856 



HYDRAULICS AND ITS APPLICATIONS 



not strictly accurate for the two cases, and that with a given velocity of 

 flow the revolutions increase as the freedom of motion decreases. 1 The 

 difference depends on the type of meter, and for velocities of 1 foot per 

 second is usually about 2 per cent. 



A further source of inaccuracy, particularly with very low velocities of 

 flow, is due to the fact that a rating carried out in still water is not quite 

 accurate when applied to the same relative velocities in moving water. 



Velocity in feet per second. 

 .6 .7 .8 .9 1.0 



1.1 1.2 1.3 



1.5 



2 3 



A 



* 



o SMI-water rating 



-water rating 



FIG. 158. Rating Curves of Price Current Meter. 



Experiments 1 indicate that the meter, particularly of the cup type, does 

 not indicate so high a velocity when dragged through still water as 

 when held in a current, the difference varying from 1 to 4 per cent, 

 at a velocity of 1 foot per second with different types of meter. The 

 difference is about 1 per cent, at a velocity of 2 feet per second. 



In calibrating a current meter it is usual to plot the curve connecting 

 velocity of current and number of revolutions of meter per second. In 



1 U. S. Water Supply and Irrigation Paper, No. 95, pp. 8389, also p. 81. 





