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Easy way to test the calibration of a DMM (Fluke 45)?
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bdunham7:

--- Quote from: Fried Chicken on April 05, 2024, 05:41:51 am ---What do meters normally use?  It's my understanding these DMMs are stable in their calibration once calibrated

--- End quote ---

Typically either a buried zener for better models or a bandgap reference at ~1.2V for handhelds and lower end bench meters.  Both are supposed to have lower drift than typical zeners.  The zener used in the 45, Fluke p/n 857201, is listed as 6.3V +/- 3% @ 2mA bias current.  They don't even actually specify that it is temperature compensated--I'm just assuming that it is.  I cannot cross-reference or match it to any common part.  This appears to not be an issue as far as temperature stability but I don't know what the long term drift implications are.  In any case, I doubt reference drift has anything to do with whatever is going on with your meter.
G0HZU:
I've got an old Fluke 45 meter here and I use it daily. I think I've had it for nearly 15 years. In all this time I've not seen any noticeable drift in the meter although I'm not a 'volt nut'. I've never sent it for calibration and I think it has not been calibrated since it was first sold.

I think it still reads just over 0.5mV low with a 5V input and this is quite impressive across this time span. I have never bothered to really put it through its paces other than to check its linearity is OK for my needs. With this in mind, the Fluke 45 owned by Fried Chicken does seem to have some significant issues.

I can post up some tests of mine if it helps? I've got some 6.5 digit DMMs here that are out of formal cal but two of them were UKAS calibrated when last calibrated a few years ago by the previous owners. I've also got a programmable PSU here that has 1mV resolution and good accuracy and this is usually good enough to do a basic health check on a meter across a huge range of test voltages and currents. It can also test current to 0.1mA up to several amps. It's not in the same class as a VI calibrator obviously but it is probably still better than most budget DMMs (and dodgy ebay references) in terms of accuracy. It's not quite accurate enough to actually calibrate a Fluke 45, but it is going to be close enough to prove it's still in pretty good shape...


bdunham7:
Alright, progress!  I had almost no time to look at it earlier so I set it up to log a 10V reference for a while (overnight) in the 10V range and then I set it up with a 20V (19.99997-ish actually) source all day in the 30V range.  This tests the stability of the 100mV and 300mV ADC ranges as well as the x100 divider circuit.  Screenshots attached, the meter is perfectly stable to 1 count or less.  I did the same thing for a shorter period in the 1V and 3V ranges, no charts but the same result--perfectly stable. 

Looking at the calibration constants it appeared to me that all of the DCV-related constants were way off while all the others seemed to reflect a reasonable amount of correction.  Proceeding on the theory that this and the punctured calibration seal might indicate that someone has tried to calibrate it and failed badly, I ran it through just the DCV calibration steps.  After that I scanned through and checked for linearity and offset on all the ranges and they were good.  As I suspected from the design, the 30,000 count ranges are better than the 100,000 count ranges--the former were perfect while the latter were still good enough but not quite as perfect.  There was a 3-count discrepancy at the negative limit of those ranges. 

However, it appears that all of the other functions--ACV, DCA, ACA and Ohms are now in spec or at least very close to it.  That's all I've got for now, more later this weekend.

CatalinaWOW:
Lots of good work there. Hope the owner appreciates the value he is getting.
Fried Chicken:

--- Quote from: CatalinaWOW on April 06, 2024, 03:49:46 am ---Lots of good work there. Hope the owner appreciates the value he is getting.

--- End quote ---

I will be thrilled once it's safely back on my bench xD  But I am excited yes
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