Author Topic: Reference Sources - For Testing Multimeters  (Read 11180 times)

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Offline Amarbir[Lynx-India]

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Reference Sources - For Testing Multimeters
« on: June 11, 2012, 03:38:46 pm »
Hello Folks ,
 My First Post here  :D .I Am Interested in calibrating Some old vintage VTVM's and VOMS That i Am Now importing from USA to india  . I would like to Know Some Cool DIY Reference Sources .Like

1 : Voltage Reference Sources
2 : Current Reference Sources
3 : Resistance Reference Sources

         If Possible Capacitance Reference Sources ,Though in VOMS and VTVM's Its of No Use  .I Will Really Appreciate if they could be variable .I would Be Importing the following To India Soon For My Restoration And Reselling Work .

VTVM's :
  • Heathkit
  • Knight
  • RCA Voltohmmyst
  • B&K Precision 177 Dynascan
VOMS :
  • AVometer [  From UK ]
  • Simpson
  • Triplett
  • General Once Like Sears , Micronta Etc
« Last Edit: June 11, 2012, 07:23:59 pm by Amarbir[Lynx-India] »
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Offline LEECH666

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Re: Reference Sources - For Testing Multimeters
« Reply #1 on: June 11, 2012, 04:10:44 pm »
Not really DIY, but here I go anyway:

http://www.gellerlabs.com/SVR%20Series.htm

http://www.voltagestandard.com/

I think the main problem with these references is, that you would need an even more accurate (at least one order of magnitude) calibrated meter to calibrate them. So you will already need some pretty expensive / high end gear to "do it yourself".

Florian
 

Offline Circuitous

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Re: Reference Sources - For Testing Multimeters
« Reply #2 on: June 11, 2012, 05:18:03 pm »
I agree with Leech666, the references from Geller Labs and VoltageStandard will probably do what you need.
There's some additional discussion here:
https://www.eevblog.com/forum/general-chat/incoming-stuff-voltage-reference/msg104528/#msg104528

These should be very accurate right out of the box.  But, be sure to follow the guidelines on the ambient temperature that they are calibrated for, and also allow for warm-up time.

On the Geller unit, you can have the device calibrated to a specific temperature, which may be handy if you don't have an air-conditioned/heated lab.
The Geller site might not be as clear about ordering and shipping, but if you have any questions, just send him a note.

On VoltageStandard, there's a new unit that puts out AC voltage and AC current (haven't tried this one yet).  Again, if you have any questions, Doug Malone is very responsive to inquiries.


alm

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Re: Reference Sources - For Testing Multimeters
« Reply #3 on: June 11, 2012, 05:32:00 pm »
I think the main problem with these references is, that you would need an even more accurate (at least one order of magnitude) calibrated meter to calibrate them. So you will already need some pretty expensive / high end gear to "do it yourself".
The main thing your buying with the SVR/VoltageStandard in my opinion is improvised calibration, so you should not need a more accurate meter. It may not be NIST traceable, but it should be plenty good for most hobbyists. Note that analog meters are not usually 0.05% accurate. If you're talking about building your own standard, then I'd certainly agree. Initial accuracy is usually much worse than stability, even the expensive voltage references may only have 1% initial accuracy, while long term stability might be in the tens of ppms.

A potential issue with VOMs is that the ~5 V from a voltage standard may not be sufficient, and that the high test current for the lower resistance ranges might heat up the reference resistors.
 

Offline Amarbir[Lynx-India]

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Re: Reference Sources - For Testing Multimeters
« Reply #4 on: June 11, 2012, 07:03:06 pm »
Not really DIY, but here I go anyway:

http://www.gellerlabs.com/SVR%20Series.htm

http://www.voltagestandard.com/

I think the main problem with these references is, that you would need an even more accurate (at least one order of magnitude) calibrated meter to calibrate them. So you will already need some pretty expensive / high end gear to "do it yourself".

Florian

Sir ,
     Hope This Is OK For VOMS And VTVM's -> http://www.voltagestandard.com/DMMCheck_Plus.html
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Offline LEECH666

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Re: Reference Sources - For Testing Multimeters
« Reply #5 on: June 12, 2012, 03:53:13 am »
[...]
If you're talking about building your own standard, then I'd certainly agree.
[...]

Yes that's what I was talking about.

Sir,
Hope This Is OK For VOMS And VTVM's -> http://www.voltagestandard.com/DMMCheck_Plus.html

I'd say yes for VOMs (Volt/Ohm Meters). I am however not the most knowledgable person to ask about this.
Never used a Vacuum Tube Volt Meter, but I don't see why it shouldn't work. Probably better to ask one of the old guys with more experience than me. Alm already hinted at some limitations of these low voltage low power references.

