Temperature Measurement

Pneumatic temperature sensor basics

Pneumatic temperature sensor systems are mostly used in HVAC systems for transmitting temperature signals to a control system.

Pneumatic Temperature Sensors setup

The bulb includes a temperature-dependent refrigerant that extends or contracts. This results in a shift in the stress in the capillary as the bulb and capillary are in a closed system.

The capillary is linked to a diaphragm that either activates a snap-action switch within a thermostat (for on / off control) or moves a slide-wire resistor center button (for control modulation).

Working of sensor:

In on / off command, at a specified temperature, the thermostat can be set to close or open the switch. Usually some differential (hysteresis) is constructed into the switch so that when the temperature of the bulb is close to the setpoint, it does not open and close rapidly.

The circuit works comparable to a Wheatstone bridge when used to modulate control with the engine equilibrium relay attempting to match the thermostat slider position.

Advantages of pneumatic temperature sensor:

  • Do not require an electric power source to operate
  • Reliable in commercial use
  • They are particularly useful for freeze protection control on a coil, since the bulb can be of any size.

Disdvantages of pneumatic temperature sensor:

  • they are difficult to fix if the bulb or capillary spring a leak, if this happen the thermostat should throw away.


Instrumentation Engineer

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