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

Thread Starter

ttttrigg3r

Joined Oct 14, 2016
20
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?
 

Reloadron

Joined Jan 15, 2015
5,934
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
 

WBahn

Joined Mar 31, 2012
26,398
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
But even if that solves the variance problem, it leaves the question open as to which one (if either) is "correct".
 

WBahn

Joined Mar 31, 2012
26,398
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?
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?
 

Reloadron

Joined Jan 15, 2015
5,934
But even if that solves the variance problem, it leaves the question open as to which one (if either) is "correct".
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
 
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,
 

ebp

Joined Feb 8, 2018
2,332
Are these devices being measured out-of-circuit?

What is the specification for test current for the meter?
 

Thread Starter

ttttrigg3r

Joined Oct 14, 2016
20
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.
 

Thread Starter

ttttrigg3r

Joined Oct 14, 2016
20
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.
 

kubeek

Joined Sep 20, 2005
5,753
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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