Cheap multimeter accuracy

[Harry Bloomfield]

As I've said, the irony is that the only major 'failure' I can recall was that of an expensive Fluke, which failure was identified/confirmed by comparison of readings with those from a number of very cheap other meters

Mine was a Sinclair early digital meter, back in the days when they were the only digital available for less the £100, which I had bought as it was a bit smaller than an AVO 8. It was showing the SP mains voltage as 150 volts, it had suffered from damp. I was lucky, I still had the AVO 8 in the car.

The absence of calibration certificates is not really relevant (other than for 'bureaucratic' reasons) if the readings are always close to those of 'expensive comparators. As for 'build quality and cost, I have plenty of (increasingly!) cheapo ones that are still functioning satisfactorily after many years/decades - and if a replacement would cost maybe 5% of the price of a mid-range Fluke, I'm inclined to regard that (for me) as a "no brainer"

No great surprise really that they are so accurate, 1% and better components, that they rely upon in the build, are much more available and no longer expensive. It's the tolerance of components, which sets the basic accuracy.

 

[bernardgreen]

A colleague in the 1970's found his "disposable" multi-meter reacted to low levels of RF radiation from transmitters on his workbench, His Avo was not noticeably affected

One of my low cost meters is affected when a PMR 446 ( UHF ) walkie talkie is fired up a couple of feet away, Adds a few milli-volts to the display on the 2 Volt AC range, The ammount of additional reading is very dependent on how the leads happen to be laid out on the bench

Repeating the test with both a Fluke 115 and a Fluke 77 and the leads in roughly the same layout shows no noticeable affect on the display.

 

[Harry Bloomfield]

I have a handy credit card sized meter, which attempts to predict what you are trying to measure, from choice of volts ac/dc, resistance, continuity, or frequency. Usually it makes the correct choice, but includes an override button if not.

 

[SFK]

handy credit card sized meter,

Is that the Amprobe DM78C ?
or a Chinese no name?

Looks interesting and i see it has the bar graph that some were asking for.

Note that I am happy to drop the fluke (a few feet) and then continue using it, but not happy to drop my £20 meter for fear that it will not be reading correctly (ie showing 0Volts when it is actually 240V).

 

[JohnW2]

No great surprise really that they are so accurate, 1% and better components, that they rely upon in the build, are much more available and no longer expensive. It's the tolerance of components, which sets the basic accuracy.

Quite so. Unless one were silly and/or very cost-cutting in one's sourcing of components, I imagine that it would be quite difficult to make a digital meter which was not accurate enough for most purposes.

What surprises me a little more is that any of them (from ultra-cheap to ultra-expensive) seem to remain pretty accurate for very long periods without 're-calibration', given that the value of even <1% components could 'drift' a little over time, particularly if subjected to appreciable thermal cycling etc. However, the oldest ones I have must be 30+ years old (and of pretty old design/manufacture) yet still seem to be pretty accurate.

When I started (a long time ago) buying ultra-cheap meters, I was very much thinking of them as 'disposable' - but I can't actually recall ever having had to 'dispose' of a single one of them through any fault of the meter (only when they have been overlooked for years in the bottom of some drawer or box with a leaking battery in them, or when I once trod on the display of one of them ).

Kind Regards, John

 

[JohnW2]

My working rules is - if you get strange reading with one meter, always have a second to hand to compare it to.

I imagine that's what we all do.

I should really have added ... although that's probably what you, I and most other people do, it's a potentially dangerous practice, which would (should) not be considered acceptable in anything approaching a safety-critical situation.

If one feels that one cannot have total confidence in the reading obtained with the 'first' meter, then one really should always confirm the reading with at least one other meter.

The problem with 'our' approach, in any 'measuring' situation, is that it assumes that any reading which appears to be 'as expected' (in the absence of faults) (i.e. "not strange") is definitely correct - but that is not inevitable, leading to potentially serious mistakes. That potential problem is probably at it's worst when the 'expected' reading is 'off-scale' - for example, if an IR meter gives a reading of, say, ">500 MΩ", then most of us would probably accept it (as 'correct'), even though it could be the result of a broken lead or broken connection within the meter!

