[volt-nuts] Semi-precision high resistance measurement
Mitch Van Ochten
mitch at vincentelectronics.com
Mon Jun 17 21:31:17 EDT 2013
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" <stevebyan at verizon.net>
To: <volt-nuts at febo.com>
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 <stevebyan at me.com>
> Littleton, MA 01460
>
>
>
> _______________________________________________
> volt-nuts mailing list -- volt-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
> and follow the instructions there.
More information about the volt-nuts
mailing list