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Wondering how to check the accuracy of a DMM
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BinaryMike
Pelagic EE
A fresh AA Duracell should read very close to 1.605 VDC. You can check resistance ranges with known-good 1% resistors.
Then with a fresh AA drycell and a "known good 1 % or better resistor and your meter all three hooked up in series you should be able to measure current and use good 'ol ohms Law to calculate the current and compare to what you meter reads. I (current) = E (voltage) / R (resistance)
We have a Fluke calibrater at work but there is not much off the shelf stuff that will really tell you a great deal about the accuracy. But they might tell you enough to know you are out of the ballpark. You could find a friend with a cal lab or a higher accuracy bench DVM and compare readings for a better idea. You could pay to have it cal'd but that would be outrageous. More than the meter costs. Best you can do is assume it is within manufacturers specs upon receipt with a fresh battery installed. After a few years that might change a little depending on the quality of the meter. But remember back in the tube days hobbyists only had off the shelf VOMS or VTVMs to use to work on their tube or transistor gear, and achieved satisfactory results with these much less accurate devices.
HackerJohn1
Well-Known Member
I'm an old retired Army calibration specialist. All those ideas are good and sensible. We ALWAYS had to keep a minimum 4:1 accuracy ratio between our "standard" and "test instrument" to calibrate (except the atomic frequency standard!). If it's something you might get "excited" about (I mean that nicely!), you may want to acquire a few different wire wound resistors (I have some that are +-.01%. For voltage, you could build an accurate regulator circuit pretty cheaply. May be overkill until you get something more accurate. I used to have a John-Fluke differential voltmeter (+-.005%). Nice until I thought "how do I check THIS?. Sold it off at a Hamfest. Anyway, welcome to the DMM owner's club!
BinaryMike
Pelagic EE
The AA Duracell voltage I posted is accurate only under 'open-circuit' conditions, i.e. with just the voltmeter connected. I checked a few of 'em with my 6.5 digit Keithley.
Quite right..Only one sure way to check accuracy.Thats with Precision standards(calibration lab equipment) or a certified calibrated DMM.
paymenow
Active Member
Others have given ways to determine if your DMM is in the ballpark but, to verify that it is meeting the accuracy specs typical of recent DMMs you need a multifunction calibrator or a really good bench DMM. To have it calibrated by a lab would cost $35 to $75 for a simple certificate of accuracy, no data.
SolderIron
Super Member
There are ICs with voltage reference outputs. I remember seeing precision voltage reference ICs back in the 1980s. There were some A/D converters with the internal reference volt output on a pin. They make excellent ref. volt for DMM cal. I have not looked for a while and don't know what is available today. The old ICs may still be around.
BinaryMike
Pelagic EE
That's a very good point, SolderIron. National's LM4040 series is available in the TO-92 leaded package for around $2 in small quantities and offers 0.1% accuracy (A grade) at room temperature. That's good enough for checking service type DMMs.
I did the same thing - bought one of those CSI solder stations for $30 and then got a bunch of other supplies to get me to $50 to get the free meter. It does look nice and I like the transistor HFE and temperature adapters.
I'd just use it and enjoy it. For your purposes I'm sure it's plenty accurate.
I'd just use it and enjoy it. For your purposes I'm sure it's plenty accurate.