Vibrating reed electrometers
Hi all
To the best of my memory Keithley never made vibrating reed electrometers, the only one that I am aware of is the Varian Cary 401 which did use Sapphire insulators. I was the European product line specialist for Varian Cary in the late 1960/1970 era and was involved with the 401.
George G6HIG
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Today's Topics:
- Re: Precision high resistance measurements / calibration of
HP 4339B high-resistance meter. (ed breya) - Re: Precision high resistance measurements / calibration of
HP 4339B high-resistance meter. (Mitch Van Ochten)
Message: 1
Date: Fri, 2 Mar 2018 09:22:32 -0800
From: ed breya eb@telight.com
To: volt-nuts@febo.com
Subject: Re: [volt-nuts] Precision high resistance measurements /
calibration of HP 4339B high-resistance meter.
Message-ID: 5f6e418c-649b-70e8-8502-facf1176f390@telight.com
Content-Type: text/plain; charset=utf-8; format=flowed
Oops - I think I didn't send this message properly yesterday - here goes
again. Ed
Yes, David, unless you go to very extreme measures, you won't see real R
values that have any practical meaning beyond E12 ohms or so. Most
practical insulation Rs may be around E12-E14 tops, unless you go to
sapphire. Up in that region, the R may be all within a material, or
include surface components like a film of dirt or moisture, or a
fingerprint.
E11 resistors can be made to fairly high precision, and maybe E12
nowadays. In the old days, higher values were made by stacking E11s -
like ten in series to get E12 with decent precision. The glass packaging
also limits how high it can go, due to leakage within and on the
surface. I once used a glass reed relay capsule as an ultra-high
resistance in a circuit. There was no precision or stability at all, but
it made a nice high resistor (probably E14-ish dry) even though there
was no element in there, and the circuit didn't care, as long as it was
very high, but not infinite.
The specs on this HP unit are likely just the most extreme capability
taking maximum voltage over minimum current resolution, but any
measurements would tend to be very noisy and unstable anyway. Also,
testing at the extreme 1 kV makes the numbers seem more impressive, but
the voltage coefficient of resistance will pretty much be unpredictable.
If this is a digital meter, then the other spec trick that tends to
obscure the real performance limit is that the ultimate resolution and
noise is that last digit - or even last two or three - that may may be
pretty jumpy, unless very long averaging time is used.
There may be newer, fancier electrometers nowadays, but Keithley used to
be the standard for these in the old days, before several digits of DVM
resolution complicated the specs. They had a vibrating capacitor
electrometer with all-sapphire input structure back in the 1970s/80s I
think, that was the epitome of electrometers. I forget the model number,
but vaguely recall that it could reach the aA region full scale - not
that last digit of resolution thing. It's long obsolete, and I don't
think they ever made anything actually better - only added DVM digits to
less capable, conventional semiconductor amplifier techniques. If you
can find info on it, it's an interesting read. I found a pdf of the
manual years ago, but have no idea where it is now, or what info may
still be around.
Ed
Message: 2
Date: Fri, 2 Mar 2018 20:41:10 -0500
From: "Mitch Van Ochten" Mitch@vincentelectronics.com
To: "'Discussion of precise voltage measurement'" volt-nuts@febo.com
Subject: Re: [volt-nuts] Precision high resistance measurements /
calibration of HP 4339B high-resistance meter.
Message-ID: 014e01d3b290$b54948f0$1fdbdad0$@vincentelectronics.com
Content-Type: text/plain; charset="UTF-8"
Here are specs from an older General Radio Bridge for observation.
GR 1644-A specifications:
Resistance Range: 1 kΩ to 1000 TΩ (10^3 to 10^15 Ω) in ten ranges.
Accuracy: 10^3 Ω to 10^10 Ω, ±1 %. After self-calibration: 10^10 to
10^12 Ω, ±1%*; 10^13 Ω, ±2%; 10^14 Ω, ±10%; 10^15 Ω, ± one scale
division.
