I have recently aquired a nice 5.1/2 digit DVM, and would like to assess
the absolute accuracy. Many years ago I had a 'Standard Cell', but that
got lost in one of my several relocations.
What is a good alternative these days? I would think that there is a
semiconductor device that has a known threshold. I am familiar with the
Wheatstone bridge method to measure voltage without affecting the reference.
Once I have a good DC reference, getting AC & RMS calibration is fairly
easy.
Having a GPS locked time and frequency reference, I am used to being at
the 1e-10 accuracy level for those, but if I can get to 1e-5 for analog
voltage I would be very happy.
Any suggestions gratefully received.

73 Alf NU8I
Scottsdale AZ DM43an
160m - 10 Gigs

Alfred Green wrote:
I have recently aquired a nice 5.1/2 digit DVM, and would like to assess
the absolute accuracy. Many years ago I had a 'Standard Cell', but that
got lost in one of my several relocations.
What is a good alternative these days? I would think that there is a
semiconductor device that has a known threshold. I am familiar with the
Wheatstone bridge method to measure voltage without affecting the reference.
Once I have a good DC reference, getting AC & RMS calibration is fairly
easy.
Having a GPS locked time and frequency reference, I am used to being at
the 1e-10 accuracy level for those, but if I can get to 1e-5 for analog
voltage I would be very happy.
Any suggestions gratefully received.

73 Alf NU8I
Scottsdale AZ DM43an
160m - 10 Gigs

I don't think there is an easy to do what you want, otherwise people
wouldn't spend lots of money getting their standards calibrated at
national standards labs like NPL. Your best bet might be to find
someone who has access to a regularly calibrated DVM and check it
against that, or send it away to get it calibrated.

Standard voltage diodes are available and are used as secondary
standards, but are probably quite expensive and difficult to find.

You should have kept your Weston cell. 8-) BTW, NPL even offers a
calibration service for them, the volt is defined using a Josephson
junction, these days.

73, Leon

Leon

I'd suggest you either find a calibrator to connect your meter to, or
make something you can use as a transfer standard. Maxim, for example,
has some reasonably low cost voltage references with good stability
(very few ppm/C). Then calibrate the transfer standard against some
known in-calibration volt meter or calibrator, at a known temperature,
and use it. I'd think in Scottsdale you could find a calibration
service that would do the job for you, or maybe you know someone who
has access to an accurate voltmeter. Of course, you should really do
it for several ranges. You can maybe calibrate a voltage divider
yourself, but it would save a lot of trouble if you could just do it
directly. If you don't know anyone who works with more accurate
instruements regularly and don't want to pay for a calibration, perhaps
you could pay a visit to the EE or physics department in Tempe and use
one of their instruments.

I've thought of making a secondary standard myself--I have several of
the reference boards from HP3455 DMMs that have single specific things
wrong with them, but combining them I have at least one good voltage
reference, and one good DC 10:1 divider.

Don't EVER expect to get to voltage measurements with accuracy or even
repeatability like you can with frequency--you would have to go to
extreme measures to avoid thermal EMFs, for example, and even with some
care they can give you part per million errors at low voltages. But
surely it's rare that you'd need that sort of resolution, let alone
accuracy, no?

Cheers,
Tom

Leon wrote:
Alfred Green wrote:

I have recently aquired a nice 5.1/2 digit DVM, and would like to assess
the absolute accuracy. Many years ago I had a 'Standard Cell', but that
got lost in one of my several relocations.


I don't think there is an easy to do what you want, otherwise people
wouldn't spend lots of money getting their standards calibrated at
national standards labs like NPL. Your best bet might be to find
someone who has access to a regularly calibrated DVM and check it
against that, or send it away to get it calibrated.


Well, maybe askimg for 1e-5 was very optimistic, but I would like to see
what can be done in a home environment.
For ham use I couldn't justify getting it professionally calibrated, but
I'll check with the test-equipment guy at work to see if we have
something recently done that I can do a comparison from.

Standard voltage diodes are available and are used as secondary
standards, but are probably quite expensive and difficult to find.


That was kind of what I was looking for. I really am prepared to pay a
few bob for something that I can reliably use as a reference in my lab.

You should have kept your Weston cell. 8-) BTW, NPL even offers a
calibration service for them, the volt is defined using a Josephson
junction, these days.


Oh yep. I think it was amongst the stuff that got lost in the move from
UK to the States back in '85. They do show up on Ebay occaisionally, but
I'd like to think there was something newer that could do the job.
Interesting to hear about the current Volt definition: I'll do some
searching for some more info on that.

