Thermocouples are used to measure temperature in many industrial processes. A thermocouple is usually mechanically mounted inside a “thermowell”, a metal sheath which protects it from damage by vibration, bending stress, or other forces. One effect of the thermowell is that its thermal mass slows the effective time response of the thermocouple/thermowell combination compared with the inherent time response of the thermocouple alone. Let the actual temperature on the outer surface of the thermowell in Kelvins be and let the voltage developed by the thermocouple in response to temperature be The response of the thermocouple to a

Question

one-Kelvin step change in the thermowell outer-surface temperature from to is where K is the thermocouple temperature-to-voltage conversion constant.



(a) Let the conversion constant be K = µ V/K. Design an active filter that processes the thermocouple voltage and compensates for its time lag making the overall system have a response to a one-Kelvin step thermowell-surface temperature change that is itself a step of voltage of 1mV.



(b) Suppose that the thermocouple also is subject to electromagnetic interference (EMI) from nearby high-power electrical equipment. Let the EMI be modeled as a sinusoid with an amplitude of 20 ?V at the thermocouple terminals. Calculate the response of the thermocouple-filter combination to EMI frequencies of 1 Hz, 10 Hz and 60 Hz. How big is the apparent temperature fluctuation caused by the EMI in each case?

Answer 1