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

[Dr. David Kirkby]

On 24 March 2018 at 17:34, Jeremy Nichols  wrote:

> I made a high-value resistor using motor oil and a couple of stainless
> bolts. It worked for what I was doing (testing an HP-425A
> Microvolt-Ammeter) but calculated as only 8,500 Megohms.
>
> Jeremy
>

I had some discussions some time ago about using oil as a dielectric in a
capacitor with someone at NPL. He said the loss of both cyclohexane and
Polydimethylsiloxane (PDMS = silicon oil) is very low. He said the only way
I would measure the loss of them was a resonate method, and detecting small
changes in Q. My thoughts on putting them as the dielectric in a capacitor
and measuring on an LCR meter would not work, nor would my transmission
line. I had already satisfied myself that using a coaxial probe and VNA
would not work.

As he said, cyclohexane is nasty stuff, but PDMS is much more
environmentally friendly.



Dave
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Re: [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
http://www.tnduction.com<-- THE source for induction heaters
http://www.neon-john.com<-- email from here
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Re: [volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?

[Jeremy Nichols]

I made a high-value resistor using motor oil and a couple of stainless
bolts. It worked for what I was doing (testing an HP-425A
Microvolt-Ammeter) but calculated as only 8,500 Megohms.

Jeremy


On Sat, Mar 24, 2018 at 10:18 AM Mark Sims  wrote:

> Many years ago,  there was an article (in Popular Electronics?) that
> needed a very high value resistor.   They built it by drawing a line
> between two terminals with Higgins India Ink.  No idea if the ink is still
> made the way it was 50 years ago...
>
> 
>
> > So how does one make ones own resistor?
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[volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?

[Mark Sims]

Many years ago,  there was an article (in Popular Electronics?) that needed a 
very high value resistor.   They built it by drawing a line between two 
terminals with Higgins India Ink.  No idea if the ink is still made the way it 
was 50 years ago...



> So how does one make ones own resistor?
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Re: [volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?

[ed breya]

Regarding making your own extreme high-value resistors - any object that 
has insulators and leads but with nothing connected inside will have 
some high R that can be perhaps be measured, but won't be stable against 
environment effects on the outer surfaces. There's not much point to 
carbonizing things for home-made ones, except for curiosity.


You can, however, use existing things that are fairly stable internally, 
have hermetic seals, and can be treated externally to reduce environment 
issues. I mentioned that reed relay capsule that I used as an unknown, 
but very high, yet not infinite R. Burned out light bulbs, vacuum tubes 
(especially something like a 5642 HV rectifier - fairly small, lots of 
glass), and xenon flashtubes are other examples of common hermetic 
glass/metal parts that can be used. But, the R is what it is, and can't 
readily be adjusted, only measured and maybe used in circuits that can 
accommodate the value. Also, along with the R, there will be some C that 
depends on the structure of whatever is used. The C can be good or bad, 
depending on the application.


At extreme values, the surface characteristics will dominate, so the 
glass envelope would have to be silicone treated. Then the measured R of 
the device will be almost all intrinsic. So, you can measure it, but you 
won't know how stable it may be with temperature and voltage and time, 
for example, so don't expect much precision.


Regarding over-voltaging electrolytic caps - you can reform caps to 
somewhat higher voltage, given enough time. They are formed 
electrolyitically to begin with, so the dielectric layer thickness is 
right for the rated voltage. If you gradually up the voltage, the 
thickness will increase and the C will go down over time. It's best to 
just use them only up to the design rating though, or the leakage will 
become unpredictable.


A good way to do voltage splitting/protecting on medium-high voltage 
series connected electrolytic caps with low leakage, is with an 
appropriate high voltage "Zener" (actually an avalanche device, not 
truly Zener) across each one. The Zeners will prevent over-voltage of 
the caps in the normal direction, and reverse protection in the diode's 
forward region. Look for transient voltage suppressors (TVS or TVSS) 
devices to get into the hundreds of volts region, and of course they can 
be stacked for more. Unipolar ones will provide intrinsic reverse 
protection for the cap, while bipolar ones will not. They are usually 
specified fairly loosely in terms of leakage current, but it should be 
possible to find ones in the low nA region at applied V reasonably below 
the knee, at room temperature. That sounds like a lot in a High-Z 
context, but it's almost certainly much less than the leakage of a 
typical electrolytic cap.


