In this article, I am discussing some important pressure transducer types, their working principle, advantages, disadvantages, and applications. So let us start.
Pressure transducers are used to measure pressure. In a pressure transducer, firstly, the pressure is converted into a displacement by an elastic element like Bourdon tube, diaphragm, bellows, etc. And then this displacement is converted into a change in some electrical parameter.
Being proportional to the applied pressure, this change in the electrical parameter is a measure of the magnitude of the pressure.
Pressure Transducer Types
Some principle pressure transducer types are as under:
Strain Gauge Pressure Transducer
Strain gauges can be used for pressure measurement also. In this method, a strain gauge is attached to a pressure sensing diaphragm.
When the pressure is applied to the diaphragm, the strain gauge is strained. This causes a change in resistance of the gauge which can be measured by a Wheatstone bridge. The detailed working of strain gauges has been discussed earlier. You can refer to that article for details.
The advantages of this type of pressure transducers are:
- good accuracy,
- no moving parts,
- simple to maintain,
- fast response.
However, there are some disadvantages too. These are:
- high cost,
- the need for fixed supply,
- sensitive to temperature variations.
Potentiometric Pressure Transducer
In this type of pressure transducer, the pressure is converted into a displacement by a suitable primary sensing element such as a diaphragm, Bourdon tube, etc.
This displacement controls the position of sliding contact (wiper)on the potentiometer. The magnitude of output voltage depends upon the position of the wiper. In this way, pressure variations are get converted in the form of output voltage and can be measured by a suitable instrument.
These are simple, cheap and can be made quite linear. But wear and tear due to frequent movement of the wiper, high noise level, and limited resolution are the main limitations of this method.
Capacitor-type Pressure Transducer
In this type of pressure transducer, the pressure causes a change in the capacitance of a parallel-plate capacitor. The change in capacitance of the capacitor is a measure of the magnitude of the pressure.
The figure shows the different parts of a capacitor type pressure transducer. It is essentially a parallel plate capacitor having one fixed plate and the other free to move as the applied pressure changes.
The movable plate is usually in the form of a diaphragm. When pressure is applied on the movable plate, the distance between the plates of the capacitor changes. This causes a corresponding change in the capacitance of the capacitor. The change in capacitance can be determined and is a measure of the magnitude of applied pressure.
Generally, capacitor transducer forms, one of the arms of a bridge circuit. When no external pressure is applied, the ammeter connected in the bridge shows no deflection i.e. output voltage is zero.
When pressure is applied, the bridge becomes unbalanced and causes some output voltage corresponding to the applied pressure. Thus output voltage of the bridge will be a measure of the magnitude of the pressure. Usually, the meter is calibrated to indicate the pressure directly.
The main advantages of this type of pressure transducer are:
- rapid response to pressure variations,
- high sensitivity,
- good frequency response,
However, they have limited range and severely affected by dirt and other contamination. They are mostly used in motor vehicles.
Inductive-type Pressure Transducer
The most commonly used inductive type transducer is the linear variable differential transformer (LVDT). The pressure can also be measured by an LDVT. It has been discussed earlier where it was used to measure displacement. You can refer to that article for details.
To measure pressure, with an LVDT, the pressure is converted into displacement by a Bourdon tube. This displacement causes the movement of the magnetic core of the LVDT.
Since the exactly similar secondary coils of the LVDT are connected in phase opposition, the magnitude of the output voltage will depend upon the position of the magnetic core.
Initially, when no pressure is applied, the core will remain in the central position and the magnitude of the induced voltage in each secondary is the same due to their symmetrical position with respect to the core.
On the application of pressure, the core shifts from its central position; causing the induced voltages in the two secondaries to differ. In this condition, the output voltage is equal to the difference in the two secondary voltages and will vary linearly with the displacement of the core. Thus output voltage of the LVDT is a measure of the magnitude of the pressure.
Advantages of this type of pressure transducers are:
- high sensitivity,
- the linear variation of output voltage with displacement up to 5 mm and,
- infinite resolution.
However, there are some disadvantages too:
- performance is affected by stray magnetic fields and temperature variations,
- a large displacement is required for reasonable amount of differential output.
Piezoelectric Pressure Transducer
The piezoelectric transducer can be used to measure pressure. The working of a piezoelectric transducer has been discussed earlier. For details, you can refer to that article.
Piezoelectric transducer transducers are used to measure high pressure that changes very rapidly e.g. pressure changes inside the cylinder of a gasoline engine, compressor, rocket, etc. The pressure changes in these devices are so rapid that ordinary pressure transducers can not respond to them.
Low-pressure Measurement by Pirani gauge
Pressure below atmospheric pressure is called low pressure. This range extends from the normal pressure of 760 mm of mercury column down to 10-8 of the mercury column. No single transducer can cover this full range. A large number of transducers have been developed for covering different ranges of low pressure. Here I will discuss the Pirani gauge only. This gauge can measure pressures ranging from 10-1 to 10-3 mm of the mercury column.
For pressures above 1 mm of the mercury column, the thermal conductivity of a gas is independent of pressure. But for the pressures below 1 mm of the mercury column, it decreases linearly with the decrease in pressure and vice-versa. This property of gases is utilized in the Pirani gauge to measure the pressure. At the pressures below 10-3 mm of Hg of the mercury column, the heat conduction becomes very small and the gauge does not work.
A Pirani gauge consists of a wire filament enclosed in a chamber having a side tube for connection to the pressure source. And the gauge is connected in the Wheatstone bridge as shown in Figure. Here, the gauge forms one of the four arms of the bridge.
Initially, when current passes through the filament, an equilibrium temperature is reached shortly. When no pressure is applied, the bridge is balanced and no current flows through ammeter.
When the source whose pressure is to measured is connected to the gauge, the pressure inside the chamber is increased. And the thermal conductivity of the gas surrounding the filament will increase. This means that gas will conduct more heat from the filament, thus lowering the temperature of the filament. The decrease in filament temperature will lower its resistance. The reverse will when the pressure inside the chamber, decreases.
This change in resistance of the filament makes the bridge unbalanced and current flows through the ammeter connected in the bridge. The deflection of the ammeter is a measure of the magnitude of the pressure. This ammeter can be calibrated to read the pressure directly.
The advantages of Pirani gauge are simplicity, easy to use, ruggedness and cheapness. However, its disadvantage is that its calibration depends upon the type of gas in which the pressure is measured.
Thanks for reading about “pressure transducer types”.