294 



THE IRRIGATION AGE. 



30.5 cubic feet 

 for each turn, the 

 rate of flow is 

 30.5 divided by 

 30, or a little over 

 one cubic foot per 

 second. 



The walls of 

 the box in which 

 the Dethridge 

 meter is set are 4 

 inches thick when 

 made of concrete. 

 Care should be 

 used in getting 

 the bottom 

 curved to the cor- 

 rect circle so that 

 the leakage 

 around the meter 

 will be small. 

 This meter is 

 somewhat compli- 

 cated in construc- 

 tion and it is bet- 

 ter for it to be 

 placed by canal 

 companies than 

 by land owners. 

 It will probably 

 cost less in this 

 way as the drums 

 can be bought in 

 quantities. In Australia the concrete box is made in 

 parts and seasoned in the material yard, the parts be- 

 ing then cemented together when placed in the field. 

 When built of concrete as shown in figures 17 and 18, 

 22.8 cubic feet of concrete and 40 board feet of lum- 

 ber are required. The drums are best made by some 

 sheet metal works. A special counter should be used 

 made of rust re- 

 sistant metal as 

 the ordinary coun- 

 ters have been 

 found to rust out 

 rapidly in use. 

 Where installed 

 in large numbers 

 in Australia the 

 cost has been 

 about $40 per 

 meter; the cost of 

 the meter in- 

 stalled at Davis 

 was about $60. 



The tests of 

 this device made 

 at Davis showed 

 quite accurate 

 under constant 

 ditch conditions 

 between rates of 

 flow of 1 to 3.5 

 cubic feet per 

 second. For both 

 larger and small- 

 er discharges the Fig. 21 Plan and elevation of 



Fig. 19 Drawing of Dethridge meter. 



( 3-6"- 



PLAN 



)- --/'-5-'--t6i -1 



na 



SECT. ELEV. 



meter passes more 

 water than it does 

 between these 

 limits. The 

 amount of water 

 going through the 

 meter varies with 

 the depth of 

 drowning. A me- 

 ter set high in the 

 ditch will dis- 

 charge less water 

 per revolution 

 than one set low. 

 Checking up the 

 ditch below a me- 

 ter so that the 

 depth is increased 

 at the meter may 

 decrease the dis- 

 charge by as 

 much as 10 per 

 cent in some 

 cases. 



The Dethridge 

 meter of this size 

 is adapted for 

 accurate measure- 

 ment of streams 

 varying from 1 

 to 3 or 4 cubic 

 feet per second ; 

 in Australia it is 



considered satisfactory up to 5 cubic feet per second. 

 Where the quantities are either larger or smaller than 

 these amounts the error will be in favor of the water 

 user. While rather expensive to install there are no 

 parts which will wear out except possibly the counter. 

 The bearings are merely oiled wooden blocks. Varia- 

 tions in the friction will not alter the discharge ; if 



fthe bearings are 

 tight a greater fall 

 will be needed to 

 drive the wheel, 

 but unless tighter 

 than they will be- 

 come if not tam- 

 pered with, the 

 discharge per rev- 

 olution will not 

 change. Where 

 larger amounts 

 are to be turned 

 out. larger meters 

 can be built or 

 more than one in- 

 stalled. The me- 

 ter has the advan- 

 tage of being 

 easily understood. 

 The wheel stands 

 up in the air and 

 has a clumsy ap- 

 pearance, yet it is 

 some advantage 

 to be able to look 

 across the field to 



6- -f 



" ra 



i 



installation of Grant-Mitchell meter. 



