A pressure sensor can detect pressure and then convert it to
electricity signal for display. So it acts as a transducer that generates an
electrical signal as a function of the pressure imposed [1]. According to
G.Carpenter’s blog, the electrical outputs of pressure sensor can be classified
into three types: (1) sensors with millivolt output. These sensors are common
and economic but need regulated power supplies and not suitable for noisy
environment because the outputs are nominally around 30mV, easily being interrupted.
(2) sensors with amplified voltage output.
By using the integral signal conditioning
the outputs are amplified, ranged
from 0-5Vdc to 0-10Vdc so they are more steady than type (1); (3) Sensors with
4-20mA output. The signal is the most steady so they are suitable for the long transition
distance(1000+ft)[2].
Pressure sensors can use different technologies to detect
pressure. The most common method is to measure strain due to applied force over
an area also named force collector. For example, as C.Meraz introduced, variable
capacitance and piezoelectric sensors are different force collector types. I
want to discuss other types of sensors using other properties to infer pressure.
Resonant wire pressure sensor uses the difference of resonant
frequency to measure pressure. The input pressure is detected by the high
pressure and low pressure diaphragms on the right and left of the unit[3]. Usually,
the resonant wire oscillates at its natural frequency. When the pressure
changes, the wire tension will change accordingly and the resonant frequency
also changes. A digital counter circuit is used to detect the shift and
transform the signal to pressure value. The advantage of this technology is it
can provide very stable readings over time[1].
Another type of sensor uses the changes in thermal
conductivity of a gas[1]. The typical application is Pirani gauge, which was
invented in 1906 by Marcello Pirani[4]. The method measures heat loss of a
filament to indirectly determine the pressure of gas. For example, within high
pressure, there should be more molecules present in the same volume and the
chance to collide with heated metal wire potentially high, resulting in more
efficiency removing heat than low pressure. Since the thermal conductivity and
heat capacity of the gas may affect the readout, the sensor needs to be
calibrated before using. The advantage of this method is its accuracy—between
0.5Torr to 10-4 Torr[4].
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