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

[NeonJohn]

On 03/22/2018 09:07 PM, Bob Albert via volt-nuts wrote:
>  I found several electrostatic voltmeters on ebay.  The brand I remember is Sensitive Research.

I have an electrostatic voltmeter (photo attached) that I no longer use
that I'd part with for $50.  This thing is big and heavy so shipping
will probably be $100.  It is mechanically sensitive enough that I'll
probably need to build a wooden crate and pack that inside a large box
filled with with padding.

A local pickup would be wonderful.  I live near Knoxville, TN and would
go maybe 100 miles to meet someone.  We have a motorhome and that would
be the start of a nice trip.

There are two kinds of electrostatics.  The first type is like the ones
currently on sleazebay.  Relatively small, phenolic block input
terminals and conventional internal wiring.  Very high impedance but no
where near infinite.  Good enough to measure, say, the screen voltage on
a tube but not suitable for measurements where the input impedance must
be essentially infinite.  Nuclear radiation ion chambers, for example.

The second one is like the one pictured.  Designed to have as close to
infinite input impedance.  That huge input tower is not for voltage
withstand - this is a 2500 volt meter - but to lengthen the creepage
path to ground.  I sent this unit to a calibration lab about 10 years
ago.  With the tower carefully cleaned and a new dessicant pack inside,
they could not measure the input impedance.  The only practical flaw is
that there are a few pF of capacitance between the plates that must be
charged.  Once the meter is deflected, there is no current flow.

To satisfy myself that it was truly infinite impedance, I connected a
2000 volt power supply to the unit through the resister used in the
Fluke 40kV high voltage probe.  It took a couple of seconds for the
capacitance to charge but once deflected, the electrostatic read the
same value as the voltage output of the supply to within a minor division.

Anyway, this instrument is available if anyone wants it.

John


-- 
John DeArmond
Tellico Plains, Occupied TN
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