| Products > Test Equipment |
| Easy way to test the calibration of a DMM (Fluke 45)? |
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| shapirus:
--- Quote from: Fungus on April 03, 2024, 12:04:43 am ---Correct, but what's the worst thing that could possibly happen? That you have to return it? --- End quote --- That you end up with N+1 devices about which you're not certain as to whether they're up to spec. Returning it and ordering another does not solve the problem of needing a known-good meter to perform the initial sanity check of the reference. |
| Fungus:
--- Quote from: shapirus on April 03, 2024, 12:06:53 am --- --- Quote from: Fungus on April 03, 2024, 12:04:43 am ---Correct, but what's the worst thing that could possibly happen? That you have to return it? --- End quote --- Returning it and ordering another does not solve the problem of needing a known-good meter to perform the initial sanity check of the reference. --- End quote --- That's why we all own several meters. :) My BM857s and my Fluke 187 both agree (to within a digit) with what's written on the sticker. Neither have been calibrated, but what are the odds of that? (that they agree with each other and with the randomly-purchased reference to three or four decimal places) |
| Fungus:
If you only own a single meter and there's a disagreement then it doesn't help, but that doesn't apply to anybody here... If it agrees then it's 1000:1 that it's just dumb luck. |
| shapirus:
--- Quote from: Fungus on April 03, 2024, 12:17:55 am ---If you only own a single meter and there's a disagreement then it doesn't help, but that doesn't apply to anybody here... If it agrees then it's 1000:1 that it's just dumb luck. --- End quote --- In the OP's situation, if the reference agrees with one of the meters, but not the other, then it's very likely, or, maybe, likely enough, that the one that it agrees with is the good one, however the chance that both the reference and that meter are off is not zero. If, however, both meters don't agree, then we end up with three untrusted devices instead of just two :). But yeah your point is valid. Once again, know the limitations and know what you're doing, and you can get useful results out of any crap. |
| KungFuJosh:
--- Quote from: shabaz on April 02, 2024, 11:35:10 pm ---"Or spend a lot more money, build a ref yourself, and still need to send it out for calibration (after hundreds of hours of runtime to confirm stability)." Or, spend $5! and not send it for calibration, nor wait hundreds of hours (a ridiculous suggestion) because it will immediately tell you which of the two multimeters has the 20 mV discrepancy. Then, you know which multimeter might actually be functioning, and you can send it for a cal, at less than the cost of a calibrated reference. --- End quote --- Not a ridiculous suggestion as thermal regulation and drift over time are things that exist. There's a reason some people leave their DMMs on 24/7. This is the part you don't seem to understand: if the POS $5 ref you buy agrees with one of your meters, it doesn't tell you anything without valid calibration. 2 items agreeing only matters if the results are repeatable on calibrated devices. 2 devices can be equally out of spec. If both meters agree with the ref, then you might be lucky. As was mentioned, most of those $5 refs come with bullshit cal sheets that are copied over and over again and are completely useless. |
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