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:
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.
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Pressure Instrument Selection
Selection of pressure instrument for a particular application must be done carefully, taking into consideration various aspects such as process conditions, turn down requirements, accuracy, installation requirements, and so on.
While selecting a pressure instrument for a particular application, the process data such as fluid phase, pressure, temperature, density and viscosity must be correctly defined for all operating conditions including start-up, emergency operations and design conditions.
Another important parameter for selection is the turn down requirements, based on which we can select the pressure instrument to suit the maximum and minimum conditions within its specified accuracy limits.
In addition to the above, the installation requirements of the selected pressure instrument shall be carefully addressed taking in to account visibility, accessibility, because these requirements may affect the piping layouts.
As shown in the figure below, the input pressure is given to the piston. This moves the piston accordingly and causes the spring to be compressed. The piston position will be directly proportional to the amount of input pressure exerted. A meter is placed outside the piston and spring arrangement, which indicates the amount of pressure exerted. As the device has the ability to withstand shock, sudden pressure changes, and vibrations, it is commonly used in hydraulic applications. Mostly, the output of the piston and spring arrangement is given to a secondary device to convert movement into an electrical signal.
The bell gauge is a type of pressure transducer that measures differential pressure between 0.06 Pa and 4 KPa. The static pressure may be as high as 4 to 6 MPa. The schematic diagram of a single element bell gauge is shown below.
The movement of the bell is taken out by link and lever mechanism or by some electrical methods. When the bell moves maximum up or down it closes the inlets of pressure p2 or p1, whereby protection to overrange and reversal of pressure are afforded. The diagram of a two element bell differential gauge or balance is also shown above. The two identical bells are suspended from the two knife edges of a balance beam. The differential weight is balanced statically by the movement of the counter weight w.