[volt-nuts] Prema 6048 zero offset problems

[David C. Partridge]

Starting with a warmed up Prema 6048, if I power it off and then on, select
20V DC range, 4 second integration and connect to the 10V output of a Fluke
732A, I get a reading of:

 +10.000047

If I reverse the polarity of the cables and allow the reading to settle down
for a minute, I get a reading of:

 -10.000006

If I short the inputs of the meter, I get a reading of:

 +0.000105

which is different from the  +0.000020 which I would have expected from the
readings of the 732A output.

Proceeding on the assumption that the meter has stored a zero offset of
-105uV

I then press the ZERO button to null out the offset and then reconnect the
732A in normal polarity I get an incorrect reading of:

 +10.000153

This is not what I would expect to see - I would expect to see a reading of
+10.000047 - 0.000105 = +09.999942  This calculation

I then reverse the inputs and get a reading of:

 -10.000113

which is close to the expected reading of -10.000006 - 0.000105 = -10.000111

I have mailed Prema about this problem, and will let you know what they have
to say.

Cheers
David Partridge