METHODS OF PRESSURE MEASUREMENT
1. By means of manometers.
2. By means of mechanical sensors.
3. By means of electrical and electronic sensors.
Today we will only discuss about manometer, in next article i will cover mechanical types.
1. MANOMETERS
All manometers operate on the principle that change in pressure will cause a liquid to raise or fail in a tube.
BASIC MANOMETERS
All manometers works on the principle that change in pressure will cause a liquid to rise or fall in a tube. A basic manometer includes a reservoir that is filled with a liquid. The reservoir is usually enclosed with a connection point that can be attached to a source in order to measure its pressure. A transparent tube is attached to the reservoir. The top of the column may be open, exposed to atmospheric pressure, or sealed and evacuated. Manometer with open columns are used to measure gauge pressure or pressure with reference to atmosphere and manometers with closed or sealed columns are used to measure vacuum. When a manometer is connected to a process, the liquid in the column will rise or fall according to the pressure of the process. The pressure exerted by the process will be equal to the change in height of the liquid. The specific gravity of the liquid must be known in order to measure the actual pressure.
Manometers are accurate; they are often used as calibration standard. Manometers are used for low pressure and low differential pressure application. For precise measurement of manometer the shape of the meniscus of the liquid must be taken into account. Mercury and water are most commonly used liquids. For water, the shape of the meniscus is concave and the shape of the mercury meniscus is convex.
Manometers are classified into three, they are:
· Well Type Manometer.
· Inclined Manometer.
· U-Tube Manometer.
WELL TYPE MANOMETER
A well type manometer has a glass column that contains the liquid that extends from a well. To measure pressure with this type of manometer the process is connected to the fitting on the well. A barometer is a well manometer. Barometer has sealed and evacuated measuring column. This type of manometer detects and measures changes in atmospheric pressure.
INCLINED TUBE MANOMETER
A variation of the well type manometer is the inclined tube manometer. This manometer also has a well that contains the liquid and a transparent column. However on an inclined manometer the tube is mounted at an angle. A small change in pressure will cause greater movement of the liquid in the column.
U TUBE MANOMETER
U tube manometer consists of ‘U’ shaped tube, half filled with liquid of known specific gravity (Water or Mercury). To measure pressure, one leg of the tube is connected to the process and the other leg of the tube is left open and exposed to atmosphere. Pressure is indicated by the amount of the liquid that falls in one leg and rise in the other leg. The total movement of the liquid in both legs is the pressure measured by the U tube manometer.
U tube manometer can also be used for vacuum measurement by sealing one leg and connecting the other leg to the process. When both the tubes are open, the liquid is at the same height in each leg. When positive pressure is applied to one leg the liquid is forced down in that leg and up in the other. The difference in the height, which is the sum of the reading above and below zero is the pressure applied. When Vacuum is applied to one leg, the liquid rises in that leg and falls in the other. The difference in the height indicates the amount of vacuum.
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