Pressure sensor-device used to sense pressure
In general terms, matter can be classified into two categories viz. solids and fluids. Fluid includes both liquids and gases. By varying pressure it is possible to change liquid into gas form and vice versa. As we know it is impossible to apply pressure to fluid in any direction except normal to its surface. At any angle except 90 degree, fluid will slide over or will flow.
For the fluid at rest, pressure can be defined as force F exerted perpendicular on unit area A of a
p = dF/dA
Pressure is mechanical term and can depend on mass, length and time. Pressure dp = -w*dh where, w is the specific weight of medium and h represents vertical height.
Pressure is unaffected by the shape of the confining boundaries and hence pressure sensors are available in various shapes and dimensions.
The kinetic theory of gases states that pressure can be viewed as a measure of the
total kinetic energy of the molecules.
p = (2/3)* (KE/V) = (1/3) * (ρ*C2 ) = N*R*T
KE = Kinetic Energy
V = Volume
C2 = average value of square of molecular velocities
ρ = density
N = no. of molecules per unit volume
R = specific gas constant
T = Absolute temperature
Whether gas pressure is above or below the pressure of ambient air, we speak about overpressure or vacuum. Pressure is called relative when it is measured with respect to ambient pressure. It is called absolute when it is measured with respect to a vacuum at 0 pressure. The pressure of a medium may be static when it is referred to fluid at rest, or dynamic when it is referred to kinetic energy of a moving fluid.
Unit of Pressure
The SI unit of pressure is the pascal: 1 Pa=1 N/m2; that is, one pascal is equal to one newton of force uniformly distributed over 1 square meter of surface. Sometimes, in technical systems, atmosphere is used, which is denoted 1 atm. One atmosphere is the pressure exerted on 1 square centimeter by a column of water having a height of 1 meter at a temperature of +4 DegreeC and normal gravitational acceleration. A pascal can be converted into other units by the use of the following relationships
Pa=1.45 X 10-4 lb/in2 =9.869 X 10-6 atm=7.5 X 10-4 cm Hg.
1 atm=760 torr (i.e. Torricelli) = 101325 Pa
1 psi = 6.89 X 103Pa = 0.0703 atm.
Pressure measurement methods
There are three different pressure measurement methods viz. absolute pressure, gauge pressure and differential pressure. The absolute method is relative to 0 pa. Gauge and differential methods are relative to some other dynamic pressure. In gauge method, reference is ambient atmospheric pressure while in differential method reference is another pressure point in the system rather than ambient pressure.
Mercury pressure sensor
A simple yet efficient pressure sensor is based on the communicating vessels principle.
Its prime use is for the measurement of gas pressure. A U-shaped wire is
immersed into mercury, which shorts its resistance in proportion with the height
of mercury in each column. The resistors are connected into a Wheatstone bridge
circuit, which remains in balance as long as the differential pressure in the tube is
zero. Pressure is applied to one of the arms of the tube and disbalances the bridge,
which results in the output signal. The higher the pressure in the left tube, the higher
the resistance of the corresponding arm is and the lower the resistance of the opposite
arm is. The output voltage is proportional to a difference in resistances ΔR of the
wire arms which are not shunted by mercury:
Vout = V * ΔR/R
The sensor can be directly calibrated in units of torr.
Bellows, membranes, thin plates
In pressure sensors, a sensing element is a mechanical device which undergoes structural changes under strain. These devices include bourdon tubes (C-shaped, twisted, and helical), corrugated, catenary diaphragms, capsules, bellows, barrel tubes and other components whose shape changed under pressure.
A bellows is intended for the conversion of pressure into a linear displacement which can be measured by an appropriate sensor. Thus the bellows performs a first step in the conversion of pressure into an electrical signal.
Apopular example of pressure conversion into a linear deflection is a diaphragm in an aneroid barometer. A deflecting device always forms at least one wall of a pressure chamber and is coupled to a strain sensor such as strain gauge which converts deflection into electrical signals. Currently a great majority of pressure sensors are fabricated with silicon membranes by using micro-machining technology.
Piezoresistive pressure sensor
To make a pressure sensor, two essential components are required. They are the plate (membrane) having known area(A) and a detector which responds to applied force(F). Both of these components can be fabricated of silicon. A silicon diaphragm pressure sensor consists of a thin silicon diaphragm as an elastic material and a piezoresistive gauge resistors made by diffusive impurities into the diaphragm.
When stress is applied to a semiconductor resistor, having initial resistance R,
piezoresistive effect results in change in the resistance ΔR.
ΔR/R = π1 * ρ1 + πt * ρt , where π1 and πt are the piezoresistive coefficients in the longitudinal and transverse direction, respectively. Stresses in longitudinal and transverse directions are designated ρ1 and ρt .
Capacitive pressure sensor
A silicon diaphragm can be used with another pressure to electric output conversion process: in a capacitive sensor. Here, the diaphragm displacement modulates capacitance with respect to the reference plate. This conversion is especially effective for the low-pressure sensors. An entire sensor can be fabricated from a solid piece of silicon, thus maximizing its operational stability. The diaphragm can be designed to produce up to 25% capacitance change over the full range which makes these sensors candidates for direct digitization.
While designing a capacitive pressure sensor, for good linearity it is important to maintain flatness of the diaphragm. Traditionally, these sensors are linear only over the displacements which are much less than their thickness. One way to improve the linear range is to make a diaphragm with groves and corrugations by applying micromachining technology.
Piezoelectric pressure sensor
Piezoelectric sensors rely on quartz crystals rather than a resistive bridge transducer. Electrodes transfer charge from the crystals to an amplifier built into the sensor. These crystals generate an electrical charge when they are strained. Piezoelectric pressure sensors do not require an external excitation source and are very rugged. The sensors, however, do require charge amplification circuitry and are very susceptible to shock and vibration.
Optoelectronic pressure sensor
When measuring low-level pressures or, to the contrary, when thick membranes are required to enable a broad dynamic range, a diaphragm displacement may be too small to assure sufficient resolution and accuracy. In addition, most of piezoresistive sensors, and some capacitive, are quite temperature sensitive, which requires an additional thermal compensation. An optical readout has several advantages over other technologies, namely a simple encapsulation, small temperature effects, high resolution, and high accuracy. Especially promising are the optoelectronic sensors operating with the light interference phenomena. Such sensors use a Fabry-Perot (FP) principle of measuring small displacements.
The sensor consists of the following essential components: a passive optical pressure chip with a membrane etched in silicon, a light-emitting diode (LED), and a detector chip.
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