Why is my DMM showing slightly different value if +/- is switched?

[David_]

Hello.

Why does my DMM show a difference of +100mV when measuring a source such as a battery while flipping to probes around.

With the positive probe to battery + and negative probe to battery - it says 1.2, I would think that changing the probe positions in regards to battery poles would show -1,2V but it does not.
It shows -1,3V...

What's up with that?

Regards

 

[schmitt trigger]

Most if not all DMMs have an auto-zero option, to remove the inherent offset in any electronic circuit.

This of course, it is not perfect, there may be remaining errors of a few millivolts or tenth of millivolts..................but 100 mV?? That is really weird. It may be defective.

My advice: borrow a good DMM, something like a Fluke or other which has had a recent calibration. Check those readings against yours.

 

[andre_luis]

Depending on if you selected a higher voltage scale, it is expected a more significant effect of the measurement error due to intrinsic inaccuracy of the circuit.

 

[Audioguru]

My Fluke 79 III measures a Ni-MH battery cell at 1.376V forward and exactly the same when the probes are reversed.
I think you have a defective or cheapo meter.

 

[FvM]

With the positive probe to battery + and negative probe to battery - it says 1.2, I would think that changing the probe positions in regards to battery poles would show -1,2V but it does not.
It shows -1,3V...

Are you using a 3.5 digit DMM in 200 V range, or a 3 digit DMM (are they made at all ?) in 10 V range so that the resolution is only 100 mV? Considering that +/- 1 least significant digit is always uncertain, this sounds like normal operation.

 

[David_]

I use a AMPROBE 5XP-A so I guess it is not expected to enable any kind of seriuse levels of accuracy but still I thought that the difference was too much to be ignored but I will have to investigate what range I last observed this on. I was not aware that +/- 1 least significant digit(is there ever LSD as an acronym?, a web search revels a lot of information about LSD but not pertaining to digital electronics )

I really do need to get my self better instruments in general but not this year though, no $$$ to speak of.

Reagrds

- - - Updated - - -

I have begun a search for borrowing a good enough voltmeter for comparison with my analog discovery in first hand since it has a "multi step"calibration routine(you calibrate a number of points/voltages) with the thought that after that is done that will have to do for now, I haven't really checked but I have opened my DMM a few times to change battery and I have not observed any pot to adjust any value but maybe that is standard to have and it is in there somewhere.

 

[schmitt trigger]

The adjustment most likely will BE NOT a pot. Nowadays it is done with a calibration table stored in EEPROM.

 

[BradtheRad]

My cheap DMM recently started displaying a similar problem. With mine it is "-.1 V", etc.

It occurs when nothing is hooked up. It throws off all measurements I take.

The error shows up on all ranges, the amount being -1 or -.1 or -10.

I find only one adjustment inside: a pot which adjusts accuracy. It does not affect the '-.1' problem.

The ADC accepts an input range of +- 200mV (I think). I believe if I could locate the ADC input pin, I could apply just the right positive voltage through a resistor divider, and bring the readout back to zero when it should read zero.