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