Thk DETHRIDGE Meter 
II 
revolutions per minute for a 4 foot wheel gives a discharge of 2.5 
second feet. 
Fig. 4 shows the approximate loss of head or droi> of water 
surface which will be necessary to allow different quantities of 
water to flow through the meter. These curves are of use in decid¬ 
ing what size of wheel to install to meet any certain ditch condition. 
To determine the total quantity which has flowed through the 
meter during any certain period of time, take the difference between 
the counter readings at the beginning and termination of this period, 
and multiply this difference by the fraction of an acre inch of water 
delivered by each revolution of the wheel, as shown in Fig. 6. For 
example, if the counter reading at the start it 21695 the stop 
is 23825, the wheel made 2,130 revolutions. If the wheel is 6 feet 
in diameter and the average depth of water in the ditch upstream 
from the wheel was 1.4 feet, it will be seen in Fig. 6 that each revo¬ 
lution of the wheel has delivered .0169 acre inches of water. Mul¬ 
tiply 2130 by .0169 and the result is 36 acre inches or three acre 
feet of water. If six acres were irrigated, the land was covered 
to an average depth of 6 inches. 
It is necessary that the wheel and box of the Dethridge meter, 
as well as the clearance between the vanes and the box, be the same 
as given in the accompanying plans, if the rating curves given in 
Fig. 6 are to be used. Under ordinary conditions of canal opera¬ 
tion a Dethridge meter should register the flow with an error not 
exceeding 2 per cent, and even under unusual ditch conditions, but 
with proper construction of the meter, the error should not exceed 
5 per cent. 
