupid tri-bnc connector on most of them.

How about just using a voltage divider with a standard electrometer?  I’m sure you thought of that though.  I like playing around with my Keithley 616 as you can show the kids how electrostatics work.

Regards,

Jerry




> On Mar 22, 2018, at 6:00 PM, Bob Albert via volt-nuts <volt-nuts at febo.com> wrote:
> 
> What about an electrostatic voltmeter?  Those have been around for decades and draw zero static current.  You didn't indicate how accurate your measurement needs to be. I am trying to recall the maker of the unit, I think ESI but not sure.
> They were somewhat popular in the 1950s as I  recall and there should be some around, gathering dust and mold, in storage places.  They were large, which is a benefit because they had a long scale with good resolution.
> Bob
>    On Thursday, March 22, 2018, 5:33:28 PM PDT, Dr. David Kirkby <drkirkby at kirkbymicrowave.co.uk> wrote:  
> 
> I want to measure a high voltage, but put minimal load on the circuit.
> Looking at the Keithley electrometers, the input resistance is at least 200
> T ohms, but they tend to have a maximum of 200 V FSD.
> 
> A 2000 V source, and a 200 T ohm resistor gives a current of 10 pA, which
> itself is easy to measure. But one can't buy 200 T ohm resistors. I looked
> at RS in the UK, and the highest value resistor I could find is 1 T ohm,
> and they are £163 each (around $200).
> 
> Maybe fabricating ones own resistor is possible, but I suspect there's a
> better way. Keithely manage to keep a 200  T ohm resistance on the 200 mV
> range, and there's no way that can be measured with an ammeter, which would
> require an ammeter with a full scale deflection of 1 fA, which is much
> smaller than the 2 pA FSD on its most sensitive range.
> 
> Any thoughts?
> 
> Dr David Kirkby Ph.D C.Eng MIET
> Kirkby Microwave Ltd
> Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, CHELMSFORD,
> Essex, CM3 6DT, United Kingdom.
> Registered in England and Wales as company number 08914892
> http://www.kirkbymicrowave.co.uk/
> Tel 01621-680100 / +44 1621-680100
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