PRESSURE GAUGES 



21 



level of the mercury columns with no water in the pipe, the pressure at 



the centre of the pipe under the conditions shown is that due to a 



column of mercury h feet high, minus that due to a column of water of 



height h' -f- J h feet, and is therefore equal to 13*596 h '5h h' = 



13-096 h h' feet of water. Where the pressure 



is in excess of 100 feet of water, such a gauge 



gives an inconveniently long mercury column 



and a type of gauge in which a large range of 



pressures may be recorded on a comparatively 



short scale is shown in Fig. 8 D. This consists 



of a series of n U tubes connected as shown, the 



lower half of each being filled with mercury and 



the upper half with water. If this gauge be 



coupled up to a pipe, and if 0' be the common 



level of the surfaces of separation of mercury and 



water when the connecting tube is full of water, 



but with atmospheric pressure in the main, and if 



be taken as the zero of the scale, the pressure 



in the main corresponding to a recorded height 



of h feet is 2 71 X 12'6 h + h h' = (25'2 n + 



1) h h' feet of water. This follows since the 



pressure at b = pressure at a = atmospheric 



pressure -f- pressure due to a column of mercury 



2 h feet high pressure due to a column of water 



2 h feet high, so that the pressure at b = 2 X 12'6 



h feet of water above the atmosphere. Again, 



pressure at d = pressure at c = pressure at b + 2 



X 12-6 h feet of water = 4 X 12'6 h feet of water 



above the atmosphere. Similarly pressure at/ = 



6 X 12-6 h, and at g = 8 X 12'6 h = ZnX 12'6 h 



feet of water. 



Differential Gauges. Where it is required to 

 determine the difference of pressure at two points 

 in the length of a pipe conveying water, some 

 form of differential gauge is commonly used. 



Such a gauge is shown in Fig. 9. Here the tubes A A are coupled up to 

 the required points in the pipe, the water from which partially fills the 

 branches TT of the inverted U tube. If the difference of pressure is 

 moderately large, the upper portion of this tube contains air, and the 

 difference of level of the free surfaces then gives the difference in pressure 



FIG. 9. 



