A pressure transducer is used to convert a certain value of pressure into its corresponding mechanical or electrical output. Measurement if pressure is of considerable importance in process industries.
The types of pressure sensors are differentiated according to the amount of differential pressure they are able to measure.
For low differential pressure measurement Liquid Column Manometers are used. Elastic type pressure gauges are also used for pressure measurement up to 700 MPa. Some of the common elastic/mechanical types are:
- Bourdon Tubes
- Piston Type Pressure Transducer
- Electrical Type Pressure Sensors
- Bell Gauge
Before going into further details regarding pressure measurement, it is important to know the different terms related to pressure.
Basic Terms Related To Pressure Measurement
Pressure is known to be the force that is exerted due to the weights of different gases and liquids. Some common examples are atmospheric pressure and the pressure implied by liquids inside the walls and underside of a container.
Pressure can be measured as the force exerted over a certain area.
Pressure = Force/Area
Pressure is not an independent variable as it is derived from force and area and it is not ideal as it depends on other factors like elevation, fluid density, temperature, flow velocity, and so on.
In instrumentation analysis, pressure is commonly expressed in pounds per square inch (psi). It can also be expressed in pounds per square feet (psf) and Pascals (Pa). Pascal is the SI unit if pressure. In many cases, pressure is expressed in terms of atmosphere which is the height of the barometric column at zero degrees Celsius, being equal to 76 cm of mercury or equivalent to 14.696 pounds per square inch absolute, 1 kg/cm2. Most of the pressures range from a little below atmosphere to hundreds of atmospheres.
- Density is the mass per unit vlume of the material. It can be expressed as kilogram per cubic meter. (kg/m3).
- Specific Weight is the weight per unit volume of the material. It can be expressed as Newon per cubic meter (N/m3).
- Specific Gravity is basically a non-dimensional value as it is the ratio of two measurements in the same unit. It can be the ratio between the density of a material and the density of water or even the ratio between the specific weight of a material to the specific weight of water.
- Static Pressure is the fluid or gas pressure that does not move.
- Dynamic Pressure is the gas or fluid pressure that is obtained when it impacts with a surface or an object due to its motion or flow.
- Impact Pressure is the total pressure or the addition of both the static and dynamic pressures.
- Atmospheric Pressure is the surface pressure of earth and is available due to the weight if the gases in the earth’s atmosphere.
- Another important aspect of pressure measurement is the measurement of very low pressure or what is known as vacuum. With the advancement of scientific research and industrial application of the results, pressure is as low as 10-6 mm of mercury is often required to be measured in some systems. Measurement of pressure, therefore, consists of two parts – that of pressure and vacuum. The force exerted by the fluid per unit area of the wall of the container is called the absolute pressure, whereas the gauge pressure is the difference between the absolute and local atmospheric pressure, and when gauge pressure is negative, it is known as vacuum.
Basic methods of pressure measurement are same as those of force measurement. For high vacuum, however, some special techniques are necessary. Primary sensors are mostly, mechanical which through secondary sensing means provide electrical outputs. Manometers and elastic element sensors are used as primary pressure sensors while secondary sensing, often called transducing here, involves resistive, inductive and capacitive changes for deriving electrical outputs.