Hi Steve,
I would suggest two things. First, place the 8050A on it's most sensitive
DC range and see if it indicates any voltage across the resistor. Assuming
there is nothing present, then I would suspect AC pickup on the meter leads
and a path to ground that is a different impedance from one side of the
resistor than the other side. In effect, it may be an effect created from a
common mode AC voltage and the meter's positive terminal responding
differently than the negative terminal.
Regards,
mitch
----- Original Message -----
From: "Steve Byan" <[email protected]>
To: <[email protected]>
Sent: Monday, June 17, 2013 1:50 PM
Subject: [volt-nuts] Semi-precision high resistance measurement
I'm restoring my old HP 410B VTVM, and I'm interested in seeing how much
the resistors have drifted since it was built, particularly the precision
resistors in the input voltage divider. I don't have volts-nuts caliber
equipment (well, there is a busted HP 3456A on the shelf waiting to be
repaired someday), just a Fluke 8050A and a Fluke 27/FM.
I didn't expect to have much trouble making consistent measurements as I
don't think the 8050A has the resolution to see temperature coefficient
changes or thermocouple effects. But I'm seeing some odd results on the
higher resistance values. First, I seem to see some contact resistance
effects: I don't get consistent measurements just using the probes as the
count varies a little with contact pressure and probe placement. The
contact resistance would have to vary by thousands of ohms for it to
affect the meter; I can't believe that could be the explanation. However,
I'm able to get consistent measurements by slipping alligator clips on to
the probe tips and clipping on to the range switch terminals. Maybe the
old solder is so oxidized that the contact resistance can really vary that
much?
Second, the Fluke 27/FM measurements track those of the 8050A better than
the spec'd limits, but I see some odd behavior in the last digit of the
8050A. The last digit of the resistance value varies with the direction of
the current through the resistor, and in one direction, it bobbles up and
down about three counts. In the other direction, the reading is stable.
The bobble doesn't seem to be sensitive to placements of the test leads.
For example, the 6.837M Ω 1% resistor (R6) measures 7.037M Ω one way, and
between about 6.994M and 6.996M Ω when I reverse the 8050A test leads.
That's a difference of nearly 0.6%
The 2.163M Ω 1% resistor (R5) measures 2.220M Ω one way, and between about
2.215M and 2.217M Ω when I reverse the leads, for a difference of about
0.2%.
The 683.7K Ω 1% resistor (R4) measures 697.9K Ω one way, and between about
697.5K and 697.7K Ω when I reserve the leads, for a difference of about
0.05%.
I did try switching off some potential nearby RFI sources - fluorescent
lights, switching power supply, laptop computer - and saw no difference in
behavior, although I didn't do an exhaustive search for RFI.
Finally, I did some quicky measurements with the Fluke 27/FM about two
months ago, and the current measurements seem to be a bit off (I don't
have the old recorded measurements handy as I write this, but I think they
are outside the accuracy limits spec'd for the 27/FM). This is a
non-climate-controlled New England basement, so the temperature is
probably up about 5 degrees C and the humidity has shot up recently. But
again, I wouldn't think my instruments are good enough to notice these
environmental effects on the components themselves.
Any ideas as to what's going on? How can I improve my measurement
procedure to get repeatable results? Do I really need better climate
control even at the 3 1/2 or 4 1/2 digit level of precision? What's with
the polarity sensitivity of the 8050A resistance measurements? Suggestions
and advice would be gratefully accepted.
Best regards,
-Steve
--
Steve Byan <[email protected]>
Littleton, MA 01460
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