|Ashcroft Indust Digital 4-1/2" Pressure Gauge|
Pressure is force per unit area. Sensing it follows the same principle as the sensing of force that is measuring the displacement of an appropriate member of the sensor in response to pressure. The range of methods is quite large and includes thermal, optical as well as magnetic and electrical principles. Earliest sensors were purely mechanical.
Pressure UnitsThe basic SI unit is pascal, where 1 pascal (Pa) = 1 (N/m2). Other often units are:
- bar: 1 bar = 0.1 Mpa
- torr: 1 torr = 133Pa
- millibar: 1 millibar = 100Pa = 1.333 torr
- microbar: 1 microbar = 0.1 Pa
Types of Pressure SensorPressure sensors come in four basic types :
- Absolute pressure sensors: pressure sensed relative to absolute vacuum.
- Differential pressure sensors: the difference between two pressures on two ports of the sensor is sensed
- Gage pressure sensors: the pressure relative to ambient pressure is sensed. (Most common)
- Sealed gage pressure sensor: the pressure relative to a sealed pressure chamber (usually 1atm at sea level or 14.7 psi) is sensed.
How Does Pressure Sensors Work
Pressure metrology is the technology of transducing pressure into an electrical quantity. Normally, a diaphragm construction is used with strain gauges either bonded to , or diffused into it, acting as resistive elements. Under the pressure-induced strain,the resistive values change.
In capacitive technology, the pressure diaphragm is one plate of a capacitor that changes its value under pressure-induced displacement.
Pressure sensing using diaphragm technology measures the difference in pressure of the two sides of the diaphragm. Depending upon the relevant pressure, we use the terms ABSOLUTE, where the reference is vacuum (1st picture), GAUGE, where the reference is atmospheric pressure (2nd picture), or DIFFERENTIAL, where the sensor has two ports for the measure of two different pressure.
The piezoresistive pressure sensor, or silicon cell
This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it, fused to a silicon or glass backplate. The resistors have a value of approx. 3.5 kOhm. Pressure induced strain increases the value of the radial resistors (r), and decreases the value of the resistors (t) transverse to the radius. This resistance change can be high as 30%. The resistors are connected as a Wheatstone Bridge, the output of which is directly proportional to the pressure.
Leadouts from the bridge
- Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through, pins of the header.
- TAB (Tape Automated Bonding). The contacts on the chip have a gold dot.
A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board, or the header. In the first method, the sensor must be fixed on the header. The TAB printed circuit, however, holds the sensor in place itself.
Low cost sensors
Figure 5. Low cost sensors
Low cost sensors are devices where the they are exposed to the media without protection. The glass feed-through and the silicon cell is mounted in a plastic housing with pressure ports for positive and negative pressure (1st picture). The silicon sensor with the TAB print is fixed between two plastic mouldings with pressure ports (2nd picture). The silicon sensor is bonded to a brass pressure port. The contacts are made either by gold wires to soldering pins, or by TAB flexible printed circuit (3rd picture).
Piezoresistive OEM Pressure Transducer
Figure 6. Piezoresistive OEM Pressure Transducer
The silicon sensor on the glass feed-through is mounted in a stainless steel housing, isolated by a thin stainless steel diaphragm and filled with silcone oil. The pressure acts on the diaphragm and is transfered through the oil onto the sensor. These transducers are fully tested for temperature and linearity and the compensation resistor values given on the individual test sheets.
Figure 7. Pressure transducer
Pressure transducers are pressure measuring instruments, ready to use. It is an OEM transducer with pressure port, integrated compensation resistors and a cable or connector. Transducers give an unamplified signal into a separate instrumentation amplifier or indicator. They can be considered as passive bridges, being interchangeable between different manufacturers.
Figure 8. Pressure transmitter
In pressure transmitters, the full signal conditioning circuitry is integrated in the housing. The sensor signal is conditioned into standard output signals of 0...100mV, 0...10V, 0.5...4.5V,and 4-20mA. Normally, the signal is independent from the excitation (i.e. 8...28V), but in ratiometric transmitters, the signal is proportional to the excitation. The accuracy of a transmitter is best described by an error band. This band covers all errors over the full pressure and temperature range. Typical errors are also given. The typical error describes the accuracy which can normally be expected in a measurement.
Handbook of Modern Sensors: Physics, Designs, and Applications