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Easy way to test the calibration of a DMM (Fluke 45)?

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bdunham7:
You didn't state the Hioki model, but assuming you have a DT4252 (my best guess) the specified tolerance is 0.3% of the reading plus 5 digits.  That works out to +/- 0.020V.  The specified 1-year tolerance of the Fluke 45 is 0.025%/rdg plus 2 digits in the 30V range that you are using which comes to about 0.003V.  So either the Hioki as at the extreme upper end of its tolerance range or the Fluke 45 is out of spec.

You could try using the slow reading 10V range on the Fluke 45 and see if anything changes.  Beyond that, it is going to be difficult to judge which meter(s) are within spec with just what you have on hand.

shapirus:

--- Quote from: Fried Chicken on March 30, 2024, 04:03:14 pm ---Consistently 0.02v ish

--- End quote ---
Well, even if that may still be within spec (I don't know what is the accuracy spec for these, that's on you to check), a 20 mV difference sounds a bit too much.

There are some good relatively inexpensive voltage references like DMMCheck plus, which are adjusted and calibrated using well-known equipment with valid calibration certificates. One option is to use these to check which of your meters is (or are) off.

Another option is much cheaper, it's to use one of these LM399 boards from Ali. The equipment with which the as-measured values written on them were obtained, and the calibration status of that equipment, are questionable, but reportedly these boards are quite decent, and in most cases good to at least 1 mV. I have one of these, and it's been staying consistently in agreement with my Brymen BM869s within 500uV in the 5V range and 1.5 mV in the 50V range, which is well within the accuracy spec of the BM869s. The question is whether the numbers written on the board are trustworthy remains, and that may be a problem in your case: ideally you would need to check this board with some trusted recently calibrated voltmeter before using it to find out which of your meters is off :).

Fried Chicken:

--- Quote from: bdunham7 on March 30, 2024, 04:27:16 pm ---You didn't state the Hioki model, but assuming you have a DT4252 (my best guess) the specified tolerance is 0.3% of the reading plus 5 digits.  That works out to +/- 0.020V.  The specified 1-year tolerance of the Fluke 45 is 0.025%/rdg plus 2 digits in the 30V range that you are using which comes to about 0.003V.  So either the Hioki as at the extreme upper end of its tolerance range or the Fluke 45 is out of spec.

You could try using the slow reading 10V range on the Fluke 45 and see if anything changes.  Beyond that, it is going to be difficult to judge which meter(s) are within spec with just what you have on hand.

--- End quote ---

Oh shoot, I thought the model would appear on the picture.  It's the DT4251 (same as DT4252 afaik, except no current functionality).

So this is where the challenge comes in, what reliable voltage source might I have laying around...  Else as shapirus suggested, one of those aliexpress/ebay voltage sources...  Unless there's some creative way to find a reliable voltage source.  Maybe from a PC computer that might have a built in multimeter for its 5v USB rail?

shapirus:

--- Quote from: Fried Chicken on March 30, 2024, 04:52:36 pm ---Maybe from a PC computer that might have a built in multimeter for its 5v USB rail?

--- End quote ---
Nope, you wouldn't expect anything like that to have the required accuracy.

You need either a voltage source that is sufficiently stable and has been measured with a sufficiently accurate (preferably better than your DMMs) calibrated (aka verified) meter, or access to such a meter to compare its readings with those of your DMMs on an arbitrary voltage source like a battery.

Realistically, DMMCheck Plus sounds like the best option (but yeah, $225, oh well), since if you get that cheap Chinese board, you still don't have a trusted DMM to verify it and use that initial verification as your reference point. Yes it is done by the manufacturer, but to what extent can that manufacturer be trusted? In case of DMMCheck Plus it is as trustworthy as the calibration certificates published on the manufacturer's website, and that's more than sufficient for your use case.

bdunham7:

--- Quote from: Fried Chicken on March 30, 2024, 04:52:36 pm ---So this is where the challenge comes in, what reliable voltage source might I have laying around... 

--- End quote ---

This is where/how USA Cal Club got started, albeit at a few more digits of precision.  You aren't likely to have anything "laying around" nearly accurate enough to exceed the Fluke 45 specs.  And a one-point voltage source isn't really all that much help when you want to verify the meter entirely.  If you really care, you need an official or trusted unofficial calibration.  The question is whether you care enough to worry about that level of discrepancy. 

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