Cheers,
Florian
 

Offline Amarbir[Lynx-India]

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Re: Reference Sources - For Testing Multimeters
« Reply #6 on: June 12, 2012, 06:32:29 pm »
[...]
If you're talking about building your own standard, then I'd certainly agree.
[...]

Yes that's what I was talking about.

Sir,
Hope This Is OK For VOMS And VTVM's -> http://www.voltagestandard.com/DMMCheck_Plus.html

I'd say yes for VOMs (Volt/Ohm Meters). I am however not the most knowledgable person to ask about this.
Never used a Vacuum Tube Volt Meter, but I don't see why it shouldn't work. Probably better to ask one of the old guys with more experience than me. Alm already hinted at some limitations of these low voltage low power references.

Cheers,
Florian


Sir ,
    Yet To Get Any Email from Them  . :-[
Regards

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alm

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Re: Reference Sources - For Testing Multimeters
« Reply #7 on: June 12, 2012, 06:55:15 pm »
It says on that page that it's sold out until June 20, maybe that's why you didn't get a response yet?

One thing I don't like is the AC signal. They appear to use a micro controller to generate a square wave. A square wave is not something I'd choose to test a multimeter. The value will be off by sqrt(2) for average-responding meters (which includes most analog meters) that measure the average amplitude and multiply it by sqrt(2) to get the RMS value for a sinusoidal input. Even for a true RMS meter, the exact value depends on the meter's bandwidth. A square wave from a micro will have fast edge rates. Some multimeters will only have bandwidth to cover the standard 50/60 Hz signals. I wouldn't attach a micro controller to a sensitive voltage reference circuit, I'd be worried that it might inject noise into its power supply rail. I hope they've decoupled it very well.

I realize that chopping a DC signal is by far the easiest way to get an accurate AC voltage. It used to be a common way to calibrate the vertical amplifiers of oscilloscopes, for example. They would generate a DC signal, verify this with an accurate voltmeter, and chop this voltage to produce an AC signal for a scope.
« Last Edit: June 12, 2012, 06:57:33 pm by alm »
 

Offline bingo600

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Re: Reference Sources - For Testing Multimeters
« Reply #8 on: June 12, 2012, 07:23:28 pm »
I just got a Geller SVR , IMHO it's more accurate than the voltagestandard one.
But also because Geller (Joe) will recalibrate it for $10 + postage.

My Agilent 34401A is showing 9.99993v , and is still within specs (cal'ed in 2003)

I also got 2 PCB's from Geller , and have build & calibrated 2 DIY's against the "Original".
Cheap Ref-Chips on *bay here : 250731549641

/Bingo
 

Offline Amarbir[Lynx-India]

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Re: Reference Sources - For Testing Multimeters
« Reply #9 on: June 12, 2012, 07:37:39 pm »
I just got a Geller SVR , IMHO it's more accurate than the voltagestandard one.
But also because Geller (Joe) will recalibrate it for $10 + postage.

My Agilent 34401A is showing 9.99993v , and is still within specs (cal'ed in 2003)

I also got 2 PCB's from Geller , and have build & calibrated 2 DIY's against the "Original".
Cheap Ref-Chips on *bay here : 250731549641

/Bingo

Well,
 The Solution i am looking out for has AC+Dc Voltage [ though ALM has put question marks there ] , AC + Dc Current +  Resistance .This Is What I am Looking For IMHO .

@ Alm

This Needs To Be answered By Them ,I Will Email Them again "One thing I don't like is the AC signal. They appear to use a micro controller to generate a square wave. A square wave is not something I'd choose to test a multimeter. The value will be off by sqrt(2) for average-responding meters (which includes most analog meters) that measure the average amplitude and multiply it by sqrt(2) to get the RMS value for a sinusoidal input. Even for a true RMS meter, the exact value depends on the meter's bandwidth. A square wave from a micro will have fast edge rates. Some multimeters will only have bandwidth to cover the standard 50/60 Hz signals. I wouldn't attach a micro controller to a sensitive voltage reference circuit, I'd be worried that it might inject noise into its power supply rail. I hope they've decoupled it very well." 

