How accurate is your digital volt meter?

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William_H

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With so much data available on battery charge state from precise voltage measurements we might wonder just how accurate a DVM is.
I have just received one of these AD 548 voltage reference devices. Just needs 15v supply and it provides 10 volts precisely for use as a calibration. It is guaranteed to 30 parts per million worst case. As a matter of interest 2 old DVMs of mine showed 9.96 and 9.95 volts on test so not too far out. Might be of interest to someone. olewill
http://www.ebay.com/sch/i.html?_fro...kw=AD548+precision+voltage+reference&_sacat=0
 
Remembering stuff I learnt more than 50 years ago, isn't the accuracy of a voltmeter related to the internal resistance (as high as possible) and the resistance across which you're measuring the voltage? Modern electronic voltmeters have extremely high internal resistance, but an older moving coil one may not have such a high resistance and hence be less accurate. The internal resistance of a voltmeter used to be (and may still be!) one of the features that the makers highlighted.
 
Doesn't matter how accurate it's claimed to be, if it's not recently calibrated, it's not to be trusted for anything important. That's why measuring kit used commercially is always calibrated, an expensive, but necessary, faffage.

I would not trust uncalibrated kit more than 1% accuracy.
 
AngusMcDoon said:
Doesn't matter how accurate it's claimed to be, if it's not recently calibrated, it's not to be trusted for anything important. That's why measuring kit used commercially is always calibrated, an expensive, but necessary, faffage.

I would not trust uncalibrated kit more than 1% accuracy.
Click to expand...

But is there a problem with that when most of the time all you want (a bit like a barometer) is a comparative reading rather than an absolute one...
 
AntarcticPilot said:
Remembering stuff I learnt more than 50 years ago, isn't the accuracy of a voltmeter related to the internal resistance (as high as possible) and the resistance across which you're measuring the voltage? Modern electronic voltmeters have extremely high internal resistance, but an older moving coil one may not have such a high resistance and hence be less accurate. The internal resistance of a voltmeter used to be (and may still be!) one of the features that the makers highlighted.
Click to expand...

I bet you're thinking of Avo 8s
 
Judging by the two digital multimeters I had 1% accuracy could be optimistic, one of mine was out by 10% or more.

I bought a third. All only cheapies for around £10 or less.
 
the spec for my cheapies is ± 0.5% of the reading, ± 2 digits.

Not quite sure what ± 2 digits means though.
 
Come on.... 5% is 0.6 volts on a 12 volt system. That is the difference between a functional lead acid battery and a nearly dead one. I have two digital volt meters and two NASA BM1's (as everyone on this forum will have read though, NASA equipment doesn't work).

These four instruments generally all agree to within about 0.02 volts on my 12 volt system and have done so for many years. I very much doubt that they are all making an agreed 5% error.
 
For my purposes, around 0.05% would be acceptable, and repeatability equally important. 0.05% might give a reading of 12.8 when it is only 12.75, and even 1% wouldn't be disastrous. My Lewmar readouts give around 0.3v error in this range, and even this varies with whether the lighting display is on or off.
 
Amulet said:
Come on.... 5% is 0.6 volts on a 12 volt system. That is the difference between a functional lead acid battery and a nearly dead one. I have two digital volt meters and two NASA BM1's (as everyone on this forum will have read though, NASA equipment doesn't work).
Click to expand...

If this is ref my post, 5% is the variation in battery capacity for a given voltage due to variables, 10 deg C will change voltage by 0.05 volt or 5/6% of capacity.

Brian
 
VicS said:
the spec for my cheapies is ± 0.5% of the reading, ± 2 digits.

Not quite sure what ± 2 digits means though.
Click to expand...
Say your meter is a typical 'three and a half digit' DMM meaning it reads up to 19.99 volts on a 20v scale
When you measure 12.00 volts, the accuracy is 0.5% of 12V or 60mV plus or minus 2 least significant digits, which is +/- 20mV on this scale.
So indicated 12.00V is anything from 11.92 to 12.08 volts.

On some meters you can get further errors if there is any ripple present. Different meters average the ripple differently.

Regarding meter impedance, a good moving coil meter used to be '20k ohms per volt' so on a 20volt scale would have a resistance of 400k ohms. If the supply wiring of your boat has got enough resistance for that to matter, you have bigger problems! It can mattter in other work of course.

I have got several of those cheap yellow meters. They are all fine enough for typical yacht work, although I did chuck the one which got very wet making the main selector switch dodgy.
I have checked them against Fluke and Isotech meters, which have been checked against traceable calibrated meters.

Brain/Halycon raises good points about the voltage of '12v' batteries being temperature dependent. Becuase the voltage chenges quite slowly in charge, a small error in voltage and/or a temperature error can be a much bigger error in state of charge.

HTH.
 
I usually only use my meter to tell if their are approx 12v present or not. I accept it is not accurate to 1/10v. Also, if there's a dodgy connection, the volts drop when load applied - thats why a test lamp is better for checking circuits.
Another thing to be aware of is that when the internal battery is low, the readings become more inaccurate. As the battery lasts a long time, its as well to calibrate against a known voltage from time to time.
It is said that an uncalibrated instrument is useless. This applies to torque wrenches, miultimeters and sphegnometers.
 
I have diagnosed a great many problems with uncalibrated meters.
When your uncalibrated voltage rises when the sun comes out, your solar panel works.
When your voltage drops a lot while cranking, the battery is flat or poorly.
Etc Etc.
 
William_H said:
With so much data available on battery charge state from precise voltage measurements we might wonder just how accurate a DVM is.
I have just received one of these AD 548 voltage reference devices. Just needs 15v supply and it provides 10 volts precisely for use as a calibration. It is guaranteed to 30 parts per million worst case. As a matter of interest 2 old DVMs of mine showed 9.96 and 9.95 volts on test so not too far out. Might be of interest to someone. olewill
http://www.ebay.com/sch/i.html?_fro...kw=AD548+precision+voltage+reference&_sacat=0
Click to expand...
It's actually AD584.
An ebay search on this gives modules from about 3 or 4 USD.
That's a bargain IMHO.

Thanks for the idea, I'm going to get one or two.
 
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