# Multimeter accuracy? <12;12.36;12.42;12.44;12.53v ???



## FoweyBoy (Dec 7, 2006)

I have 5 multimeters (4 digital ones). I know, why 5? 1 for motorhome, 1 for yacht, 1 for home, 1 I thought defunct; 1 old analogue.

I got the following readings on a 12v battery

Analogue <12v - difficult to get more precise reading - hence now buy digital ones.
£5 Rolson 12.36v
£20 Tandy (many years old and does not always work) 12.42v
£15 Duratool replacing Tandy 12.44v
£~10 Draper 12.53v

This is a bit like the old 11+ question - one clock face says 1032, one 1043, one 1037 - what is the correct time? Answer - don't know the clock has stopped!

Without buying a professional multimeter ( assuming they are more accurate) how do I find the correct voltage?

I am using the meters mainly to monitor leisure batteries and decide when to charge - particularly on our yacht with a small compressor fridge and using main engine to charge battery. I guess the answer is not to worry about the actual voltage but monitor the trend. The fridge does flash a warning red light when the voltage reaches a low value.

Any ideas on what I could use to provide a "standard" voltage to calibrate the meters?


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## SilverF1 (Feb 27, 2009)

FoweyBoy said:


> I have 5 multimeters (4 digital ones). I know, why 5? 1 for motorhome, 1 for yacht, 1 for home, 1 I thought defunct; 1 old analogue.
> 
> I got the following readings on a 12v battery
> 
> ...


The quality of a multimeter is determined by it's impedance and in DC circuits this equates to straight resistance. When you measure a voltage some current from the measured circuit, in this case your battery, is used by the multimeter. The multimeter acts as a parallel resistance.

The less current used by the multimeter the better the accuracy of the reading, in general. Your multimeters clearly have different impedances as they give different voltage readings.

Discounting the old analogue and Rolson meters, the rest are in the same ballpark. I'd use these others and monitor the trend, without worrying about exact accuracy.

HTH


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## Chudders (Jul 5, 2008)

If you really want accuracy guaranteed you have to have meters calibrated and certified. Because of my job (electrical inspection and testing) I have to have all my testers calibrated annually and verified by me monthly. Each unit costs in the region of £40 to £60 dependant on type to get calibrated by specialists. Even then there is a tolerance which is permissable.
I can still have differing readings within the allowable tolerances on different testers. Even contact pressure can create differences as well as length of test lead (which should be nulled before test).
Dave


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## TR5 (Jun 6, 2007)

High accuracy comes at a price.

£15 - £20 multimeters are an acceptable guide only.


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## H1-GBV (Feb 28, 2006)

One little test is to link all of them in parallel.

Theoretically, they should all read the same and that should be the potential difference of the battery, allowing for the current being drawn from it. (V = E - I r, where E is the EMF [electromotive force = maximum voltage you can get from the cell], I = current, r = internal resistance [caused b y the chemicals in the cell: lead acid cells have a much lower resistance than "dry" cells eg C,D - "duracell" type])

Then disconnect each voltmeter in turn, record the value(s) of the others and re-connect. The "worst" meter will produce the biggest change, because it is drawing the biggest current.

However, we are not really talking accuracy here.

Alternatively, set one meter to "resistance", connect it to the other meters in turn and see which has the biggest resistance when it is on "voltmeter" setting.

As you appreciate, the trend might well be the most important information - Gordon


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## safariboy (May 1, 2005)

Digital meters on d.c used to measure a battery voltage or probably anything else in a motor home draw so little current that the load will not bring the voltage down. 

As a physics teacher I used to have to check the accuracy of voltmeters from time to time and we did it with a zenner diode set up to produce a specific voltage. You need a constant current circuit to get accuracy. there are special ones that have zero temp coef. We had £20 is meters and I cannot remember any noticeable deviations.


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## bubble63 (Sep 30, 2009)

its a while since i worked in industry 
but i used to calibrate instruments. 

you start off with decent kit, ie fluke or AVO meters 

I think we then used a stable battery as a reference and a wheatstone bridge and a galvometer. 

in practice the flukes were always spot on, but the AVO mark 8 was a hard act to beat, you can watch a needle dance and get a lot from it. 

you pay the price you get the quality me thinks. 

I still have , and use, an AVO Mk 8, in a lovely leather case  

regds neill 
under the weather in cambridge


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## listerdiesel (Aug 3, 2012)

We use Fluke DVM's which are periodically calibrated.

They don't vary much between them, and even a couple of cheapie DVM's come up well by comparison.

Analogue meters are still useful for trending, although the Flukes do have a bar graph at the bottom of the display area.

Even the old stuff can be quite accurate, when I repaired and cleaned some old Crompton Parkinson meters for our Ruston engine, they gave a very good account of themselves:










Peter


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## FoweyBoy (Dec 7, 2006)

Well - that was a "can of worms" I opened! I did not appreciate measuring voltage could be so complex.

There is obviously no simple answer for non-electricians Iike me. I will take the digits to the right of the decimal place with a pinch of salt and follow the trend, noting the critical value on a particular multimeter for future reference.

Thanks for all the replies.

David


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