Which of the following is NOT a temperature-sensing element that outputs an electrical signal?

Which of the following is NOT a temperature-sensing element that outputs an electrical signal?

• A. RTD
• B. Thermistor
• C. Thermocouple
• D. Mercury thermometer

Answer: (D)

In building automation, temperature sensors that feed controllers are expected to convert temperature into an electrical signal. An RTD changes its resistance with temperature, which the controller reads as an electrical value. A thermistor behaves similarly through resistance changes, often with higher sensitivity. A thermocouple produces a small voltage due to the Seebeck effect when its two metals are at different temperatures, and this voltage is read directly by the control system. A mercury thermometer, by contrast, works by the physical expansion of liquid mercury in a glass tube and is read from a scale. It does not generate an electrical signal on its own, so it isn’t an electrical-signal temperature-sensing element.

In building automation, temperature sensors that feed controllers are expected to convert temperature into an electrical signal. An RTD changes its resistance with temperature, which the controller reads as an electrical value. A thermistor behaves similarly through resistance changes, often with higher sensitivity. A thermocouple produces a small voltage due to the Seebeck effect when its two metals are at different temperatures, and this voltage is read directly by the control system.

A mercury thermometer, by contrast, works by the physical expansion of liquid mercury in a glass tube and is read from a scale. It does not generate an electrical signal on its own, so it isn’t an electrical-signal temperature-sensing element.