What two physical quantities are sensed by typical bellows in HVAC control?

What two physical quantities are sensed by typical bellows in HVAC control?

• A. Temperature and Humidity
• B. Pressure and Humidity
• C. Temperature and Flow
• D. Pressure and Temperature

Answer: (D)

Bellows in HVAC control are flexible chambers that convert a physical change into motion. They primarily respond to pressure changes, since the control signal is a pneumatic pressure that makes the bellows expand or contract to move linkages or valves. Temperature comes into play because the gas or fluid inside the bellows changes pressure with temperature, so the element can sense ambient temperature as well and drive a corresponding mechanical response. Humidity and flow aren’t typically sensed by a bellows—the former needs a humidity sensor and the latter is usually measured by differential pressure across an orifice or a flow sensor. So the two quantities detected by typical bellows are pressure and temperature.

Bellows in HVAC control are flexible chambers that convert a physical change into motion. They primarily respond to pressure changes, since the control signal is a pneumatic pressure that makes the bellows expand or contract to move linkages or valves. Temperature comes into play because the gas or fluid inside the bellows changes pressure with temperature, so the element can sense ambient temperature as well and drive a corresponding mechanical response. Humidity and flow aren’t typically sensed by a bellows—the former needs a humidity sensor and the latter is usually measured by differential pressure across an orifice or a flow sensor. So the two quantities detected by typical bellows are pressure and temperature.