Reader Feedback: On SATs, Correction Factors, & Possible Findings

READER QUESTION: Just read your article in regards to system accuracy test. I just had a question that maybe you can help me understand. When applying the correction factor of the test instrument and test sensor, is the correction factor to be used based on the furnace set point (operating temperature) at the time of the test or the recording instrument reading during the test? Any insight is appreciated!

Jason Schulze (Conrad Kacsik) for HTT:

This is a question that comes up often in the pyrometry courses I teach.

The Nadcap Pyrometry Reference Guide, question #6, addresses this question, although from a TUS standpoint. The premise is the same for the SAT process though.

Correction factors applied to any test results (TUS & SAT) should be determined based on the setpoint temperature during testing.

One thing to remember is that you may be testing at a temperature which does not fall directly at a temperature indicated on the test wire/test instrument calibration certificate. In this case, you would have two options;
1) Linear Interpolation
2) Pick the adjacent calibration temperature closest to your setpoint during test.

Either way, you would need to establish if you execute #1 or #2 above and document that in an internal procedure.

READER QUESTION: Thanks for the feedback, I have another concern. I perform an SAT on a refrigeration unit that operates at -20 degrees. My test instrument and test sensor are both calibrated at a low temperature of -20, but sometimes the recording instrument indicates a temp of -21 degrees or so. My operating temperature is -20 (setpoint), but as I stated it might indicate a lower temperature. Is there a possible finding here? Although setpoint is -20, recorder shows -21, which the test instrument/sensor does not cover (calibration point).

Jason Schulze for HTT:

You should be in no danger of a finding. The pyrometry guide states the correction factors are based on setpoint.