Thanks for your comments,

73 Alf NU8I (aka G4ABB, amongst others)

K7ITM wrote:
I'd suggest you either find a calibrator to connect your meter to, or
make something you can use as a transfer standard. Maxim, for example,
has some reasonably low cost voltage references with good stability
(very few ppm/C).

Yes, that's pretty much what I'm looking for. The only problem with
Maxim is that they have SO MUCH good stuff it makes it hard for me to
find what I'm looking for. I'll try a more directed search.

Then calibrate the transfer standard against some
known in-calibration volt meter or calibrator, at a known temperature,
and use it.

Yes, I can do that. I already have a temperature controlled environment
for my frequency sources.

I'd think in Scottsdale you could find a calibration
service that would do the job for you, or maybe you know someone who
has access to an accurate voltmeter. Of course, you should really do
it for several ranges. You can maybe calibrate a voltage divider
yourself, but it would save a lot of trouble if you could just do it
directly. If you don't know anyone who works with more accurate
instruements regularly and don't want to pay for a calibration, perhaps
you could pay a visit to the EE or physics department in Tempe and use
one of their instruments.


All good points. I have friends at ASU that could help with this, but at
the end of the day it still has to come home with me. Back to the
transfer standard issue.

I've thought of making a secondary standard myself--I have several of
the reference boards from HP3455 DMMs that have single specific things
wrong with them, but combining them I have at least one good voltage
reference, and one good DC 10:1 divider.


Hmm. The unit in question is a HP3478A. It has a 'CAL' button, but I
haven't hunted for a manual yet so I dont know what it does. Any ideas?

Don't EVER expect to get to voltage measurements with accuracy or even
repeatability like you can with frequency

Oh no, I wasn't expecting that. Even my comment about 1e-5 was really
tongue-in-cheek. However, I really can't point to anything in my shack
that can even approach 1e-2 in any repeatable way as far as voltage or
current measurement goes. I just feel that in this day and age I should
be able to do better that that.

73 Alf NU8I

Alfred Green wrote:
That was kind of what I was looking for. I really am prepared to pay a
few bob for something that I can reliably use as a reference in my lab.

I'm no expert, but I wonder what you might be able to do with a
temperature-compensated design around a LMV431BI.

--
All relevant people are pertinent.
All rude people are impertinent.
Therefore, no rude people are relevant.
-- Solomon W. Golomb

Thaler has the best voltage references -- there was a design in one of the
rags (Nuts n Volts or QEX) a few years ago in which 3 Thaler references
were placed in parallel for a super reference. Email me privately if you
would like a copy.

Also, get a copy of Linear Technology Application Note 86 "A Standards Lab
Grade 20 bit DAC with 0.1ppm/degC Drift" -- Jim Williams is the author --
and make sure to read the appendices. Williams also did an ApNote with a
comprehensive history of voltage measurement techniques, but I forget the
application number.

jack wrote:
Thaler has the best voltage references -- there was a design in one of the
rags (Nuts n Volts or QEX) a few years ago in which 3 Thaler references
were placed in parallel for a super reference. Email me privately if you
would like a copy.

Also, get a copy of Linear Technology Application Note 86 "A Standards Lab
Grade 20 bit DAC with 0.1ppm/degC Drift" -- Jim Williams is the author --
and make sure to read the appendices. Williams also did an ApNote with a
comprehensive history of voltage measurement techniques, but I forget the
application number.



Thanks for the suggestion. I must admit to never having heard of of
'Thaler', but a quick search has thrown up some really interesting
stuff. No online ordering, but an 800 number to find the distributors.
I'll follow up on that.

I'll get the LT AppNote. Thanks for the info.

73 Alf NU8I

clifto wrote:
Alfred Green wrote:

That was kind of what I was looking for. I really am prepared to pay a
few bob for something that I can reliably use as a reference in my lab.


I'm no expert, but I wonder what you might be able to do with a
temperature-compensated design around a LMV431BI.


That looks like an interesting part, cheap enough and available. I'll
give it a try!

Hi Alf,

I'm not familiar with that particular model (HP3478A). We used to have
a complete set of microfiche for all the HP product line, back to the
HP200 oscillators, but unfortunately someone else coveted it and we
don't have it any longer. Sorry I can't be more specific help. You
should be able to find a manual somewhere--there are places that
specialize in used manuals. Or you may find it online--if not now, it
may show up before too long (I just don't know how accessible it will
be...)

Cheers,
Tom