Page 133 - MEMS Mechanical Sensors

122 Pressure Sensors

10 10

5 5
Pressure
h
0 0

Figure 6.9 U-tube manometer.

6.3.2 Aneroid Barometers

Aneroid barometers essentially consist of an evacuated metal capsule with flexible
top and bottom faces. The shape of the capsule changes with variations in
atmospheric pressure and this deformation is usually mechanically amplified via a
series of levers or gears. The pressure capsule can be fabricated in the form of bel-
lows to provide further deflection. The measurement of deflection is done visually
by a pointer connected to the levers aligned to an appropriate scale. Alternatively,
they can be connected to a plotter for recording pressure against time (known as a
Barograph). These devices were first practically realized in the mid-nineteenth cen-
taury as an alternative to the manometer and, while not as accurate, offered the
advantages of ruggedness, compactness, and no liquid (aneroid meaning without
liquid). Dynamic response of these devices is poor due to the mechanical mass of the
sensor element, and they are not suited for dynamic pressure sensing applications.
They are still widely used today.

6.3.3 Bourdon Tube
Bourdon tubes operate on the same principle as the aneroid barometer, but instead
of an evacuated capsule or bellows arrangement, a C-shaped or helical tube is used
(see Figure 6.10). The tubes are closed at one end and connected to the pressure at
the other end, which is fixed in position. The tube has an elliptical cross-section, and
when pressure is applied, its cross-section becomes more circular, which causes the
tube to straighten out until the force of the fluid pressure is balanced by the
elastic resistance of the tube material. Different pressure ranges are therefore

Motion

Pressure
Pressure
Figure 6.10 Bourdon tube sensor elements.