Regarding This "I realize that chopping a DC signal is by far the easiest way to get an accurate AC voltage. It used to be a common way to calibrate the vertical amplifiers of oscilloscopes, for example. They would generate a DC signal, verify this with an accurate voltmeter, and chop this voltage to produce an AC signal for a scope. " .Can You Explain Me Better Please
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alm

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Re: Reference Sources - For Testing Multimeters
« Reply #10 on: June 12, 2012, 10:48:50 pm »
Generating an accurate DC voltage is relatively easy, both the Geller and VoltageStandard ones do a decent job. Chopping the DC voltage with something like a MOSFET produces a relatively accurate low voltage (very close to 0 V because of the high off resistance of the MOSFET) and high voltage (very close to the DC voltage because of the very low RDS(on) and high input resistance of the meter). The duty cycle then determines the RMS voltage (50% duty cycle of 5 V = 2.5 V). A similar technique was discussed in more detail in Richard (amspire's) power supply design thread.

Doing the same for a sinusoidal signal usually involves an oscillator and an amplifier with a feedback mechanism which compares the output amplitude to some reference signal and adjusts the gain. This is much more complex and requires more components.
 

Offline Kilroy

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Re: Reference Sources - For Testing Multimeters
« Reply #11 on: June 12, 2012, 11:32:44 pm »
If you are going to go very far with this I would be more inclined to procure good low ripple, laboratory grade power supplies first, and make sure they are fully calibrated and referenced. That way you will have 99% of your verification plus the main tool required for calibration work.

Reason is, you are going to need to input specific, accurate and stable voltages and currents into these meters in order to perform proper setup and calibrations. Just the full scale movement calibration will require a stable 50uA current for typical 20kohms/volt analog VOMs like the Simpson 260 and Triplett 630/60 series. Something like the Simpson 269 or Triplett 630-NS will need even lower current values due to their higher sensitivities.


 
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Online amspire

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Re: Reference Sources - For Testing Multimeters
« Reply #12 on: June 13, 2012, 01:18:49 am »
Doing the same for a sinusoidal signal usually involves an oscillator and an amplifier with a feedback mechanism which compares the output amplitude to some reference signal and adjusts the gain. This is much more complex and requires more components.
I did post a stable circuit for making an AC reference for Marius (king.oslo).

https://www.eevblog.com/forum/general-chat/safe-to-repair-hp-agilent-3456a-6-d-dmm-any-ideas/msg92012/#msg92012

I didn't get into calibration though. With AC, the most accurate way to calibrate is with a Thermal Transfer standard that basically matches the heat produced by an AC voltage to the heat produced by a DC voltage.

It is not hard to find an electronic way to derive an accurate AC level from DC sources or meters to better then 1% accurate, but when you start needing 0.1% or better, it is amazing how many sources of errors there are. Very few DMM's quote better then 0.1% AC accuracy for very good reasons. Squarewaves have a heap of high frequency harmonics that can cause problems, and you are never quite sure how a squarewave reference reading will correlate to a true sinewave source.

One of my to-do projects is to experiment with an AC calibrator circuit that I can check against my Fluke 540A Thermal Reference standard. I would love to find a cheap circuit the does a pretty accurate and reliable AC calibration that I could use to set the level on my AC reference circuit.

I did also discuss with Marius a precision 100V DC reference circuit that was self calibrating from a 10V reference.

https://www.eevblog.com/forum/projects-designs-and-technical-stuff/100vdc-reference-circuit/

Richard.
 

alm

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Re: Reference Sources - For Testing Multimeters
« Reply #13 on: June 13, 2012, 01:40:09 am »
I did post a stable circuit for making an AC reference for Marius (king.oslo).
Still a significantly higher parts count and design effort than sticking an 8-pin uC in there, connect it to the reference voltage and program the PWM to generate a 50% duty cycle signal at some fixed frequency. Any idea how much error the distortion introduces? How much difference would there be between an average-responding meter, a true RMS meter with only a few kHz bandwidth and a true RMS meter with 1 MHz bandwidth?

It is not hard to find an electronic way to derive an accurate AC level from DC sources or meters to better then 1% accurate, but when you start needing 0.1% or better, it is amazing how many sources of errors there are. Very few DMM's quote better then 0.1% AC accuracy for very good reasons.
AC specs are usually much worse than DC specs. Many 6.5 digit meters are slightly better than 0.1% at audio frequencies. Datron 1081 quotes 0.02% 1 year 10 Hz-2 kHz and 0.005% AC/DC transfer 90 days.
 

