handbook of modern sensors

354 10 Pressure Sensors
tocurrent, and when the pressure changes, the photocurrent is cosine modulated with a
period defined by half the mean wavelength of the light source. The photodiode without
the FP filter serves as a reference diode, which monitors the total light intensity
arriving at the detector. Its output signal is used for the ratiometric processing of the
information. Because the output of the sensor is inherently nonlinear, a linearization
by a microprocessor is generally required. Similar optical pressure sensors can be
designed with fiber optics, which makes them especially useful for remote sensing
where radio-frequency interferences present a serious problem [14].
10.9 Vacuum Sensors
Measurement of very low pressures is important for the processing of the microelectronic
wafers, optical components, chemistry and other industrial applications. It also
vital for the scientific studies, for instance, in space exploration. In general, vacuum
means pressure below atmospheric, but usually the term is used with respect to a near
absence of gas pressure. True vacuum is never attained. Even the intrastellar space is
not entirely free of matter.
Vacuum can be measured as negative pressure compared to the atmospheric pressure
by conventional pressure sensors, yet this is not quite efficient. Conventional
pressure sensors do not resolve extremely low concentrations of gas due to the poor
signal-to-noise ratio. Whereas the pressure sensors in most cases employ some kind of
membrane and a displacement (deflection) transducer, special vacuum sensors operate
on different principles. They rely on some physical properties of gaseous molecules
that are related to the number of such molecules per volume of space. These properties
may be a thermal conductivity, viscosity, ionization, and others. Here, we briefly
describe some popular sensor designs.
10.9.1 Pirani Gauge
The Pirani vacuum gauge is a sensor that measures pressure through the thermal
conductivity of gas. It is one of the oldest vacuum sensors. The simplest version of
the gauge contains a heated plate. The measurement is done by detecting of amount of
heat lost from the plate that depends on gas pressure. Operation of the Pirani gauge is
based on the pioneering works by Marian Von Smoluchowski [15]. He established that
when an object is heated, thermal conductivity to the surrounding objects is governed
by
G = G 0 + G g = G s + G r + ak PP T
, (10.16)
P + P T
where G s is thermal conductivity via the solid supporting elements, G e is the radiative
heat transfer, a is the area of a heated plate, k is a coefficient related to gas properties
and P T is a transitional pressure which is the maximum pressure that can be measured.
Figure 10.14A illustrates different factors that contributes to a thermal loss from
a heated plate. If the solid conductive and radiative loss is accounted for, the gas