ΔR% Dial: ±5% range; accurate to ±0.2% or, for small changes,
to ±0.1%.
Test Voltage: Voltage accuracy is ±3% ±0.5 V.
Fixed Voltages** 10 20 50 100 200 500 1000 v
Best regards,
mitch
-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces@febo.com] On Behalf Of ed breya
Sent: Friday, March 02, 2018 12:23 PM
To: volt-nuts@febo.com
Subject: Re: [volt-nuts] Precision high resistance measurements / calibration of HP 4339B high-resistance meter.
Oops - I think I didn't send this message properly yesterday - here goes again. Ed
Yes, David, unless you go to very extreme measures, you won't see real R values that have any practical meaning beyond E12 ohms or so. Most practical insulation Rs may be around E12-E14 tops, unless you go to sapphire. Up in that region, the R may be all within a material, or include surface components like a film of dirt or moisture, or a fingerprint.
E11 resistors can be made to fairly high precision, and maybe E12 nowadays. In the old days, higher values were made by stacking E11s - like ten in series to get E12 with decent precision. The glass packaging also limits how high it can go, due to leakage within and on the surface. I once used a glass reed relay capsule as an ultra-high resistance in a circuit. There was no precision or stability at all, but it made a nice high resistor (probably E14-ish dry) even though there was no element in there, and the circuit didn't care, as long as it was very high, but not infinite.
The specs on this HP unit are likely just the most extreme capability taking maximum voltage over minimum current resolution, but any measurements would tend to be very noisy and unstable anyway. Also, testing at the extreme 1 kV makes the numbers seem more impressive, but the voltage coefficient of resistance will pretty much be unpredictable.
If this is a digital meter, then the other spec trick that tends to obscure the real performance limit is that the ultimate resolution and noise is that last digit - or even last two or three - that may may be pretty jumpy, unless very long averaging time is used.
There may be newer, fancier electrometers nowadays, but Keithley used to be the standard for these in the old days, before several digits of DVM resolution complicated the specs. They had a vibrating capacitor electrometer with all-sapphire input structure back in the 1970s/80s I think, that was the epitome of electrometers. I forget the model number, but vaguely recall that it could reach the aA region full scale - not that last digit of resolution thing. It's long obsolete, and I don't think they ever made anything actually better - only added DVM digits to less capable, conventional semiconductor amplifier techniques. If you can find info on it, it's an interesting read. I found a pdf of the manual years ago, but have no idea where it is now, or what info may still be around.
Ed
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End of volt-nuts Digest, Vol 103, Issue 3
The Keithley model 640 was a vibrating capacitor electrometer.
It was available in the 1970's.
Bruce
On 04 March 2018 at 06:34 george <g_einst@hotmail.com> wrote:
Hi all
To the best of my memory Keithley never made vibrating reed electrometers, the only one that I am aware of is the Varian Cary 401 which did use Sapphire insulators. I was the European product line specialist for Varian Cary in the late 1960/1970 era and was involved with the 401.
George G6HIG
________________________________________
From: volt-nuts <volt-nuts-bounces@febo.com> on behalf of volt-nuts-request@febo.com <volt-nuts-request@febo.com>
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Subject: volt-nuts Digest, Vol 103, Issue 3
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Today's Topics:
1. Re: Precision high resistance measurements / calibration of
HP 4339B high-resistance meter. (ed breya)
2. Re: Precision high resistance measurements / calibration of
HP 4339B high-resistance meter. (Mitch Van Ochten)
----------------------------------------------------------------------
Message: 1
Date: Fri, 2 Mar 2018 09:22:32 -0800
From: ed breya <eb@telight.com>
To: volt-nuts@febo.com
Subject: Re: [volt-nuts] Precision high resistance measurements /
calibration of HP 4339B high-resistance meter.
