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A thermocouple installation reads 20 degrees low on a hot day. The likely cause is

  • A.the resistance of the extension leads having risen with the temperature
  • B.the hot junction having oxidised and lost part of its output
  • C.the amplifier gain drifting upward with the ambient temperature
  • D.a cold junction compensation that has not tracked the ambient temperature

Answer: D

The emf reports the difference between the junctions, so if the reference is warmer than the compensation assumes, the computed hot junction temperature comes out low by exactly that amount. Lead resistance hardly matters, since the measuring circuit draws almost no current.

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