So far all the civil engineering projects are constructed on the earth. The earth has an already existing omnipresent component of pressure acting on the fluids on it. That is the atmospheric pressure, the pressure due to the weight of the air column above the level where pressure measurement is being done. The absolute pressure in any fluid, thus, is the sum of atmospheric pressure and the pressure due to the weight of the fluid column above the level of measurement.
Generally the pressure indicated by pressure measuring devices or pressure gauges is the quantity in excess to the atmospheric pressure. The pressure gauges are calibrated so that they read zero at atmospheric pressure. Any pressure above atmospheric pressure is read as positive and anything below the atmospheric pressure is read as negative. The pressure readings of these devices, difference from the atmospheric pressure, is known as Gauge Pressure. The summation of gauge pressure and the atmospheric pressure (reference pressure) is the absolute pressure at the particular point.
Pressure Measurement Devices
Although there is a complete range of pressure measuring devices for different applications, ranging from simple gauges with low accuracy to complex gauges with high accuracy, instrument to measure low pressures and some to measure very high pressures, gauges using fluids and gauges without fluids, mechanical, electrical and even optical gauges, but I will present a few commonly used pressure gauges.
Piezometer: It is a tube connected vertically to the liquid system whose pressure is to be measured. The liquid rises in the tube up to the point when the gauge pressure of the system is balanced by the weight of the liquid column in the tube. Pressure shown by the piezometer is the gauge pressure as the tube is open to the atmosphere on the other side.
U-Tube Manometer: It is a U shaped tube open at both ends filled with an immiscible liquid. One end is connected to the fluid system whose pressure is to be measured and the other end is open to the atmosphere. Initially the manometer liquid columns in both the arms of the U-tube are at the same level. One end is connected to the system under consideration. The manometer liquid level falls in one arm and rises in the other arm. The difference in the manometer liquid levels in the two arms gives the gauge pressure of the system as the other end is open to the atmosphere. For measuring high pressures using a U-tube manometer, a heavy manometer liquid is used, and for measuring low pressures, a light manometer liquid is used.
These two are the very basic and simple pressure measurement devices useful for simple applications. In the next article more advanced types of pressure measurement devices are taken up.