Products > Test Equipment
Easy way to test the calibration of a DMM (Fluke 45)?
J-R:
--- Quote from: bdunham7 on April 04, 2024, 08:12:23 pm ---The Fluke 45 arrived today and I had a bit of time between tasks to set it up atop the wreckage on my bench and take some initial readings. It didn't get a full warmup, but that's probably not an issue. Here's the numbers, read 'em and weep! As posted elsewhere, my 5220A current amp has gone up in flames for the moment. When it arises from the ashes, or at least I manage to get it off my bench, I'll get back to this meter.
--- End quote ---
I think that this explains the 5.1V discrepency:
"In the medium and fast measurement rates, the a/d converter uses one of two ranges: ±300 mV and ±3 V full scale.
In the slow rate, the a/d converter uses one of two additional ranges (±100 mV and ±1000 mV full scale), for a total of four ranges."
bdunham7:
--- Quote from: J-R on April 04, 2024, 09:49:43 pm ---I think that this explains the 5.1V discrepency:
"In the medium and fast measurement rates, the a/d converter uses one of two ranges: ±300 mV and ±3 V full scale.
In the slow rate, the a/d converter uses one of two additional ranges (±100 mV and ±1000 mV full scale), for a total of four ranges."
--- End quote ---
Possibly, but since it measures 10V using a scaled (divided) range (10V or 30V, not 3 as I wrote) it could be either the scaling constant for that range or the ADC calibration.
If I measure 1V using both the 1V and 3V ranges, I get 0.99460 and 0.9969 respectively. 0.99460 x 5.1 = 5.07246 and 0.9969 x 5.1 = 5.084. So according to that, the range scaling factor is still way off on 10V but close on 30V. And this is where it might get interesting as the DCV calibration adjustment process sets all 4 ADC ranges, but only sets range constants on the 30/300/1000V ranges not the 10/100.
100.000 mV on the 100mV and 300mV ADC ranges gives me 96.535mV and 98.84mV respectively.
So what now? Does this patient need calibration (adjustment) or repair?
shapirus:
--- Quote from: mawyatt on April 04, 2024, 06:27:13 pm ---Anyway, hope this helps understand what we did way back then in measuring Mains Frequency Deviation.
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Sounds like one hell of a complicated system, but I guess it was the way to go back then.
Mine is much more simple (by today's standards, of course): input divider -> MCP3304 -> single-board computer. Frequency measurement is as (in)accurate as the SBC's clock generator is, plus any inaccuracy added by the finite sampling rate, which is 100 kSa/s. I guess that's good enough for my home/hobby use. At least it agrees with my Brymen BM869s to within ~2 mHz. The latter's accuracy spec, however, is +/- 0.014 Hz at 50 Hz line frequency, so not an ideal comparison, but what are the odds of them both being off by the same amount? But I think I can verify this: I have a GPSDO, and I can build a frequency divider circuit to divide its 10 MHz to get a 50 Hz output, and measure that with the BM869s to see what it's showing.
p.s. whenever I measured AC line frequency, I don't remember seeing exactly 50 Hz. It was 49.98-ish every time. Right now its 3 a.m., and it's fluctuating around 49.97-49.99. I don't think I would want to sync my wall clock to that :).
p.p.s. I also log the value of the crest factor (\$\frac{|V_{peak}|}{V_{RMS}}\$) of the actual AC line waveform divided by the crest factor of the ideal sine wave to be used as a representation of how much the sine wave in the wall outlet is distorted. I'm usually seeing values around 0.97-0.98, which means that the wave's peaks are a bit flattened (and this can actually be observed with an oscilloscope). I guess this can be expected. I completely forgot that I had this value logged too -- will of course plot it along with frequency. Time to find (or rather remember) a suitable time-series data acquisition and storage backend for Grafana. There was one that I thought was a perfect fit for the job, but forgot it, maybe someone can remind me.
J-R:
Repair or adjust... I admit I'm on the fence. It does seem a little odd that so many readings are low. There is a decent amount of information in the service manual, so checking a few things out seems reasonable.
bdunham7:
Before I take it apart and start poking at its guts, I'm going to test it for stability and linearity. I wouldn't anticipate problems with either, but I may as well check. I'll be logging a 10 reference overnight, although the amount it is off by is pretty ridiculous--it reads 9.7470, or 2.53% of nominal. If that is stable, even being way off, that would be a good sign.
Looking at this meter closely, I'm seeing it is really a 30,000 count meter with a 100,000 count "hi-res" mode, similar to the Fluke 87 and relatives with their 6,000 counts and 20,000 counts in hi-res. It is also the only meter I know of that has modern closed-case calibration but still uses a plain 6.3V zener (presumably a selected temperature compensated model) as a reference.
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