Resistance changes when reversing leads for RTD device and potentiometer. Why?

Discussion in 'Test & Measurement Forum' started by ttttrigg3r, Jun 5, 2018.

  1. ttttrigg3r

    Thread Starter New Member

    Oct 14, 2016
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    So I have an resistance temperature detector (RTD) that when I measure the resistance, it's 100 Ohms, then when I switch the leads around and measure, it reads 95 Ohms. This is not a cheap part, it's being used by the Airforce, so why does the resistance differ when I just switch the leads around?
    This also happened on this precision potentiometer I bought. It's 200 Ohms and 3%. I set it to 100 ohms and measured. Then switched the leads around and measurements went off by 4 Ohms.
    These two things are basically resistors right? Why would just switching the leads around give a different reading?
     
  2. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    Are the contact materials, at the point of measurement, the same on both contacts.?
    E
     
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  3. KeepItSimpleStupid

    AAC Fanatic!

    Mar 4, 2014
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    Switch the leads at the meter and not the device.
     
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  4. Reloadron

    Distinguished Member

    Jan 15, 2015
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    My guess and thinking runs with that of Eric and KISS in that if the actual alloys are not the same at the point of contact (your meter leads) you can get a thermal or Seebeck effect and depending on your ohmmeter the difference you see would be easily explained. Should that be the case then doing as KISS suggested should give the same reading. Swap the leads at the meter rather than the source.

    Ron
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    But even if that solves the variance problem, it leaves the question open as to which one (if either) is "correct".
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    How repeatable is this? If you switch the RTD back do you get 100 Ω again? Or was this a single measurement that might be explained by the RTD temperature changing, perhaps as a result of handling it?
     
  7. Reloadron

    Distinguished Member

    Jan 15, 2015
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    Yes, very true, but I was hoping to see if that was the problem before worrying about the which one is true. Sort of like having two watches and wondering the correct time. :)

    Ron
     
  8. KeepItSimpleStupid

    AAC Fanatic!

    Mar 4, 2014
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    I'm "more suspecting bad leads". Why? Swapping them at the meter would "generally" cause less disturbance.

    1-4 ohms is on the order of lead resistance anyway. I wanted to wait for an answer before commenting further.

    Do, take the leads one at a time and short to the other banana jack of the meter and wiggle the leads at the ends particularly. Also look for loose banana plug connections. Look at the probe connectors for signs of oxydation,
     
  9. ebp

    Well-Known Member

    Feb 8, 2018
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    Are these devices being measured out-of-circuit?

    What is the specification for test current for the meter?
     
  10. ttttrigg3r

    Thread Starter New Member

    Oct 14, 2016
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    It wasn't repeatable today. Yesterday, it was very repeatable. I'm suspecting because I was using the RTD and potentiometer all day long in a circuit and it warmed up? Today, went in and measured them cold, the resistance was virtually the same.
    I'm going to do some more testing today with these components and I'll try the swapping methods prescribed.
    Thanks all.
     
  11. ttttrigg3r

    Thread Starter New Member

    Oct 14, 2016
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    After a whole day of testing, I wasn't able to repeat the original measurements. Thanks for the help everyone. I'm going to pass it off as loose banana plugs or something because the problem went away after I took them out and put them back into the multimeter.
     
  12. kubeek

    Expert

    Sep 20, 2005
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    The range switch on a multimeter can also be the culprit, lately mine sometimes seems to require a good few rotations of the switch to get normal values.
     
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