DMM Voltage Calibration

Morning Guys

Over the past few days I've been troubleshooting a circuit, looking for a fault, which involved a great many checks for signal and voltage at different points, using a DMM and my 'scope. I've noticed a significant discrepancy between the voltage reading of my DMM and the 'scope.

I have 3 handheld DMM's none of which would be described as a quality instrument, but the 'scope is fairly new and reasonable quality.. .. .. I don't yet possess a quality bench DMM, but it is on my christmas list

I have seen a number of small circuits which include a reference voltage chip, designed to give an accurate output voltage at 10 vdc for calibration purposes .. my question ?? ... .. .. are they any good ? Do they work ? Are they accurate ?

In case I don't get another chance, I would like to wish all participants in ETO, whether Administrators; MOD's; Very Helpful Members; Helpful Members; Active Members; Members; Guests and anybody else who isn't in that list a very Happy Christmas & New Year.

There is an absolutely huge Thank You for all the help, advice and patience you have given me in this my first year as an electronics hobbyist. I have learnt such a lot from you all, not least, how much there is to learn. I have a great deal of respect ... .. .. .

S

Oh Dear .. have I qualified for the 'Ignore' button ?

Scopes aren't that accurate for reading voltages, it's likely your meters are more so.

Don't get carried away, you almost certainly don't need any more accuracy than you have.

Really ? That does surprise me .. .. .. I thought that the scope was the dogs doodahs for measurement .. .. .

Mind you, some reading I've done this morning suggests that things like the ambient temperature of the workshop and the length of time a test instrument has been powered up will affect the test value .. .. .. and that would never have occurred to me !

Well Thanks for that, I'll put that one to bed .. .. I've got enough to keep me busy without it

S

I guess the problem is which one to trust?
For a start, write down the specs of each.
The scope was never meant to be accurate. Let's say it has a 8 bit converter and your measuring a 9V battery. 9/255 is 0.0352941176 (yea right). +- 1 bit. SO, you have a +-35 mV error right there. Analog scope?

A reference alone will only test one range and possibly two polarities. The reference should be of higher performance than your instruments. You won't get NIST or a traceable calibration.

I found a nice article: https://steelcityelectronics.com/2015/11/11/analog-devices-ad587-10v-reference/

Does it matter for you?

Ignore button. Sure, if you want one.



Don't say I didn't get you something for Xmas.

Nigel Goodwin said:

Scopes aren't that accurate for reading voltages, it's likely your meters are more so.
Don't get carried away, you almost certainly don't need any more accuracy than you have.

Click to expand...

But not the point to use the one below either. . .



Once, I put in series all the multimeters I got and measured a nominal current of 1 mA. What a variety!!!

To put things into context with a practical demonstration, I connected three DMMs in parallel to a power supply.

I trust this several hundred £/$/Euro Fluke to give the most accurate reading:



On the left here is a £30/£40 20 year old Korean meter, on the right is a £3 10 year old cheapie from some big DIY store.


So it looks like the cheapy has quite good agreement with the mighty John Fluke.

You pays your money and you takes your choice.

JimB

Thanks Guys .. .. ..

.. .. . they say a little knowledge is a dangerous thing .

S

MM , I would take an average of the 3 DMM and scope ? ( is 5v the same anywhere on the planet ? )

The yellow meter is like $5.00 USD. Oops $3.99 USD and an updated model. The price of 2.5 candy bars. I got one similar to the yellow one free. I have a bunch of meters. Never compared them.

https://www.harborfreight.com/7-function-digital-multimeter-69096.html

grandad: Ignore the scope. It will thow off the average.

I plan to buy sometime in the future, one of these:

http://www.voltagestandard.com/

Members of another forum liked these units very, very much.
But again, it all depends the accuracy level your work/hobby requires

schmitt trigger said:

I plan to buy sometime in the future, one of these:

http://www.voltagestandard.com/

Members of another forum liked these units very, very much.
But again, it all depends the accuracy level your work/hobby requires

Click to expand...

Hi ST ... that is exactly the sort of thing I had in mind .. ..however, I have now been persuaded that they are unnecessary .. .. .. I must say though, there seems to be times when a very precise measurement is crucial to a circuit functioning as it should and equally times when you can lick the side of your thumb and hold it up to the light as near enough !

The trick is knowing which is which .. ..

Having said that .. .. I was given a boxful of test gear etc over a year ago and I'd forgotten about it until very recently .. .. I had a bit of a poke about in it yesterday and I've found 2 bench multimeters, one of which looks very new and unused ... .. .. I may be in the pound seats after all

S

Some of this really depends on the voltage you are reading and the meter being used. This becomes especially true when reading an AC voltage which is other than sinusoidal in wave shape. For example we have good hand held meters which are RMS Responding RMS Indicating and we have lesser cost meters which are Average Responding RMS Indicating. When measuring DC levels we have concerns with the meters stated uncertainty (accuracy) and also the meter's resolution (ability to be read). So really often times it is a matter of choosing the best instrument for the best job. That may include a scope tossed in there.

Ron