Type K Thermocouple drift

C

chriswhitehead

#1
I have calibrated 7 k type thermocouples that were used in an autoclave at 350°C for 12 months

Each thmcpl was around 6°C high. I check with brand new thmcpls and my kit was okay.

I did some research and found this:

“The reasons for the instability in Type K thermocouples are due to some inherent properties in the chromel/alumel material.
One problem that occurs with this thermocouple is an effect called short-range ordering. It occurs in a temperature
range of about 500°F to 1020°F (260°C to 549°C) when nickel and chromium atoms in the chromel leg tend to form an ordered crystalline
structure. The ordering produces a different metallurgical structure and if a temperature gradient exists, an erroneous
EMF is produced”



It doesn’t say over what period of time this may occur

So my question is: is it normal for the k types to drift that much over such a short period of time if used at 350°C


Great site.


:nope::confused::bonk:
 
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Jerry Eldred

Forum Moderator
Super Moderator
#2
Re: Type K Thermocouple drift?

The general presumption (erroneous) is that thermocouples don't change. Probing the back of my memory, I recall there is an oddity in K thermocouples wherein there is some add shift after prolonged use at certain temperatures. 260 C seems to ring a bell. I also seem to recall that they can be refreshed through annealing at a higher temperature. But I really can't remember the particulars.

My lab among other things calibrates high temp B and R thermocouples and they definitely do drift over time due to outgassing of Rhodium where platinum stays much more stable.

My intuitive answer about whether it is normal for K thermocouples to drift that much over 12 months is I think yes. Based upon that temperature range, the oddity in K (that I think you referenced) and that you apparently use them in a temp range that produces that oddity. And if I understand my inference in your question correctly, perhaps you are asking that if this is an anomaly, whether you should consider it as such and not be concerned with the interval.

Sounds like the appropriate thing may be to establish a calibration interval commensurate with the amount of change and your process tolerances. You may just need to regularly change out the K thermocouples. Or if your hardware allows, perhaps you could switch to another thermocouple type (I'm guessing that may not be feasible).
 

BradM

Leader
Admin
#3
Re: Type K Thermocouple drift?

Excellent response as usual, Jerry.

Sounds like the appropriate thing may be to establish a calibration interval commensurate with the amount of change and your process tolerances. You may just need to regularly change out the K thermocouples. Or if your hardware allows, perhaps you could switch to another thermocouple type (I'm guessing that may not be feasible).
This is what I was thinking. I have always considered process thermocouples expendable (as opposed to standard thermocouples like R and S; $$$). They do wear out and develop error over time.

I don't know how many T/C's we're talking about. Maybe you can justify replacing all (or a portion of them) during periodic maintenance. If you are verifying them, if you have the data and can demonstrate when they begin to drift, that should be adequate justification for replacement.
 
C

chriswhitehead

#4
Re: Type K Thermocouple drift?

:magic: Hi, thanks for the advice. Interestingly enough this has come back to haunt me with a new project manager on the job. He has asked how we can dictate when any of the large number k types has drifted enough to produce erroneous results. My answer is to shorten the calibration period but the tests that are run in the associated autoclaves run for two years. I also said we need to find a pattern in the failure rates ie age of thermocouples. Another suggestion I made wast to use one r type amongst the k's in each autoclave. This could high light the amount of drift in the k's themselves. However I know r types are very expensive. I'm at a loss to suggest anything else now :deadhorse:
 
P

PaulJSmith

#5
Re: Type K Thermocouple drift?

Assuming you've been collecting data in the three years since the original post, that in itself could make a compelling argument.

As for your "very expensive" argument against the R-type; are they more expensive than your potential loss in time, material, and on-time delivery should you end up with a run of non-conforming product? Product impact? Customer confidence?
 

dwperron

Trusted Information Resource
#6
Jerry has a good memory (and good counsel).

I went looking in AMS 2750E (the SAE bible on pyrometry) and found that they severely restrict the acceptable use of type K thermocouples in controlling, monitoring, and recording instruments:
If you are using expendable thermocouples (plastic or fabric covered) they are not permitted to be re-calibrated - they are to be replaced.
If you have non-expendable type K thermocouples (metal sheathed or ceramic insulators) they can be recalibrated at 3 month intervals IF USED AT OR BELOW 500?F (260?C) - otherwise not permitted

It looks like regular replacement may be your best option.
 
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