U-Tube Manometer

About the U-tube manometer

The U-tube manometer consists of a glass vessel in the shape of a letter U. When the manometer is used for measuring gauge pressure, both ends of the tube are open, with an unknown pressure being applied at one end and the other end being open to the atmosphere. The unknown gauge pressure of the fluid (P1) is then related to the difference (h) in the levels of fluid in the two halves of the tube by the expression:
 

P1 = hpg


where P is the specific gravity of the fluid.


In a third way of using the U-tube manometer, each open end of the tube is connected to different unknown pressures (P1 and P2), and the instrument thereby measures the differential pressure (P1 - P2) according to the expression:
 

(P1 -P2) = hpg


U-tube manometers are typically used to measure gauge and differential pressures up to about 2 bar. The type of liquid used in the instrument depends on the pressure and characteristics of the fluid being measured. Water is a convenient and certainly cheap choice, but it evaporates easily and is difficult to see. It is nevertheless used extensively, with the major obstacles to its use being overcome by using colored water and regularly topping up the tube to counteract evaporation.


Situations where water is definitely not used as the U-tube manometer fluid include the measurement of fluids which react with or dissolve in water, and also where higher-magnitude pressure measurements are required. In such circumstances, liquids such as aniline, carbon tetrachloride, bromoform, mercury or transformer oil are used.


The U-tube manometer, in one of its various forms, is an instrument commonly used in industry to give a visual measurement of pressure which can be acted upon by a human operator. It is not normally possible to transform the output of a U-tube manometer into an electrical signal, however, and so this instrument is not suitable for use as part of automatic control systems.


More on Pressure Measurements

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