handbook of modern sensors

348 10 Pressure Sensors
(A)
(B)
Fig. 10.7. Absolute (A) and differential (B) pressure sensor packagings. (Copyright Motorola,
Inc. Used with permission.)
10.6B. For the same diaphragm dimensions and the same overall thickness of the
chip, the SFB device is about 50% smaller.
Pressure sensors are usually available in three basic configurations that permit
measurement of absolute, differential, and gauge pressures. Absolute pressure, such
a barometric pressure, is measured with respect to a reference vacuum chamber. The
chamber may be either external or it can be built directly into the sensor (Fig. 10.7A).
A differential pressure, such as the pressure drop in a pressure-differential flowmeter,
is measured by applying pressure to opposite sides of the diaphragm simultaneously.
Gauge pressure is measured with respect to some kind of reference pressure. An
example is a blood pressure measurement which is done with respect to atmospheric
pressure. Thus, gauge pressure is a special case of a differential pressure. Diaphragm
and strain gauge designs are the same for all three configurations; the packaging makes
them different. For example, to make a differential or gauge sensor, a silicon die is
positioned inside the chamber (Fig. 10.7B), which has two openings at both sides
of the die. To protect them from a harsh environment, the interior of the housing is
filled with a silicone gel which isolates the die surface and wire bonds while allowing
the pressure signal to be coupled to the silicon diaphragm. A differential sensor may
be incorporated into various porting holders (Fig. 10.8). Certain applications, such
as a hot water hammer, corrosive fluids, and load cells, require physical isolation
and hydraulic coupling to the chip-carrier package. It can be done with additional
Fig. 10.8. Examples of differential pressure packagings. (Copyright Motorola, Inc. Used with
permission.)