W
4
Pharmaceutical
Engineering
Where h is height to which the liquid rises in the glass tube. w is the specific weight of the liquid Limitations of Piezometer 1.
Piezometers can only measure gauge pressures. Not suitable for measuring negative pressures
2.
Not used to measure Gas pressures.
3.
Piezometers cannot be used when large pressures measured on lighter liquids as this would require very long tubes.
1.2.2 V - tube
manometer
The limitations of piezometers can be overcome by the use of V-tube manometers.
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Figure1.2 : V - tube manometer It consists of a glass tube folded like the letter 'U'. In this type of manometer One end of the U'-tube is attached to the point where the pressure is to be measured and the other end is open at atmospheric pressure. The pressure at point B is given by: (1.6) Where PI = density of light liquid h, = height of light liquid above reference line. P2 = density of heavy liquid h2 = height cif heavy liquid above reference line
1.2.3 Single column manometer It is a modified form of a U-tube manometer in which a reservoir, having a large cross-sectional area (approximately 100 times) compared to the tube area is connected to one of the manometer's limbs. For any variation in pressure, the change in liquid level in the reservoir will be very small, which can be neglected