What does the “initial calibration tolerance” mean?

Materials for “letter designated thermocouples” (as designated by ASTM) and certain other common thermocouple types are produced to meet nominal emf vs. temperature curves that are accepted by industry. Since no pairs of thermocouple wires can be expected to fall precisely on these nominal curves, standards organizations have specified tolerance bands around these nominal curves.

New, homogeneous thermocouple materials are expected to produce thermoelectric outputs that fall within these “initial calibration tolerances.” They are referred to as “initial” tolerances because it must be recognized that, after first use, thermocouples may change in output (for a variety of reasons) and, therefore, may no longer meet these initial tolerances.

(This is the first in a series of discussions on common questions relating to thermocouple applications. Check for future posts, or subscribe to our blog for notifications of them. You are welcome to contact us with your specific application questions.)

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Figure 1 | Type C thermocouple calibration and tolerances

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