Why are type K and type E thermocouples not pemitted to be recalibrated ?

G

ggdjr

#1
ams 2750D states that type K and type E t/c cannot be recalibrated after use over 500F

anyone know why that is?

or why type J and N need to be calibrated twice as often as the noble types (s,r,b)

thanks,

GD
 
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Jerry Eldred

Forum Moderator
Super Moderator
#2
Re: Why are type K and type E not pemitted to be recalibrated ?

I am not at all familiar with the document you mentioned AMS 2750D). Could you tell me it's full title and who issued it?

Regarding Type K and E, Type K is rated to 2282 Deg F, and Type E is rated to 1652 Deg F. so I can't imagine why either of them could not be recalibrated after use over 500 Deg F. The only thing I could GUESS would be that because there likely is some drift in their emf (mV/Deg F or c), that some local policy may prescribe that to ensure they maintain some high level of accuracy (certainly more strict than their normal specs). Or... if this document prescribes use in some sort of caustic or oxidizing environment, perhaps there is evidence they degrade above that temperature in those more challenging environs.

Regarding why Type J or N have to be re-calibrated twice as often as B/R/S, the only guess I would have is the same as above. As J and N are lower max temperatures (similar to K), I don't understand why the interval would be shorter on them than on B/R/S T/C's. Matter of fact, many places don't even calibrate J/K/T/E (due to the lower temperature ranges, and drift being precipitated from oxidation and/or outgassing that occurs at higher temps). In the lab I used to work in, we had B and R on typically 6 month intervals; and replaced wire with new wire each six months (didn't use S - although its similar to R).

My thought would be J/K/T/E/N would likely be on at least 12 months (unless they are exposed to high temperatures). If exposed to high temperatures, I would gather and analyze some data as to drift rate versus calibrated spec, and assign an interval based on 2 or 3 sigma confidence of remaining in spec. Or even spec the interval based on hours and temps exposed to.

I would definitely like to see some data as to change in exposure to >500 F. I personally don't believe that is an extreme temp for J/K/T/E/N. However, if exposed to >500F for months, I could see that it might deteriorate.
 
G

ggdjr

#3
Re: Why are type K and type E not pemitted to be recalibrated ?

I am not at all familiar with the document you mentioned AMS 2750D). Could you tell me it's full title and who issued it?

Regarding Type K and E, Type K is rated to 2282 Deg F, and Type E is rated to 1652 Deg F. so I can't imagine why either of them could not be recalibrated after use over 500 Deg F. The only thing I could GUESS would be that because there likely is some drift in their emf (mV/Deg F or c), that some local policy may prescribe that to ensure they maintain some high level of accuracy (certainly more strict than their normal specs). Or... if this document prescribes use in some sort of caustic or oxidizing environment, perhaps there is evidence they degrade above that temperature in those more challenging environs.

Regarding why Type J or N have to be re-calibrated twice as often as B/R/S, the only guess I would have is the same as above. As J and N are lower max temperatures (similar to K), I don't understand why the interval would be shorter on them than on B/R/S T/C's. Matter of fact, many places don't even calibrate J/K/T/E (due to the lower temperature ranges, and drift being precipitated from oxidation and/or outgassing that occurs at higher temps). In the lab I used to work in, we had B and R on typically 6 month intervals; and replaced wire with new wire each six months (didn't use S - although its similar to R).

My thought would be J/K/T/E/N would likely be on at least 12 months (unless they are exposed to high temperatures). If exposed to high temperatures, I would gather and analyze some data as to drift rate versus calibrated spec, and assign an interval based on 2 or 3 sigma confidence of remaining in spec. Or even spec the interval based on hours and temps exposed to.

I would definitely like to see some data as to change in exposure to >500 F. I personally don't believe that is an extreme temp for J/K/T/E/N. However, if exposed to >500F for months, I could see that it might deteriorate.
jerry,

i was hoping you would help out.
the standard is Aerospace Material Specification 2750 Rev D. It involves Heat Treating of Steel, mostly, and the sensors/instrumentation/recording and monitoring systems that are involved.

It gives specs on life of sensors based on temp used, length of use at certain temps, and the cal intervals (if applicable)

thanks,

GD
 
G

ggdjr

#4
Re: Why are type K and type E not pemitted to be recalibrated ?

an excerpt (in its infancy)for a seminar i'll be giving on these and related topics

i was hoping that some folkes here might know the 'why' behind some of these specs (besides self serving modes to drive more business)

- Calibration frequency:
- Recalibration after use above 500F for type K and type E are prohibited
- TUS and SAT T/Cs
- B, R, and S – before first use and after 6 months
- J and N – before first use and after 3 months
- others – not permitted
- Control, Recording and Monitoring T/Cs
- Before each use
- Load T/Cs
- B, R, and S – before each use and after 6 months
- Others – not permitted
 
Last edited by a moderator:

Jerry Eldred

Forum Moderator
Super Moderator
#5
Re: Why are type K and type E not pemitted to be recalibrated ?

Based on the statements below, and considering the nature of the application, my GUESS would be that this is somehow based on a high reliability requirement. It is definitely true that all thermocouples age when exposed to high temperatures. The relative impact of that aging is a factor of the

a. thermocouple type (i.e.: what metal alloys are used in the thermocouple)
b. maximum temperature exposure
c. exposure time.

After reading your quote below, it does make a little more sense. The noble T/C's (B/R/S) all have the longest interval (which matches that seen in the semiconductor industry; although I can't actually cite anything for proprietary reasons).

The base metal thermocouples have shorter max intervals. However, in normal, knowledgable use, base metal T/C's are used at MUCH lower temps and are more stable (at those lower temps) for longer. It almost seems they expect the T/C's to all be used at the same max temps.

If it were me, I would definitely HAVE to presume these numbers are based on a combination of application and collected reliability data. If I were nit-picking, I might even disagree with the policy about B (as I am somewhat an "Anti-Type-B" person: in my recent thread I detailed how my experimental work on B T/C's showed they have an unpredictable decay in emf after long term high temp exposure (become non-linear across their operating range as well as decayed).

It appears this is a known set of factors for this application, and (based on prior history - in my opinion) produces a known level of reliability for the process. Most probably a high reliability standard (as the limits are so stringent).

- Calibration frequency:
- Recalibration after use above 500F for type K and type E are prohibited
- TUS and SAT T/Cs
- B, R, and S – before first use and after 6 months
- J and N – before first use and after 3 months
- others – not permitted
- Control, Recording and Monitoring T/Cs
- Before each use
- Load T/Cs
- B, R, and S – before each use and after 6 months
- Others – not permitted
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#6
Re: Why are type K and type E not pemitted to be recalibrated ?

Jerry is correct, in that the thermocouples are well within the requirements.....

Based on subsequent replies, this may be a regulator requirement.....if so, smile, and do it.....
 
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