RTD temperature elements - heavier extension wire gauge effects? 1

[timsch]

Does anyone have experience with RTD elements that use heavier gauge extension wire than that which is typically used?

I have an application where customer has specified 18 gauge wire where our standard is 22-26 gauge wire. We are having more difficulty with calibration than we typically do. The lengths of the wire connected to the RTD element varies from 3 - 27 meters.

Our calibration is performed by inserting the element about 4" into a calibration block, with the remainder of the wire outside the block.

 

[Gr8blu]

Resistance measured is going to be a function of wire cross-section, (total) length of run, temperature - and the design of the sensor itself (different materials have different "slopes" when it comes to temperature).

Know all three and adjust your "as measured" data accordingly.
Note that this may mean a different coefficient for EACH sensor run, as lengths may vary from one to another.

It's not uncommon to use a larger gauge wire (i.e. larger cross-section) for sensor extensions to limit the effect of the run length.

The bigger question - how accurate does your measure have to be? Did you pick the correct sensor in the first place to even have a chance at being within the allowed variance?

 

[timsch]

We use Class A resistors by default in our applications. With our typical 22-26ga extension wire, we rarely have problems meeting class A tolerances and never fall outside class B, with this wire we are having difficulty with class A half the time and are even falling outside of class B occasionally. The further away from ambient temperature that the calibration temperature is, the greater the deviance of the reading to the actual temperature is.

 

[3DDave]

Is this a 2 wire system?

 

[timsch]

3-wire with a PT100 wire-wound ceramic element

 

[waross]

Strange. I would expect greater accuracy.
#18 AWG has less than 1/2 the resistance per unit length than #22 AWG.
I would expect that a run of #18 AWG would be equivalent to a run of #22 AWG of a little less than i/2 the length.

timsch
our typical 22-26ga extension wire​

The PU resistance ratio between #22 AWG and #26 AWG is the same as between #18 AWG and #22 AWG.

Thermal conduction via the extension wires?
Is your physical arrangement such that the extension wires may be conducting heat away from the sensor?
Try inserting about a meter of #26 AWG between the RTD and the #18 extension wires. If that cures your problem, then we may have identified the root cause as thermal conduction.
Once the root cause is identified we may work on a solution.

 

[EdStainless]

I think that Waross hit it, thermal conductivity of the heavier wire is playing a role.
This also means that your calibration will be more immersion dependent than you are used to.

 

[timsch]

I think that Waross hit it, thermal conductivity of the heavier wire is playing a role.
This also means that your calibration will be more immersion dependent than you are used to.

I agree. That's been my hypothesis. We had issues calibrating at -196C, being off by 5-8C when 1' of wire was immersed. When we immersed the majority of the wire, we were much closer. The further from ambient we've been, the greater the error, and the error has been to the ambient side of the measured temperature for the most part. I'll be going over all the data in the next few days to verify that.

I posted here just in case there might be something else I could be missing. Thanks for the replies.

 

[danw2]

What direction is the error? Is the error a low error, below the expected temperature value?

I have a buddy who runs an ISO 17025 cal lab. He once discovered that the MgO mineral insulation (MI) used to insulate the RTD element from its protection sheath had entrained moisture in it, which lowered the resistance between the element and its sheath. The lowered resistance allowed the excitation current to have a leakage path or paths in parallel with the RTD element, with the result of an overall lower resistance createsing a lower-than-expected temperature reading.

He got a low resistance reading from either lead to the sheath, in the low tens of KOHms, far lower than an expected 100 KOhms.

The 'transition' at the open end of the RTD sheath was not sealed well enough to keep moisture or water out.

 

[timsch]

For high calibration temperatures, the reading is low, for low calibration temperatures, the reading is high. The further from ambient temperature, the greater the deviation, which supports the thermal conduction along the wire hypothesis.

Moisture in the MgO is always on our minds, particularly since we are in a very humid climate. Our assemblies using MgO are put into bakeout ovens before sealing.

 

[waross]

The 'transition' at the open end of the RTD sheath was not sealed well enough to keep moisture or water out.

If that happens with a power cable, the termination may short out and with enough fault current the termination may be blown off of the cable.
"not sealed well enough to keep moisture or water out" That is unacceptable for MI terminations.
Find another source for your RTDs.