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Dethridge meter - cumulative flow


Stein, T.-M., 2008 (*)

http://www.vl-irrigation.org

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Introduction




Figure 1:
Typical Dethridge meter outlet in the Murrumbidgee Irrigation Area of New South Wales, Australia.
Source: © Dr. Thomas-M. Stein

The Dethridge meter is a volumetric water meter widely used as an outlet in Australia for the measurement of irrigation water. It was invented in 1910 by the Commissioner at that time, the late J. S. Dethridge, of the State Rivers and Water Supply Commission, Victoria, and adopted by the Commission after tests for three years under field conditions. The original Dethridge meter was a simple measuring device giving a positive measurement of volume discharged, and recording it directly in acre feet.

 

The Dethridge meter has been widely used in Australia providing a robust, cheap, simple to use and reasonably accurate flow measuring device for nearly a century now. ANCID (2002) has estimated that there are still approximately 60,000 units in use in Australia today. In comparison with other types of metering devices in surface water systems the Dethridge meter is still dominant and being used on 70% of the supply outlets followed by mechanical meters (20%), MagFlow™ (6%) and a smaller number of other devices such as FlumeGate™, Ultrasonic, etc. (ANCID, 2007). Some Dethridge meter testing and comparison with other flow measuring devices was commissioned by G-MW (2007).

 

However, many Australian State Water Providers have decided to phase out the use of the Dethridge meters replacing them with more accurate and better manageable meters.

 

A typical Dethridge meter outlet with gate, inspection grid and protection screen, as currently found in the Murrumbidgee Irrigation Area of Australia, is shown in Figure 1.


Flow and construction




Figure 2:
Dethridge meter outlet details with dimensions for both large and small units. Values for small units are given in brackets.
Source: Kraatz and Mahajan, 1975
Note: Click to enlarge

The Dethridge meter consists of an undershot water wheel operating with small clearances in a specially shaped reinforced concrete flume. The wheel is geared to a counter (or more recently the counter was attached to the inside surface of the wheel) which registers the revolutions made by the wheel, which when used with the wheels calibration gives cumulative volume of water discharged. Accuracy is to ± 3-5% over a wide range of upstream water levels when the meter is installed and calibrated correctly and operated within the design range of discharge, i.e. 14 to 70 l/s for the smaller and 42 to 140 l/s for the larger model. The bottom clearance is 6 mm and side clearance 9 mm. Each installation should be calibrated in situ.

 

The construction of the Dethridge meter has remained basically the same over the years, i.e. concrete emplacement with minor variations to head wall and transition shape. Laboratory and field tests have resulted in a standard setting of the wheel in relation to channel flow level and mechanical improvements for the wheel and fittings, e.g. wheel with a mild steel plate drum, vanes and water pipe axle, originally with timber spokes and rivetted, now all steel and welded; timber bearing blocks now replaced by a more robust sealed unit; steel gate in a timber frame, now replaced by neoprene guides set in concrete.

 

Outlet design details with dimensions for both large and small units are given in Figure 2. More details and additional drawings are given by Kraatz and Mahajan (1975) (see also Dethridge Meter (Excerpt of Chapter 5.5) [3.3MB!] Pages 245-265).

 


References


ANCID, 2002: Know the Flow - Flowmetering Training Manual .- Australian National Committee on Irrigation and Drainage and National Program on Irrigation Research and Development.
ANCID, 2007: Australian Water Provider - Benchmarking Data Report 2005/2006 .- Australian National Committee on Irrigation and Drainage (ANCID), Torrens ACT.
G-MW, 2007: Future of the Dethridge Meter . - Goulburn-Murray-Water, Victoria, Final Report, May.
ICID-CIID, 2000: Multilingual Technical Dictionary on Irrigation and Drainage. - CD Version September 2000, International Commission on Irrigation and Drainage, New Dehli.
Kraatz, D.B. and I.K. Mahajan, 1975: Dethridge Meter (Excerpt of Chapter 5.5) [3.3MB!] in Small hydraulic structures. - Irrigation and Drainage Paper No 26/1, Food and Agriculture Organization of the United Nations, Rome, pp. 245-265.
Kraatz, D.B. and I.K. Mahajan, 1975: Small hydraulic structures. - Irrigation and Drainage Paper No 26/1 [49 MB!] and 26/2 [35 MB!], Food and Agriculture Organization of the United Nations, Rome.
Withers, B. and Vipond, S., 1988: Irrigation design and practice. - BT Batsford Limited, London.

(*) Versions


Stein, T.-M., 2008 (revised)

- First published 2003

 




       

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