MEASUREMENT OF DISCHARGE 



217 



Morgantown and Millway, with light oil displacing heavy oil, showed 

 a length of admixture varying from 10 to 12 per cent. 



ART. 68. MEASUREMENT OF PIPE DISCHARGE. 



The volume actually discharged by a pipe may be determiLed 

 approximately in several ways. 



(1) The most accurate method is that of collecting and weighing the 

 quantity discharged in a definite time, but this is impossible with any but 

 the smallest of pipes. 



(2) The mean velocity may be computed from a knowledge of the 

 hydraulic gradient and of 



the diameter and internal 

 condition of the pipe by an 

 application of one or other 

 of the formulae of Art. 65. 



Where these data can be J 



accurately obtained an error 

 not exceeding 10 per cent, 

 may be expected, with pipes 

 ranging from 8 inches to 



Datum 



level 



FIG. 99. 



6*0 feet in diameter. The 

 hydraulic gradient may be 

 obtained by observing the 

 difference in the free level 

 of the columns in two 



piezometers, or pressure tubes, placed at a known distance apart, or 

 by the use of one of the types of differential gauge described in Art. 9. 

 (3) The velocity may be deduced from Pitot tube observations, this 

 being used to give the velocity at the centre, or at the radius of mean 

 velocity (p. 214), or at a series of radii across the pipe. In the first case 

 the mean velocity is approximately *84 times that observed. In the third 

 case the mean velocity is given by 



1 /"" 



v = H / 2 TT v r d r, 

 TT a 2 / o 



where a is the pipe radius, and v the velocity at radius r, the integration 

 being performed graphically. (See p. 110.) 



The Pitot tube consists of a tube of fine bore bent at right angles 

 (Fig. 99), having both ends open, and so arranged that while one leg 

 remains vertical, the other may be rotated so as to point either up, 



