multimeter doesn't zero

[stevestk]

I bought a meter at a yard sale today. The ac/dc voltage varies and does not zero when when the probes aren't touching anything (I've seen it 10 - over 100 mV) When the probes are crossed, the dc reads zero and the ac reads .7 mV. Is this abnormal? Is it broken or not properly calibrated?

[alm]

Probably the leads acting as antenna's. It doesn't take a lot of current through 10Mohm to generate a few mV, as long as it doesn't occur when actually connected to something, it should be fine. All meters with an 'infinite' (>10Gohm) input impedance have the same effect. As long as the output impedance of your test point isn't too high, this shouldn't matter. 100mV in DC mode is a bit high for 10Mohm I believe, my Fluke 189 is <1mV unless I'm moving the leads, in AC it's like 500mV with the leads close to power cords. The high reading in DC mode might indicate poor CMRR. Not a calibration issue in my opinion, possibly a design issue.

If it's a true-RMS meter, it's perfectly normal that it doesn't read zero in AC mode with the leads shorted. The response of the True-RMS converter is not exactly linear close to 0V, so they adjust it so it's accurate in the linear part. The accuracy specs are usually not valid below 20% of full-scale or so. You should not subtract this from other readings. There's a note on the Keithley website somewhere that explains this. Not sure about non-true-RMS (false-RMS? ) meters, I don't own any decent meter without true-RMS. I can't see any reason why it wouldn't read close to zero.

[EEVblog]

Some meters have a very high impedance (Gohms) mV mode, so it's normal for it to read all over the place with no source connected.
Have you tried it with some known source voltages?

Dave.

[Simon]

should be ok even my sparkling new amprobe AM220 does it but has 10 Mohm input and I think 100 Mohm for mV

[Waifian]

Yup, this is normal. Just goes to show you what is in he air.
like others said if it is still crazy on a circuit measurement that you know is solid then I would be concerned.  Enjoy your meter!

[Simon]

try holding a scope probe in the air in a room and don't be surprised to see some volts of 50 Hz sinusoidal waveform: you mains power

[Waifian]

Oh yes! I can get 60 hz on my frequency display all the time here in the USA shen not connected to anything.

[stevestk]

Thanks for the responses and  info.