Measurement of gas pressure by the manometer
Earlier in this chapter it was stated that the pressure of the atmosphere was about 100 kNzm? (or kPa). Before explaining how this is measured let us make a study of the manometer, an instrument for measuring the pressure of gas. The manometer consists of a U-tube containing water. When both arms are open to the atmosphere the same atmospheric pressure is exerted on the water surfaces and B, and these are at the same horizontal level (Fig. 10.10 (a)). In order to measure the pressure of the gas supply in the laboratory, the side A is connected to a gas-tap by a length of rubber tubing (Fig. 10.10 (b)). When the tap is
turned on the gas exerts pressure on the surface A, with the result that the level B rises until the pressure at C on the same horizontal level as A becomes equal to the
gas pressure. Thus,
pressure of gas = atmospheric pressure + pressure due to water column BC
It follows that the excess pressure, in N/m2, of the gas above that of the atmosphere is given by the pressure of the water column BC, and is therefore equal to hpg as explained on page 108.
The height, h, is called the head of water in the manometer and it is often convenient to express the excess pressure simply in terms of h only. In this case the units generally used are millimetres of water (mmH20). For measuring higher pressures than in the example above, mercury (density 13.6 g/cm”) is used in the manometer, in which case the units are mmHg.
H20 and Hg are the chemical symbols for water and mercury respectively. Doctors use an instrument called a sphygmomanometer (Fig. 10.11) for measuring a tient’s blood pressure in mmHg which they find adequate for their purpose. As the readings are not used in calculations, the strict use of SI units (pascals) is necessary.