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Measurement Mystery #12 — Solved!

THE PROBLEM WAS that the thermocouple’s millivolt output was directly translated into temperature from the thermocouple tables. But the inlet thermocouple was massive. It had a tremendous time constant. So during a transient, its own temperature was considerably less than the gas temperature. 

After the test engineers determined the thermocouple time constant, and then corrected for its effect, they confirmed that the gas temperature had exceeded the 2500 F limit by a considerable margin.

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The Unified Approach to Measurement Systems Engineering
reliably enables not only the methodical diagnosis of such problems,
but also their prevention.

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