[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.



----- 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
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