Can a DMM be trusted?

[boater sam]

Do you trust your Digital Multi Meter?

If you put a diode mains dropper on a radio, then the voltage reading will be incorrect because its half sine wave but neither Ac or DC.

But what about capacitor droppers?  My meters give widely incorrect reading then too!

Sam.

[ppppenguin]

A capacitor dropper should be OK. But don't expect the voltage across the cap and the voltage across the heater chain to add up to mains voltage. They are 90 degrees out of phase.

[PerdioPal]

The "True RMS" ones are believable for lower frequencies, the question is what do you want to measure? Non sinusoidal filament voltage/current at AC then RMS is what you want, sine wave AC or DC any meter will do.

[Mike Watterson]

Any test instrument has limitations and can give misleading results.
DMMs vary widely in performance and how they work. Some can't measure wiring resistance of a large value inductance or higher voltage winding on a transformer as well as the cheapest analogue meter if they are not using static DC energising.
I've a couple that take too long to respond to shorts (continuity tests) or low resistance. You have to hold the probes on far too long. IMO those are junk even though nearly new. Also I have one that not just has 10A on a separate socket but irritatingly has Ohms and mA separate from volts.

I had one with a switch and a 750V DC range that died with a blue flash, set to that and connected to a 450V HT. I keep an old AVO to measure more than 250V or resistance of massive inductuctor/transformers.
I've a VVM* that unlike the DMM (which are 1M, 10M or 20M on volts depending on model) can measure AC up to about 150 MHz ( though not very accurately at some frequencies). Due to the 100M Ohm approx input impedance it can be a bit mental. It also can measure huge resistances, though my 400V via 2M leakage tester can roughly measure 20M Ohms to maybe several Gigaohms by looking at flash rate. 1M ohm looks like solid on and 100 M Ohms is a slow flash.

[*A VVM unlike a FET based one is maybe more robust a really high input impedance one is really an Electrometer. A cheap one is a strip of aluminium foil folded over a plastic cocktail stick across top of a glass jar]

[joebog1]

I use my AVO 8 all the time, its my everyday meter.
If I need to measure a HT of 321.9373545487436324 volts I use digital.
I have a HP 410B if I want really high input impedance, and a HP400D if I want high frequencies.

I have a variety of meters of course, but in my opinion you cant beat an AVO.

Joe

[Mike Watterson]

The Avo has too low an input impedance on particular ranges and poor for measuring uA or resistance. It's also huge. I'd only use my AVO in specialist situations when the DMM isn't suitable.
I've never had a DMM that could do more than about 5 digits (most are only accurate for 4) never 18+ digits.

[joebog1]

Ohh Mike !!!.
I thought you would pick up on my bent humour.
The CHEAPEST digital is not worth the four digits it can do, UNLESS I am measureing some bridge or sensitive equipment.
I have a Fluke 8050A digital. Like ALL digital the LCD is black, BUT still works. Cost $1300.00 Au, 40 years ago.
( I also have a few "throw away digital meters ", that I only use to show people who cant tell the time what volts we are reading. )
I work on valve gear by choice, I avoid the sand pit like the plague.

I expressed an opinion, half tongue in cheek. I DONT like digital at all, its not really "cosher ".
I bought a new 250 megacycle two channel digital oscilloscope, its crap !!!.
I dont have any control over it at all. The computer tells me what IT THINKS is important, NOT what I am trying to see.
( NOT to mention the digital noise that it creates itself, and puts lots of little negative going "spikes " on the trace )

OOHH yes it has lots of pretty colours, and if you actually read the 1mm tall text it does show a few things, that I already knew from using my AVO.
I play vinyl. I have an old record grinder made by Garrard. My speakers are English Tannoy and work superbly. They were made in 1974.
I designed and made my amp. 6BW6, stereo, in pushpull. A staggering 7 watts RMS per channel. Thats 475,539 PMPO watts, whatever THEY are.
Its like 15 years old, and I havent changed a single component so far.

The origional question was "trust " of a digital meter. My answer is no.

Thanks for reading, and I hope Sam read my rave.

Joe

[BusyBee]

It's horses for courses, I suppose, as the old AVO would be useful when checking some of the vintage stuff where the circuit diagram shows voltages read using such a meter. A digital meter would be difficult to compare without some thinking. As mentioned above, it's too easy to get into a false sense of precision too.

A revelant experience I once had was a 9V battery which gave a perfect voltage at the terminals, measured with a digital meter, but almost nothing when measured with an AVO. High internal resistance or what! Obviously they should be checked under load but how many of us just use a quick meter check.

I use both, although have abandoned cheap digital meters. I have always found them to have issues of one sort or another. I also find that with most modern digital equipment, in general, that there are usually caveats to their descibed functions. Somewhat hidden issues which would reduce the good selling points.

Don't get me onto digital meters for testing Germanium transistors either. I use an old Russian analogue multimeter for that. It works

Tracy

[Refugee]

I have got a Maplin digital meter that has an on load battery test function.
Even an AVO 8/9 does not really put enough load on for a good battery test. You really need an AVO 7.

[Doodlebug]

To see a fluctuating voltage it's a non starter with digital.
A moving needle shows the variations, a digital just has a jumble
of rapidly changing numbers which make no sense of how
the voltage is moving.