As useful as thermowells are, they are not without their caveats. All thermowells, no matter how
well they may be installed, increase the first-order time lag of the temperature sensor by virtue of
their mass and specific heat value. It should be intuitively obvious that a few pounds of metal will
not heat up and cool down as fast as a few ounces’ worth of RTD or thermocouple, and therefore the
addition of a thermowell to the sensing element will decrease the responsiveness of any temperaturesensing
element. What is not so obvious is that such time lags, if severe enough, may compromise
the stability of feedback control. A control system receiving a “delayed” temperature measurement
will not see the live temperature of the process in real time due to this lag.