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Here is a piece of information from a datasheet of a DMM. The accuracy rates seemed interesting for me. I wonder what causes difference in the accuracy, especially the changes after 24 hrs? Is it only shunt related, or another components related issue?

Second question, what kind of equipment do you need to calibrate such DMMs for the best accuracy?

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  • \$\begingroup\$ Not sure if you're considering buying one but I bought a 34461A several months ago and have been very happy with it. \$\endgroup\$ – PeterJ Oct 8 '14 at 11:37
  • \$\begingroup\$ Yes, I am considering to buy a 34461A, it is good to know your feedback.Thank you. \$\endgroup\$ – Angs Oct 8 '14 at 12:10
  • \$\begingroup\$ RS components offers a calibration service in the UK. Something similar may be available locally for you. It might be cheaper and easier to use than buying the equipment to calibrate the DMM. \$\endgroup\$ – gbulmer Oct 8 '14 at 13:12
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The DC volts reading on a DMM is typically dependent on two things- the DC voltage reference and resistor ratios (for most ranges). Both of those drift with time.

AC volts is similar. Current is dependent on absolute resistor values as well as the reference and (probably) resistor ratios. All those drift with time. Ohms is mostly dependent on the absolute value of a reference resistor. Again, it drifts with time.

Wherever I mentioned time, also include humidity, mechanical stress, air pressure etc.

The actual physical reasons for the change with time might be impurities in the device, migration of metal or something else entirely. There seems to be a natural law that real things have a 'noise' (power spectral density of noise to be specific) that has a 1/f characteristic, which is typically what we call drift with time. This has been observed in many natural phenomena. You'll see this characteristic in op-noise too.

How do you calibrate a DMM? Keithley, Fluke and others will be happy to sell you a calibrator which will provide outputs you can use to calibrate your DMM. Of course the calibrator drifts with time too, but less than most DMMs because it's frighteningly expensive and uses very good parts and good techniques to reduce drift. Still, there will be a calibration period that you should have it calibrated by a lab. Their references they use to calibrate the calibrator also need to be calibrated, and they should be traceable to some internationally recognized standards lab such as NIST. If this is done properly you can trace the calibration of your $100 meter back to the standards lab (doesn't mean it will be accurate or reliable, but it should ensure that 1,000 DMMs are not calibrated with a 1.999V reference that is off by 5%.

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    \$\begingroup\$ "The man with two voltmeters is never sure." \$\endgroup\$ – Phil Frost Oct 8 '14 at 13:56
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    \$\begingroup\$ It may be worthwhile to note that the drift specs don't say that something will drift a certain amount with time, or even that it's likely to do so; they merely indicate an inability to guarantee that it won't; saying that something is only guaranteed to be within 0.1% when it turns out to be accurate to within 0.02% is much better than saying it's guaranteed to stay 0.01% and having it turn out to drift by 0.02%. \$\endgroup\$ – supercat Oct 8 '14 at 20:03

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