Message-ID: <5f6e418c-649b-70e8-8502-facf1176f390@telight.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Oops - I think I didn't send this message properly yesterday - here goes
again. Ed
Yes, David, unless you go to very extreme measures, you won't see real R
values that have any practical meaning beyond E12 ohms or so. Most
practical insulation Rs may be around E12-E14 tops, unless you go to
sapphire. Up in that region, the R may be all within a material, or
include surface components like a film of dirt or moisture, or a
fingerprint.
E11 resistors can be made to fairly high precision, and maybe E12
nowadays. In the old days, higher values were made by stacking E11s -
like ten in series to get E12 with decent precision. The glass packaging
also limits how high it can go, due to leakage within and on the
surface. I once used a glass reed relay capsule as an ultra-high
resistance in a circuit. There was no precision or stability at all, but
it made a nice high resistor (probably E14-ish dry) even though there
was no element in there, and the circuit didn't care, as long as it was
very high, but not infinite.
The specs on this HP unit are likely just the most extreme capability
taking maximum voltage over minimum current resolution, but any
measurements would tend to be very noisy and unstable anyway. Also,
testing at the extreme 1 kV makes the numbers seem more impressive, but
the voltage coefficient of resistance will pretty much be unpredictable.
If this is a digital meter, then the other spec trick that tends to
obscure the real performance limit is that the ultimate resolution and
noise is that last digit - or even last two or three - that may may be
pretty jumpy, unless very long averaging time is used.
There may be newer, fancier electrometers nowadays, but Keithley used to
be the standard for these in the old days, before several digits of DVM
resolution complicated the specs. They had a vibrating capacitor
electrometer with all-sapphire input structure back in the 1970s/80s I
think, that was the epitome of electrometers. I forget the model number,
but vaguely recall that it could reach the aA region full scale - not
that last digit of resolution thing. It's long obsolete, and I don't
think they ever made anything actually better - only added DVM digits to
less capable, conventional semiconductor amplifier techniques. If you
can find info on it, it's an interesting read. I found a pdf of the
manual years ago, but have no idea where it is now, or what info may
still be around.
Ed
------------------------------
Message: 2
Date: Fri, 2 Mar 2018 20:41:10 -0500
From: "Mitch Van Ochten" <Mitch@vincentelectronics.com>
To: "'Discussion of precise voltage measurement'" <volt-nuts@febo.com>
Subject: Re: [volt-nuts] Precision high resistance measurements /
calibration of HP 4339B high-resistance meter.
Message-ID: <014e01d3b290$b54948f0$1fdbdad0$@vincentelectronics.com>
Content-Type: text/plain; charset="UTF-8"
Here are specs from an older General Radio Bridge for observation.
GR 1644-A specifications:
Resistance Range: 1 kΩ to 1000 TΩ (10^3 to 10^15 Ω) in ten ranges.
Accuracy: 10^3 Ω to 10^10 Ω, ±1 %. After self-calibration: 10^10 to
10^12 Ω, ±1%*; 10^13 Ω, ±2%; 10^14 Ω, ±10%; 10^15 Ω, ± one scale
division.
ΔR% Dial: ±5% range; accurate to ±0.2% or, for small changes,
to ±0.1%.
Test Voltage: Voltage accuracy is ±3% ±0.5 V.
Fixed Voltages** 10 20 50 100 200 500 1000 v
Best regards,
mitch
-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces@febo.com] On Behalf Of ed breya
Sent: Friday, March 02, 2018 12:23 PM
To: volt-nuts@febo.com
Subject: Re: [volt-nuts] Precision high resistance measurements / calibration of HP 4339B high-resistance meter.
Oops - I think I didn't send this message properly yesterday - here goes again. Ed
Yes, David, unless you go to very extreme measures, you won't see real R values that have any practical meaning beyond E12 ohms or so. Most practical insulation Rs may be around E12-E14 tops, unless you go to sapphire. Up in that region, the R may be all within a material, or include surface components like a film of dirt or moisture, or a fingerprint.
