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how do you calibrate a dvm with 6.5 digits?
Joop01:
--- Quote from: NoisyBoy on May 01, 2023, 05:14:08 pm ---If you are truly serious about calibration to bring it within spec, it simply isn’t feasible for most labs, let alone home labs. There are numerous calibration point within each function, each requiring an extreme stable source. My 6.5 digit meters requires Fluke 5720A and 5725A or later for proper calibration, these equipment would far exceeds the cost of most home labs.
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What amazes me is how difficult they do about what a product cost. With some trouble I found the 5720A, way above my head or what I want to spend on it. Didn't found 5725A. Did found that the 6.5 digit is no longer in catalog. They have now only a 5.5 digit Bench meter. Back to my question, if it is not possible to calibrate your own equipment, then what is the point in having, say, a 6.5 digit bench multi meter?. What I looking at?
alm:
--- Quote from: Joop01 on May 01, 2023, 06:17:37 pm ---What amazes me is how difficult they do about what a product cost. With some trouble I found the 5720A, way above my head or what I want to spend on it. Didn't found 5725A. Did found that the 6.5 digit is no longer in catalog. They have now only a 5.5 digit Bench meter. Back to my question, if it is not possible to calibrate your own equipment, then what is the point in having, say, a 6.5 digit bench multi meter?. What I looking at?
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Calibration is about having readings that are traceable to national standards. For many applications you need resolution (to determine how much a value changes with temperature, for example), but not necessarily accuracy. Also, good quality 6.5 digits like the classic HP/Agilent/Keysight 34401A are known to stay within their stated 1 year accuracy or most ranges for years, though if you want to certify this, you will need to have it calibrated every year. But for many hobbyists, knowing that a voltage is 5V +/- 1% but it's varying at most 0.5 mV over load or temperature is good enough.
If you need traceability, for example because your customers demand it, then you'd send it to a cal lab at the interval the manufacturer recommends (often 1 year, sometimes if you want higher accuracy). I haven't kept track of prices, but I'd think that calibration of a 6.5 digit meter would be in the ball park of $200.
The problem with calibrating it yourself is a) that you need another reference with traceable calibration with lower uncertainty than the desired uncertainty of the calibration and b) that you need stable sources for all signals that the meter needs, unless you want to do a limited calibration of only DCV and Ohms, let's say. For a full calibration might need DC voltages from 100 mV to 1000 V, resistance from 100 Ohm to 100 MOhm, DC currents from 10 mA to 2 A, AC voltages from 100 mV to 750 V from 10 Hz to 50 kHz and AC currents at 1 A and 2 A at 1 kHz (this is for the 34401A, check the service/calibration procedure for your DMM). Especially high frequency, high voltage AC is often difficult in your average lab. This all needs to have a stability much better than the desired uncertainty. So a basic 1/4W resistor will definitely not be stable enough for this comparison.
So either you would need a more accurate DMM that was calibrated before and stable sources for all these signals, or a calibrated multi-function calibrator that can source all these signs with a low enough uncertainty. This is how the cal labs do it, but they might be investing $100k in equipment and another $10k or so for yearly calibration. They make this back in the volume of equipment they calibrate.
bdunham7:
--- Quote from: Joop01 on May 01, 2023, 06:17:37 pm --- Back to my question, if it is not possible to calibrate your own equipment, then what is the point in having, say, a 6.5 digit bench multi meter?.
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If you actually need a current traceable calibration certificate then you have to ship it to a certified (to whatever standard you require) calibration lab. To actually set that up yourself for a typical 6.5-digit bench meter like Keithley DMM6500 or Keysight 34465A would take an enormous investment in equipment and then you'd still have to get whatever accreditation you require--and then you'd still have to have that equipment calibrated periodically at an exponentially higher cost. Nobody does that unless they have hundreds or thousands of instruments to calibrate. The point of having a calibrated 6.5-digit meter is that you can take measurements with the meter and be fairly sure that the are accurate within the specifications of the meter.
Now if you wanted to check the calibration unofficially, you could get a few standards or DMM-check type units that would at least verify that your meter is not broken and hasn't drifted grossly out of spec. However, to get the precise high-voltage and high-frequency calibration stimuli that is required for a full calibration would still be a very daunting task. Some of us do this sort of thing as hobby, but I would never guarantee results although I'm pretty sure I can get pretty close.
I have no idea why you think that nothing can ever be shipped--how do you get your new meters? If getting a new meter is the only way you can get something without it being smashed, just buy a new meter every year or two, depending on your calibration cycle. It will be way cheaper, especially if you sell your old one.
Fungus:
Yeah, is this calibration needed for legal reasons or are you just doing it as a hobby?
As noted, these things are designed not to go out of calibration. Maybe all you need is a way to convince yourself of that fact.
Adjusting it if it ever goes out of spec usually needs a lot of fancy equipment. You usually need several very precise references, one for each range.
David Hess:
I usually settle for periodically checking the calibration as best I can against my best references and my best other multimeter.
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