Kind Regards, John

 

[SUNRAY]

A colleague in the 1970's found his "disposable" multi-meter reacted to low levels of RF radiation from transmitters on his workbench, His Avo was not noticeably affected

One of my low cost meters is affected when a PMR 446 ( UHF ) walkie talkie is fired up a couple of feet away, Adds a few milli-volts to the display on the 2 Volt AC range, The ammount of additional reading is very dependent on how the leads happen to be laid out on the bench

Repeating the test with both a Fluke 115 and a Fluke 77 and the leads in roughly the same layout shows no noticeable affect on the display.

I have to say that all of my MM's have shown the intolerence to RF including the AVO8 the worst I recall was the fluke and the Wayne Kerr displaying wildly varying random digits whereas most of the others tend to settle to a value. That said I think i'd prefer the 'unreadable' value as opposed to an incorrect stable value which looks sensible (ie showing 14V but is actually 12V).

I don't reaaly see how a meter with a metre or four of vulnerable single wire aerial attatched is expected to differentiate between the required part of the received signal and the unwanted part of the received signal. The meters that really surprised me have been MI ammeters showing wrapped round the endstop the moment a tiny current passed, such as the 8A MI meter on a 5A 13.8V psu running a CB rig which moved from a few mA on receive to off the scale on transmit.

The meters which don't seem to be affected are the low impedance moving coil types such as my cheap Jap 5KΩ/V MM from 1960's, Avo14 (I think 500Ω/V) and SA9083 (1KΩ/V).

 

[JohnW2]

I have to say that all of my MM's have shown the intolerence to RF including the AVO8 ...

I would say that my experience has been similar.

I don't reaaly see how a meter with a metre or four of vulnerable single wire aerial attatched is expected to differentiate between the required part of the received signal and the unwanted part of the received signal.

I suppose it depends upon what one is measuring. If it's DC voltage, DC current or resistance etc., then it shouldn't be difficult to minimise the effect to which it is affected by RF. Measurement of AC voltage is probably the most vulnerable but, even then, (and assuming one is only interested in extremely low frequency AC, like 50/60 Hz - or, at least, only up to a few hundred Hz) it ought not to be difficult to keep RF out.

However, if one wanted a meter that could measure AC voltage at RF frequencies then, as you say, there's no way that it could be expected to be able to distinguish between 'what you wanted to measure' and 'interference'!

Kind Regards, John

 

[bernardgreen]

It may be that tracks on the meter's PCB are one route for RF energy to get into the analogue to digital conversion circuitry.

 

[JohnW2]

It may be that tracks on the meter's PCB are one route for RF energy to get into the analogue to digital conversion circuitry.

Maybe, but that could presumably be addressed, to at least some extent, in much the same way (at it's simplest, just with a capacitor) as for 'RF energy' getting in via the test leads?

Kind Regards, John

 

[Harry Bloomfield]

Is that the Amprobe DM78C ?
or a Chinese no name?

Just a cheap Chineseium one, bought many years ago at a radio rally, as a novelty. I wasn't expecting much, but it is very handy.

 

[SUNRAY]

Is that the Amprobe DM78C ?
or a Chinese no name?

Looks interesting and i see it has the bar graph that some were asking for.

Note that I am happy to drop the fluke (a few feet) and then continue using it, but not happy to drop my £20 meter for fear that it will not be reading correctly (ie showing 0Volts when it is actually 240V).

Just a cheap Chineseium one, bought many years ago at a radio rally, as a novelty. I wasn't expecting much, but it is very handy.

Oh yes ... I'd completely forgotten I had one of those.
Sadly that was my DMM that wasn't worth having, I don't think the display ever stabilised, with or without volts applied. It kinda lived in a toolbox for a while then tried to take a reading with it one last time... rubbish box and mini DMM lived together in harmony, then the rubbish truck ate them both in one gulp.
Mine came from a radio rally too, Sandown Park about the turn of the millenium, perhaps mine had the Y2K bug