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6.4 Diaphragm-Based Pressure Sensors 123

accommodated by using different materials such as phosphor bronze or stainless
steel. Changes in pressure move the closed end of the tube to which a linkage arm
and a gear and pinion assembly are attached. These rotate a pointer around a gradu-
ated scale, providing visual reading of the pressure. Bourdon tubes are usually used
by gauge pressure sensing applications, but differential sensing is possible by con-
necting two tubes to one pointer. By correctly arranging the linkages, the pointer
can be made to measure the pressure difference between the tubes. Helical tubes are
more compact, reliable, and offer performance advantages over the more tradi-
tional C-shaped devices. Bourdon tubes are used throughout the industry and are
available in a wide range of pressure specifications.

6.3.4 Vacuum Sensors
Vacuums are pressures that are below atmospheric. Vacuums are difficult to meas-
ure by mechanical means, and therefore, different sensing techniques have been
developed. These techniques are suited for different vacuum levels. Within the low
–3
vacuum region (atmosphere to ∼10 mbar) the Pirani gauge is normally used. This
is based upon a heated wire, the electrical resistance of which is proportional to its
temperature. At atmospheric pressure convection occurs and heat energy is lost
from the wire. As the vacuum increases, gas molecules are removed and less convec-
tion occurs, causing the wire to heat up. As it heats up, its electrical resistance
increases, and this can be calibrated against pressure to provide a reasonably accu-
rate measure of the vacuum.
Below the range of the Pirani gauge, Ion gauges (also known as Penning or Cold
Cathode gauges) are used. These consist of a filament, a grid, and a collector. The
filament produces thermonic emission of electrons, and a +ve charge on the grid
draws the electrons away from the filament. The electrons circulate around the grid,
which has a fine structure enabling the electrodes to pass through many times until
they eventually collide. Any gas molecules present around the grid may collide with
circulating electrons, which results in the gas molecule being ionized. The collector
inside the grid is –ve charged and attracts these +ve charged ions. The number of
ions collected is directly proportional to the number of molecules inside the vacuum
system, and therefore, the collected ion current gives a direct reading of the pressure.

6.4 Diaphragm-Based Pressure Sensors

Diaphragms are the simplest mechanical structure suitable for use as a pressure-
sensing element. They are used as a sensor element in both traditional and MEMS
technology pressure sensors. In the case of MEMS, due to the planar nature of many
established fabrication processes, the diaphragm is the main form of sensor element
developed. This section will first review basic diaphragm theory before analyzing in
more detail particular aspects relating to MEMS pressure sensors. This review of
traditional diaphragm theory is particularly relevant in the packaging of MEMS
technology pressure sensors. Stainless steel diaphragms are routinely incorporated
into the package to isolate the sensor from the media. The behavior of the stainless
steel diaphragm will affect the performance of the sensor and must be considered