Can a DMM be trusted?

[Murphyv310]

During my time in the trade all test equipment was send to a specialist instrument calibration lab in Glasgow (long gone) every AVO 8 & 9 were done and each had a written report of faults and errors, surprisingly many were inaccurate at arrival to the Lab. Interestingly the DVMs in the main were reliable and accurate and just had routine checking.
A Weston cell gives an accurate 1.018638v and can still be found to this day from sellers in the Ukraine, I had one but it failed many moons ago.

[The Philpott]

I've certainly found the magnet/s in an AVO have often 'sagged' by the time i get hold of them, but generally not by a large amount. Latter day innovation with neodymium magnets will save those few that have really poor magnets.

On the rare occasions that i open up a (budget) DMM i'm pleased if i can see trim pots.. they're even present in those rather farty old Tandy Microntas, although with mine trimming the 20vDC range put the 2vDC out by a huge amount. Poor chipset.

Dave

[Murphyv310]

Those cheap Aneng DMMs are really decent for the money and pretty damn near as accurate as a Fluke.
These days I only use an Avo 8 for current measurement.

[BusyBee]

I only tend to use the Avo 8 for current measurement too. The talk of magnet sag reminds me of the user handbook I have with the Russian multimeter I have. In it there is suggestion of remagnetizing the magnet to get the calibration correct (or something like that). Last I used it, which is seldom since I now have the Avo and good digital meters, it appeared reasonably accurate. There were some spare diodes with it too and what may be a spring for the meter movement?

Tracy

[ppppenguin]

A question to Trevor and Tracy. Since you use a DMM for volts and ohms, why do you use an AVO for current?

[Murphyv310]

Hi Jeffrey.
I use the current on the Avo while testing repairs to amateur radio transmitters often up to 50 watts. On the odd occasion DMMs are upset by stray RF whereas the AVO seems pretty immune. Plus its often easier to see in some cases.

[ppppenguin]

Analogue is, of course, excellent for seeing how I or V varies. The bargraph displays on some DMMs are a reasonable substitute.

I've never worked with lots of RF around (though I once did some EMC work on a product that had to be used, I think, at Crystal Palace TX) so am not aware of how it affects DMMs. I know that looking at, for example, the boost voltage on a valve TV can be misleading with some DMMs.

True RMS DMMs, such my Fluke 87, need to be treated with caution when measuring awkward waveforms. A notable recent case was checking the heater voltage on a video monitor. The heater supply was derived from the LOPT and gave daft readings on the 87. I think it was Mark Hennessey who reminded me that my Rigol scope could do RMS voltage measurements and that solved the problem. An AVO would not have helped Nor would a VTVM. A thermocouple meter would have worked but I don't have access to one.

[BusyBee]

A question to Trevor and Tracy. Since you use a DMM for volts and ohms, why do you use an AVO for current?

In the main it is when I am doing valve tests as I use multiple meters. I like to see the current flow at a glance with the swing of the needle. It also makes me think more closely as to what I am doing rather than use a digital meter, thinking it will cope with whatever happens. A digital analogue display meter may be useful in this case but I have never had, or used, one.

I think the swing of an analogue needle conveys more than a less obvious rise of a number. The rate of change can be a good indicator (especially when the needle flys to the endstop which luckily I am wary enough and careful to avoid ). I tend to use my oscilloscope in a similar way, watching the input and output waveforms as I turn on the power and watch the rise in output. I think it's easier to cope with relative levels than understand the precise relationship between a clutch of numbers in an instant.

Tracy

[Kalee20]

True RMS DMMs, such my Fluke 87, need to be treated with caution when measuring awkward waveforms. A notable recent case was checking the heater voltage on a video monitor. The heater supply was derived from the LOPT and gave daft readings on the 87..

That's probably the same effect I noted in an earlier post, it's really annoying and to my mind massively diminishes the usefulness of the meter!

... my 'true RMS' Fluke 177 isn't any good!

It filters out any DC and does a true RMS on the AC content only. That means if there's any DC component, such as a diode dropper, you won't get a true RMS on the whole - and your valves will overheat.
It's easy to see if this is the case. Just select AC, and measure a DC voltage or current, True 'true RMS' will give the DC value; AC-filtered 'true RMS' will show zero (after a brief reading).

[ppppenguin]

When you know or suspect an awkward waveform and/or AC superimposed on DC you need to understand what you want to measure and the limitations of your testgear. On these difficult waveforms AVO type meters will give the wrong result. DMMs, whether true RMS or otherwise, may or may not give the right result.

Taking the opposite case to a diode dropper, let's measure the unsmoothed ripple on a DC supply. You would normally want the DC blocked, whether you are looking for a mean or true RMS value. Better to use a 'scope and see the frequency and peak-to-peak voltage. Which is the voltage that's of most value.