Online amspire

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Re: Reference Sources - For Testing Multimeters
« Reply #14 on: June 13, 2012, 02:20:19 am »
I did post a stable circuit for making an AC reference for Marius (king.oslo).
Still a significantly higher parts count and design effort than sticking an 8-pin uC in there, connect it to the reference voltage and program the PWM to generate a 50% duty cycle signal at some fixed frequency. Any idea how much error the distortion introduces? How much difference would there be between an average-responding meter, a true RMS meter with only a few kHz bandwidth and a true RMS meter with 1 MHz bandwidth?
You do not know the error and that is the problem. You can never be sure if the error is 0.1% or 1% unless you can compare it to a true sinewave reference signal. Calibration is all about confidence, and a square wave is not a good waveform for a high confidence calibration.
Quote
It is not hard to find an electronic way to derive an accurate AC level from DC sources or meters to better then 1% accurate, but when you start needing 0.1% or better, it is amazing how many sources of errors there are. Very few DMM's quote better then 0.1% AC accuracy for very good reasons.
AC specs are usually much worse than DC specs. Many 6.5 digit meters are slightly better than 0.1% at audio frequencies. Datron 1081 quotes 0.02% 1 year 10 Hz-2 kHz and 0.005% AC/DC transfer 90 days.
They are amazing specs for a DMM, and I know seeing that that Datron have done a massive amount of work to achieve those results. I do wonder if Datron specs are as conservative as Fluke.

Definitely though, specs like the Datron specs are the exception. If you were trying to calibrate the Datron, I am sure you would have to use a precision multi-frequency sinewave reference source (or another calibrated meter as a transfer standard), as to get those specs, I am sure there is frequency compensation that needs to be calibrated. That would rule out a square wave for calibration.

In general, you want a very stable sinewave source for calibration, and some way for accurately determining the true level of the sinewave source.

Richard.
« Last Edit: June 13, 2012, 03:00:51 am by amspire »
 

Offline Amarbir[Lynx-India]

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Re: Reference Sources - For Testing Multimeters
« Reply #15 on: June 13, 2012, 08:54:38 am »
@ All,
 I got the following Message From Doug "  I have been noting the discussion on eevblog.com and would like to say that the AC reference voltage output is NOT the DC reference voltage "chopped" by the microcontroller - it is a true AC (bi-polar) waveform whose amplitude is 5V rms - as verified by our calibrated Hewlett Packard 3458A." .

PS :  Can a Pro disect It
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alm

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Re: Reference Sources - For Testing Multimeters
« Reply #16 on: June 13, 2012, 02:37:11 pm »
You do not know the error and that is the problem. You can never be sure if the error is 0.1% or 1% unless you can compare it to a true sinewave reference signal. Calibration is all about confidence, and a square wave is not a good waveform for a high confidence calibration.
If the error is something like 1%, than you might be better off building a less stable sine wave oscillator (eg. Wien bridge oscillator).

They are amazing specs for a DMM, and I know seeing that that Datron have done a massive amount of work to achieve those results. I do wonder if Datron specs are as conservative as Fluke.
I'm sure that takes a lot of effort, I'm not suggesting one could easily achieve this performance level. Fluke bought Datron (through Wavetek) to get their hands on the 7.5/8.5 digit technology, that's where the 8508 came from after years of reselling the HP 3458A, so it was apparently worth something to Fluke.

In general, you want a very stable sinewave source for calibration, and some way for accurately determining the true level of the sinewave source.
Agreed. But this is hard to build for a few bucks in parts, as is probably the case for the VoltageStandard.

@ All,
 I got the following Message From Doug "  I have been noting the discussion on eevblog.com and would like to say that the AC reference voltage output is NOT the DC reference voltage "chopped" by the microcontroller - it is a true AC (bi-polar) waveform whose amplitude is 5V rms - as verified by our calibrated Hewlett Packard 3458A." .
Doug is welcome to join the discussion. I would love to be proven wrong, but the only information available are the fairly terse specs on his website. OK, so chopped DC might be a little too simplified, there is also some AC coupling and amplification involved. It is still a square wave as far as I can see. The fact that it can be used to check scope probe compensation suggests it's a square wave with a fairly fast edge rate. This is confirmed by the statement that it has a variable duty cycle, a sine does not have a variable duty cycle (if it has a duty cycle at all).
 

Offline Amarbir[Lynx-India]

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Re: Reference Sources - For Testing Multimeters
« Reply #17 on: June 13, 2012, 04:23:29 pm »

Quote
Doug is welcome to join the discussion. I would love to be proven wrong, but the only information available are the fairly terse specs on his website. OK, so chopped DC might be a little too simplified, there is also some AC coupling and amplification involved. It is still a square wave as far as I can see. The fact that it can be used to check scope probe compensation suggests it's a square wave with a fairly fast edge rate. This is confirmed by the statement that it has a variable duty cycle, a sine does not have a variable duty cycle (if it has a duty cycle at all).


