New DMM... Malfunction or Normal?

[HiGhGuY]

Hi, I just got a new DMM, the Amprobe AM-240.  When I turn it on to "V" with the test leads in, the voltage is constantly fluctuating.  When the dial is turned to VAC its fluctuates roughly between 20-50mV and when the dial is on VDC it fluctuates roughly between 100-500mV

Is this normal or did I receive a defective product?  other DMMs I have used have never done this, they have just stayed at 0 until connected to a voltage source.  Then again, the ones I have used before didn't measure down to mV... only V

I would appreciate any help, thanks.

[AndyC_772]

What happens if you short the probes together, does it read close to zero then?

[HiGhGuY]

yes, when touching the probes, in VAC i get down to .2mV and in VDC 0

[Simon]

pretty much normal, the input impedance is very high (10+Mohms) and the leads act as aerials picking up mains power that emanates a magnetic field all around you. Same happens on oscilloscopes, you will see an almost perfect 50Hz sine wave if you just wave the probes in the air. i remember being on the floor playing with my rigol and my dad walked past, the whole wave shifted up and down as he passed as his movement upset the magnetic field and effectively modulated it at a very low frequency

[G7PSK]

Normal mine reads 2.4 mv ac and 6mv valts dc without any probes even its just picking up stray voltage zeros as soon as the leads are shorted, often wondered if those stray voltages are added to whatever voltage is being measured.

[Simon]

no, that voltage is picked up due to the sensitivity of the meter, as soon as you connect it to something that will have a low impedence by comparison so it will just go away

[alm]

True RMS meters are expected to have a slight offset with the inputs shorted. A true RMS converter has poor linearity near 0V, so the manufacturers choose to spec the meter only from 5% of full scale and up, rather than adjusting for zero offset and compromising accuracy at higher voltages. See this Keithley document for more information.