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Dethbridge Wheel
Dethbridge Wheel
Dethbridge Wheel
Dethbridge Wheel at rest
Top. The sluice gate in the bank of the supply channel carries the wheel serial number. One vane is painted red so it is easy to see from a distance that the wheel is turning.
Lower. The concrete enclosure for the wheel provides an exit race for the water to the left.
The sight of a Dethbridge Wheel (or Dethbridge meter) steadily rotating at the intersection of a main supply irrigation channel and a side channel is a sure sign of an irrigation scheme and is often considered as an irrigator's icon. At first sight Dethbridge Wheels can look as if they are pumping water into the smaller channels but the opposite is true; water flowing under gravity from the supply channel into a smaller channel pushes vanes on the wheel making it rotate. The number of rotations, measured by a cyclometer attached to the inside of the wheel, is used to calculate the volume of water delivered to a property. Large Dethbridge Wheels are designed to handle water flow of 42 to 140 litres per second, a smaller version is designed to handle 14 to 70 litres per second.

The volume of water flowing past the wheel is controlled by a sluice-gate on the side of the supply channel adjacent to the wheel.

The Dethbridge Wheel was invented in 1910 by Mr J. S. Dethbridge, then Commissioner of the Victorian State Rivers and Waters Supply Commission, and is now widely used in Victoria and in other states including New South Wales (Murrumbidgee Irrigation Area) and Western Australia (Ord River scheme). Generally, a single Dethbridge Wheel measures water delivered to a single property; large farms, exceeding 130 hectares, may need a second wheel to measure the total volume of water supplied.

The Dethbridge Wheel comprises an undershot water wheel mounted in a specially shaped flume made of 100 mm thick reinforced concrete. Outside diameter of the wheels across the vanes is 152.4 cm (five feet), small wheels are 121.9 cm (four feet) in diameter. To ensure the most accurate water measurement there is very small clearance between the rotating wheel and the bottom (0.64 cm) and sides (0.64 cm in small wheels, 1.92 cm in large wheels) of the enclosure. Even with this small clearance some water passes the wheel without being measured; the amount of unmeasured leakage varies with changes in water levels on the supply and delivery side and on water depth at the wheel. Each wheel is calibrated before first use to determine an average correction for raw readings when calculating water volumes; this correction is accepted as accurate to within about 5%. Once correctly installed no further adjustment is required and wheels do not have to be re-calibrated.

Information.
   Small hydraulic structures, published FAO Irrigation and Drainage Paper 26/1, Food and Agriculture Organisation of the United Nations, Dethbridge Wheel at pp 245-265.
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