Ed

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

[ed breya]

I'm guessing the application relates back to your leaf electrometer 
project discussed earlier - trying to assess how the bias charge on the 
capacitor holds up from leakage and use of the instrument. If this is 
the case, then it's for a one-time use for design of the item, so 
shouldn't be too fancy or expensive. I believe the original goal was to 
have the cap get charged up and then not need any electric support for 
the leaf electrometer, appearing totally passive, for some amount of 
operating time. If built-in monitoring of the cap voltage is now 
desired, that's a different story.


If the measurement is just for design, to roughly see the cap 
charge-holding time situation, then I'd recommend using methods that 
Chris described, comparing to a variable HV supply at various times and 
settings - all manual iterations, but doable. You can always say, 
recharge the cap, then guess what the voltage may be after so much time, 
then set the test supply and compare - over and over and over.


If continuous, long-term, fairly accurate monitoring is desired, then 
you'd have to go with some sort of non-contact electrostatic voltmeter 
or such, as others have mentioned.


Relating back to recent discussions, it's pretty clear that you're not 
going to find an actual specified resistor in the hundred T-ohm region. 
You can certainly make your own from T-ohms to infinite, but you won't 
be able to know the "exact" value. The commercial instruments that have 
say "200 T-ohms" input R don't actually have that resistor value inside 
- it's an "effective" or "equivalent" derived value that depends on a 
real resistance of maybe E11-E12, multiplied by system gain.


Some electrometers like the old Keithleys have a voltage mode where the 
high-Z input amplifier is bootstrapped up as a voltage follower, but 
have less range than you want. It's conceivable that you could build the 
same thing, but with a HV amplifier follower that can reach the desired 
level. This would not be trivial.


Again, if the purpose is just to measure the droop in bias voltage of 
the charged cap over certain time intervals, there may be another 
option. Since this is a dv/dt rather than DC measurement, you could 
possibly set up an electrometer to view the change of the bias voltage 
via current through another capacitor, and conceivably even rig it up to 
directly measure the total change in cap voltage over a given time.


Let's say the charge storage cap is 1 uF, and you put a much smaller, 
less leaky, test cap plus some protective series R from the HV node to 
the input of the electrometer, and also clamp the input with a low 
leakage diode circuit. The test cap could be say 100 or 1000 times 
smaller than the main cap, so its effect will be small. This could be in 
the 10 nF or less range, where it should be fairly easy to find 3 kV or 
so rated metalized film plastic capacitors with suitably low leakage. 
Any constant DC leakage from the cap could be zeroed out or accounted 
for, at least for short-term measurements.


The electrometer could then read the test cap current directly 
proportional to dv/dt, or integrate it back up to delta V in the charge 
mode. There are limits to the reasonable measuring ranges, of course. 
For example, 1 nF would provide 1 nA at 1V/sec - a fairly easy 
measurement. But 1V/1000 seconds could be tricky - only 1 pA to work with.


Ed


On 3/22/2018 7:12 PM, kc9ieq via volt-nuts wrote:

I guess I don't see what the issue is.  No, impedance is not infinate when not 
nulled, but this is why V supply #2 Is adjustable by whatever convenient means. 
 Rough adjust, connect, adjust for null, measure.  Rinse and repeat.  If it 
were my project, I'd just run up an HV transformer on a variac, with a 
rectifier, cap, and probably some series R thrown at it to limit current 
through the meter.  Curious to know what the application is, if this will not 
work.
Good luck with whatever solution you choose.
Regards, Chris


Sent from my SMRTphone
 Original message From: "Dr. David Kirkby"  
Date: 3/22/18  8:58 PM  (GMT-06:00) To: kc9ieq , Discussion of precise voltage 
measurement  Subject: Re: [volt-nuts] How can I make a 2000 V DC meter with an 
input resistance of at least 100 T ohms?
On 23 March 2018 at 01:49, kc9ieq via volt-nuts  wrote:
How about using (or building) an additional 2kV power supply and a sensitive 
meter movement like a differential voltmeter, adjusting for/measuring the null? 
 Impedance at null will be theoretically infinate, current will be 
theoretically zero, and you can measure/monitor the voltage of your second 
supply directly with the probe/meter of your choice.

