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Sensor Types
Pressure Transducers compress with increasing external pressure and expand as the pressure drops. This movement can be used to apply mechanical control force.
Differential Pressure sensors reference one pressure to another rather than to the atmosphere.
Aneroid sensors sense pressure changes relative to absolute zero pressure by creating a vacuum on one side of the bellows. Aneroid sensors sense change in barometric pressure to bias or reference a system to absolute pressure, or to sense changes in altitude and introduce corresponding motion and|or force to a system, such as directly operating a valve or opening a parachute.
Sudden Pressure Relays, or Rapid Rise Relays, are used to detect sudden surges in pressure.
Bellows Sensor Functions
Gas Filled bellows can make the sensor sensitive to both external pressure and temperature in vastly different applications, including:
- Fuel Controls
- Air Density Controls
- Passive actuation in a vacuum or pressure chamber
Liquid Filled bellows can function in a number of transducer applications, including:
- Temperature-Motion transducer
- Temperature-Force transducer
- Pressure-Motion transducer
- Force-Pressure transducer
- Motion-Motion transducer
- Force-Force transducer
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Design Considerations
Pressure Transducers: For pressure transducers, usually two small bellows joined with a capillary tube comprise the liquid-filled system, one the sender, the other the receiver. In pressure-motion transduction, the sender converts pressure of the system that surrounds it to displacement of the equilant through the capillary. Flow of the equilant into the receiver bellows is transduced into motion. An example would be a submarine depth gauge where the sender bellows is in seawater, the capillary passes through the hull and the motion of the receiver bellows moves the linkage on a pressure gauge. In pressure-force transduction, there is almost no motion in either bellows. Pressure on the sender produces force from the receiver.
Temperature Transducers: The higher coefficient of expansion of liquids over metals will cause a liquid-filled metal bellows to expand with increasing temperature and contract conversely. This is akin to an alcohol-filled thermometer, but with a bellows we can drive force or motion. The liquid (the equilant) can be confined strictly to the bellows, or for greater motion, additional equilant can be in a near or remote bulb. In temperature-force transducers, force can be appreciable with larger bellows. For temperature-motion transduction, motion can be linear to less than 0.5% of full scale for low force applications. Flexial builds liquid-filled transducers as small as 0.250 in. (6mm) OD.
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