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Mechanical Pressure Elements - Bellows Sensor |
Bellows Definition
1 a device with an air bag that emits a stream of air
when squeezed together with two handles, used for blowing air into a
fire. Ø a similar device used in a harmonium or small organ.
2 an object or device with concertinaed sides to allow it to expand and
contract, such as a tube joining a lens to a camera body.
 The bellows
pressure sensor is made of a sealed chamber that has multiple ridges
like the pleats of an accordion that are compressed slightly when the
sensor is manufactured. When pressure is applied to the chamber, the
chamber will try to expand and open the pleats. The figure on the left
shows an example of a bellows sensor, which uses a spring to oppose the
movement of the bellows and provides a means to adjust the amount of
travel the chamber will have when pressure is applied. In low-pressure
bellows sensors, the spring is not required. The travel of the bellows
can be converted to linear motion so that a switch can be activated, or
it can be connected Io a potentiometer. This type of sensor is used in
low-pressure applications usually less than 30 psi. The bellows sensor
is also used to make a differential pressure sensor. In this application
two bellows are mounted in a housing so that the movement of each
bellows opposes the other. This will cause the overall travel of the
pair to be equal to the difference of pressure that is applied to them.
All of the foregoing pressure sensors provide a change of position or
motion as pressure changes. This type of sensor limits the type of
electrical interface to either an on/off switch or to a potentiometer.
When a more accurate indication is required, other types of pressure
sensors that produce a change of voltage or resistance will be used.
Absolute ranges : 0 – 100 mmHg
Gauge ranges : 0 – 5 inH20
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Axially flexible, cylindrical enclosure with folded sides Increased pressure causes the closed end to extend axially |
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Movement rotates a pointer or actuates a controller or transmitter by
mechanical linkage |
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Movement of the bellows is opposed by
- Spring action of the bellows material
- Pressure surrounding the bellows
- External force by spring
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Accurately measures LOW pressure compared to Bourdon tubes as
well as Vacuum and differential pressure |
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