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Water-measuring Devices 



By G. K. Larrison, Superintendent of Hydrography. 



This paper is prepared for the purpose of providing for watel 

 users a brief non-technical description of the various devices now 

 used for measuring water, and of the essential conditions undel. 

 which each type should be used. In many instances it is impos- 

 sible to determine the correct type of measurement device to be 

 used without a careful investigation by a competent hydraulic 

 engineer, and the reader is warned against relying on the sugges- 

 tions contained herein for any specific problem without further 

 technical advice. 



All kinds of water measurements will be made under one of 

 two conditions — either when the water is under pressure, as in 

 pipes or other closed conduits, or when it is flowing with a free 

 open surface, as in the case of streams, ditches, flumes, etc. The 

 flow of water in the first case may change to the latter, as when 

 the head on a pipe line has lowered so that the pipe is only par- 

 tially filled. 



MEASUREMENT OF FLOW IN PIPES UNDER PRESSURE. 



A few of the basic principles of hydraulics are as follows : 



1. Water confined in a tank, pipe, or other vessel will exert 

 a pressure per square inch equal to the weight of a column of 

 water one inch square and as long as the vertical distance between 

 the surface of the water and the area under pressure, as long as 

 this column of water is at a standstill. This pressure is also 

 exerted in all directions, or against the sides of the vessel as well 

 as the bottom. When the water is allowed to flow through the 

 pipe or vessel, the pressure at any point decreases, due to the 

 velocity of the flowing water. 



2. It is commonly supposed that water will "seek its own 

 level" or in case of an inverted pipe syphon the water will flow 

 out of one end at the same level at wdiich it enters the other end. 

 This would be true if it w^ere not for the fact that the sides of the 

 pipe and the bends of the pipe exert a slowing down efifect on 

 account of friction, and in reality the water, when flowing, will 

 not reach the same level as that at which it enters the pipe, al- 

 though the same amount of water will flow out of the pipe as 

 enters at the other end. 



3. The friction factor is also of primary importance in de- 

 termining the flow of water through pipes laid under all condi- 

 tions of gradient and curvature. For pipes of the same mate- 

 rials, the larger the diameter of the pipe — all other things being 

 equal — the less friction in proportion to the area of the pipe will 

 occur and less loss of head will be the result. 



For example: If two pipes, one of 2-inch and the other of 

 1-inch diameter, were laid from a common source, with identical 

 gradients, and curvature, to similar tanks, the 2-inch pipe would 