E11 resistors can be made to fairly high precision, and maybe E12 nowadays. In the old days, higher values were made by stacking E11s - like ten in series to get E12 with decent precision. The glass packaging also limits how high it can go, due to leakage within and on the surface. I once used a glass reed relay capsule as an ultra-high resistance in a circuit. There was no precision or stability at all, but it made a nice high resistor (probably E14-ish dry) even though there was no element in there, and the circuit didn't care, as long as it was very high, but not infinite.
The specs on this HP unit are likely just the most extreme capability taking maximum voltage over minimum current resolution, but any measurements would tend to be very noisy and unstable anyway. Also, testing at the extreme 1 kV makes the numbers seem more impressive, but the voltage coefficient of resistance will pretty much be unpredictable.
If this is a digital meter, then the other spec trick that tends to obscure the real performance limit is that the ultimate resolution and noise is that last digit - or even last two or three - that may may be pretty jumpy, unless very long averaging time is used.
There may be newer, fancier electrometers nowadays, but Keithley used to be the standard for these in the old days, before several digits of DVM resolution complicated the specs. They had a vibrating capacitor electrometer with all-sapphire input structure back in the 1970s/80s I think, that was the epitome of electrometers. I forget the model number, but vaguely recall that it could reach the aA region full scale - not that last digit of resolution thing. It's long obsolete, and I don't think they ever made anything actually better - only added DVM digits to less capable, conventional semiconductor amplifier techniques. If you can find info on it, it's an interesting read. I found a pdf of the manual years ago, but have no idea where it is now, or what info may still be around.
Ed
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Yup - Keithley 640 - that must be the one. This stirred my memory
somewhat, and I just located info on the model 642 also, which was
apparently newer. The 642 went to (or back to) ultra-low bias MOSFETs,
while keeping sapphire insulation and a separate input head. The MOSFETs
need all kinds of offset, temperature, and bias current compensation.
The 642 also uses a few digits of DVM that obscure the real
capabilities, as I mentioned previously. The specs apparently show the
most sensitive range as 1 pA FS, so all stuff below E-12 A depends on
those digits to resolve. The manual recommends that extremely small
currents below 1 fA (the third digit down) be measured in charge mode. I
think this is to compensate for bias current and to average out some of
the 1/f noise.
The 640 on the other hand, can apparently reach 1 fA FS (1000x lower)
with 100 aA p-p (+/- 5%) noise on analog readout. Given a choice between
the two, I think I'd pick the 640, and hook a DVM to the output, and
average a whole bunch if necessary. I think the 640 uses a superior
front-end technology that maybe could be even further improved in the
middle and back end, while the 642 probably is as good as it can ever be
already.
Ed
Another thing I noticed in these instruments - the highest R value used
is E12, even though decades higher would have been appropriate in
certain ranges. It shows that was about the practical limit for somewhat
decent precision and cost. Filling in the desired higher ranges had to
be done by adding complexity like more gain elsewhere, and writing the
specs accordingly.
Ed
Curiosity forced me to look around a little more for info on the 640.
The first link includes some info on the 640 with pictures. The
vibrating capacitor thing looks like a sort of vacuum tube. I thought
maybe it was custom made by or for Keithley, but it seems to actually
have been an off-shelf commercial product. The second link shows
pictures of what it likely is - XL7900, made by Philips back then. The
third link is to the data sheet that explains a lot.
https://doc.xdevs.com/doc/_Metrology/femto_ampere_current_source.pdf
http://lampes-et-tubes.info/sp/sp003.php?l=e
http://frank.pocnet.net/sheets/009/x/XL7900.pdf
I also found some info on the Varian/Cary 401 vibrating reed type
electrometer. It seems it was very common and used in a lot of
applications from the late 1960s on. From sparse technical info and
pictures, it appears to be actually built up from parts, into an
interesting machine. I could not find any manuals (without signing up
for some kind of subscription at document websites) or detailed info.
Ed