Regards,Chris

No, that will not work for me, as while the impedance at null is infinite, it 
is not when not nulled, and that will mess up the measurements.

Absolute accuracy is not important. +/- 10% or even 20% would be 

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

[Bruce Griffiths]

Vibrating capacitor non contact field probe in feedback loop to create a 
voltage follower:

https://www.monroe-electronics.com/ESD/pdf/244a_m.pdf

Bruce

> 
> On 23 March 2018 at 15:12 Neville Michie  wrote:
> 
> Improvise by holding a metal disk over an electronic balance and 
> measuring the force of attraction.
> Calibrate it with a lower known voltage.
> 
> cheers,
> Neville Michie
> 
> > > 
> > On 23 Mar 2018, at 12:58, Dr. David Kirkby 
> >  wrote:
> > 
> > On 23 March 2018 at 01:49, kc9ieq via volt-nuts 
> >  wrote:
> > 
> > > > > 
> > > How about using (or building) an additional 2kV power supply 
> > > and a
> > > sensitive meter movement like a differential voltmeter, 
> > > adjusting
> > > for/measuring the null? Impedance at null will be 
> > > theoretically infinate,
> > > current will be theoretically zero, and you can 
> > > measure/monitor the voltage
> > > of your second supply directly with the probe/meter of your 
> > > choice.
> > > Regards,Chris
> > > 
> > > > > 
> > No, that will not work for me, as while the impedance at null is 
> > infinite,
> > it is not when not nulled, and that will mess up the measurements.
> > 
> > Absolute accuracy is not important. +/- 10% or even 20% would be 
> > okay. I
> > want to measure a couple of voltages and compare them. As long as 
> > the meter
> > reads the same with identical input voltages, that is fine.
> > 
> > Dave
> > 
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Re: [volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?

[Neville Michie]

Improvise by holding a metal disk over an electronic balance and measuring the 
force of attraction.
Calibrate it with a lower known voltage.

cheers,
Neville Michie
> On 23 Mar 2018, at 12:58, Dr. David Kirkby  
> wrote:
> 
> On 23 March 2018 at 01:49, kc9ieq via volt-nuts  wrote:
> 
>> How about using (or building) an additional 2kV power supply and a
>> sensitive meter movement like a differential voltmeter, adjusting
>> for/measuring the null?  Impedance at null will be theoretically infinate,
>> current will be theoretically zero, and you can measure/monitor the voltage
>> of your second supply directly with the probe/meter of your choice.
>> Regards,Chris
>> 
> 
> No, that will not work for me, as while the impedance at null is infinite,
> it is not when not nulled, and that will mess up the measurements.
> 
> Absolute accuracy is not important. +/- 10% or even 20% would be okay. I
> want to measure a couple of voltages and compare them. As long as the meter
> reads the same with identical input voltages, that is fine.
> 
> Dave
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Re: [volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?

[kc9ieq via volt-nuts]

I guess I don't see what the issue is.  No, impedance is not infinate when not 
nulled, but this is why V supply #2 Is adjustable by whatever convenient means. 
 Rough adjust, connect, adjust for null, measure.  Rinse and repeat.  If it 
were my project, I'd just run up an HV transformer on a variac, with a 
rectifier, cap, and probably some series R thrown at it to limit current 
through the meter.  Curious to know what the application is, if this will not 
work.  
Good luck with whatever solution you choose.  
Regards, Chris 


Sent from my SMRTphone
 Original message From: "Dr. David Kirkby" 
 Date: 3/22/18  8:58 PM  (GMT-06:00) To: kc9ieq 
, Discussion of precise voltage measurement 
 Subject: Re: [volt-nuts] How can I make a 2000 V DC meter 
with an input resistance of at least 100 T ohms? 
On 23 March 2018 at 01:49, kc9ieq via volt-nuts  wrote:
How about using (or building) an additional 2kV power supply and a sensitive 
meter movement like a differential voltmeter, adjusting for/measuring the null? 
 Impedance at null will be theoretically infinate, current will be 
theoretically zero, and you can measure/monitor the voltage of your second 
supply directly with the probe/meter of your choice.  