Well,
      Might Be Someone Has This Can Can Check the Signal With The scope and post here ,Would Be Very Interesting To Know That
« Last Edit: June 17, 2012, 04:44:11 pm by Amarbir[Lynx-India] »
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Offline Amarbir[Lynx-India]

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Re: Reference Sources - For Testing Multimeters
« Reply #18 on: June 17, 2012, 04:47:07 pm »
Hello guys ,
 Can anyone Check This Please And Post For Us All With a Scope  ? .
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Offline Circuitous

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Re: Reference Sources - For Testing Multimeters
« Reply #19 on: June 17, 2012, 08:14:27 pm »
The DMMCheck Plus is currently sold out, looks like they will be restocked in a week.
I plan to order one, and I can check it with a 34410A and a scope.

I'll post details when I have the unit.

Offline Amarbir[Lynx-India]

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Re: Reference Sources - For Testing Multimeters
« Reply #20 on: June 19, 2012, 02:17:09 pm »
The DMMCheck Plus is currently sold out, looks like they will be restocked in a week.
I plan to order one, and I can check it with a 34410A and a scope.

I'll post details when I have the unit.

Hii,
 Nice ,Do post here when you order also i will also order one unit i was also thinking of taking the penta unit so that i could have some variation in dc voltages

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Offline ModemHead

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Re: Reference Sources - For Testing Multimeters
« Reply #21 on: July 12, 2012, 01:01:01 am »
I recently received one of the new DMMCheck Plus units.  The reference AC voltage is indeed a 100Hz square wave that swings from -5V to +5V, with 50% duty cycle.  All of my RMS-responding meters read at or near 5.00 and the average-responding ones read about 5.55, due to the 1.11 Vrms/Vavg fudge factor for sine waves.  I find it curious that the included documentation did not mention this little caveat at all.  Nonetheless, a very useful little device.
 

Offline codeboy2k

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Re: Reference Sources - For Testing Multimeters
« Reply #22 on: July 12, 2012, 10:38:05 pm »
I recently received one of the new DMMCheck Plus units.  The reference AC voltage is indeed a 100Hz square wave that swings from -5V to +5V, with 50% duty cycle.  All of my RMS-responding meters read at or near 5.00 and the average-responding ones read about 5.55, due to the 1.11 Vrms/Vavg fudge factor for sine waves.  I find it curious that the included documentation did not mention this little caveat at all.  Nonetheless, a very useful little device.

I think people see "AC" and think "Sine Wave", when in fact what you have is a 5V rms bipolar AC source.
It's not a sine, but it's still AC, and calibrated 5V rms, so it can be used to test a true RMS DMM.

It is just that, a caveat, and the documentation would be wise to include this note about the AC source's waveform.

 

Offline Hrvoje-CRO

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Re: Reference Sources - For Testing Multimeters
« Reply #23 on: July 19, 2012, 06:21:45 pm »

Quote
Doug is welcome to join the discussion. I would love to be proven wrong, but the only information available are the fairly terse specs on his website. OK, so chopped DC might be a little too simplified, there is also some AC coupling and amplification involved. It is still a square wave as far as I can see. The fact that it can be used to check scope probe compensation suggests it's a square wave with a fairly fast edge rate. This is confirmed by the statement that it has a variable duty cycle, a sine does not have a variable duty cycle (if it has a duty cycle at all).


Well,
      Might Be Someone Has This Can Can Check the Signal With The scope and post here ,Would Be Very Interesting To Know That

Hello there!

Well, i will post it.
 

Offline ErikTheNorwegian

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Re: Reference Sources - For Testing Multimeters
« Reply #24 on: July 20, 2012, 06:33:33 pm »
Nice, an excellent source of calibrating oscilloscopes with to!   :)
 

Offline Magicmushroom666

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Re: Reference Sources - For Testing Multimeters
« Reply #25 on: July 20, 2012, 09:37:34 pm »
Surely the cheapest way to start calibrating multimeters would be to just buy a lab multimeter that you get calibrated which is an order more accurate than the calibration you provide to use as a reference? Buying a load of off the shelf sources etc is going to be a bit of a nightmare keeping them all in cal etc? with a precision meter you can setup your own sources, or automate by making a test fixture.
 

Online IanB

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Re: Reference Sources - For Testing Multimeters
« Reply #26 on: July 20, 2012, 09:46:58 pm »
Surely the cheapest way to start calibrating multimeters would be to just buy a lab multimeter...

You can buy a calibrated lab multimeter for $50?

This thread is not about calibrating, it is about testing or verifying.
I'm not an EE--what am I doing here?
 


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