Regards,Chris 

No, that will not work for me, as while the impedance at null is infinite, it 
is not when not nulled, and that will mess up the measurements. 

Absolute accuracy is not important. +/- 10% or even 20% would be okay. I want 
to measure a couple of voltages and compare them. As long as the meter reads 
the same with identical input voltages, that is fine. 

Dave 

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

[Dr. David Kirkby]

On 23 March 2018 at 01:49, kc9ieq via volt-nuts  wrote:

> How about using (or building) an additional 2kV power supply and a
> sensitive meter movement like a differential voltmeter, adjusting
> for/measuring the null?  Impedance at null will be theoretically infinate,
> current will be theoretically zero, and you can measure/monitor the voltage
> of your second supply directly with the probe/meter of your choice.
> Regards,Chris
>

No, that will not work for me, as while the impedance at null is infinite,
it is not when not nulled, and that will mess up the measurements.

Absolute accuracy is not important. +/- 10% or even 20% would be okay. I
want to measure a couple of voltages and compare them. As long as the meter
reads the same with identical input voltages, that is fine.

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

[kc9ieq via volt-nuts]

How about using (or building) an additional 2kV power supply and a sensitive 
meter movement like a differential voltmeter, adjusting for/measuring the null? 
 Impedance at null will be theoretically infinate, current will be 
theoretically zero, and you can measure/monitor the voltage of your second 
supply directly with the probe/meter of your choice.  
Regards,Chris 


Sent from my SMRTphone
 Original message From: "Dr. David Kirkby" 
 Date: 3/22/18  7:33 PM  (GMT-06:00) To: 
Discussion of precise voltage measurement  Subject: 
[volt-nuts] How can I make a 2000 V DC meter with an input
  resistance of at least 100 T ohms? 
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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Re: [volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?

[Dr. David Kirkby]

On 23 March 2018 at 01:07, Bob Albert via volt-nuts 
wrote:

>  I found several electrostatic voltmeters on ebay.  The brand I remember
> is Sensitive Research.
> Bob
>

So something like this?

https://www.ebay.com/itm/Singer-ESD-7-Sensitive-Research-Electrostatic-Voltmeter-1500-Volt-Range/122976453378

1500 V would probably do. I must admit, I don't understand how these work.
Do you need to have a power supply to balance the voltage? I can't see how
taking almost no current can physically make a meter needle move, as that
needs energy.

I see some are contact, and some non-contact.

It is 01:45 here, so I am just about to go asleep, but will read any
replies later today.

I would be interested in making a more modern version, but really don't
know how.

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

[Dr Joe Palsa P.E. via volt-nuts]

How about using a high voltage probe that used to be commonly used to measure 
anode voltages of TV picture tubes. There were probes that had a KV meter built 
into the probe and there are ones that were used with VOM'S.
Joe
K3WRY
 
In a message dated 3/22/2018 8:59:08 PM Eastern Standard Time, 
bruce.griffi...@xtra.co.nz writes:

 
 Electrostatic voltmeter?

Either the classical version or the modern electronic variant perhaps?

Bruce

> 
> On 23 March 2018 at 13:33 "Dr. David Kirkby"  
> 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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Re: [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  
> 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 
>  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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Re: [volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?

[Bob Albert via volt-nuts]

 I found several electrostatic voltmeters on ebay.  The brand I remember is 
Sensitive Research.
Bob
On Thursday, March 22, 2018, 6:00:39 PM PDT, Bob Albert via volt-nuts 
 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 
 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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Re: [volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?

[Bob Albert via volt-nuts]

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

[Bruce Griffiths]

Electrostatic voltmeter?

Either the classical version or the modern electronic variant perhaps?

Bruce

> 
> On 23 March 2018 at 13:33 "Dr. David Kirkby" 
>  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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[volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?

[Dr. David Kirkby]

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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