There are tens of thousands of Dethridge Wheels throughout Australia. They were designed in 1910 by engineer Mr John Dethridge, an innovator who was also involved in the Eildon (Sugarloaf) reservoir. For over 60 years the Dethridge Wheel has been used across Australian irrigation systems to measure the volume of water passing onto farms from supply channels. Water delivery authorities/companies are accountable for these volumetric measurements and farmers are billed accordingly.
The Dethridge Wheel has been a robust measuring device, however a combination of maintenance issues, OH&S risks and increased requirements on accuracy have dictated the end of this era. It is likely that the Dethridge Wheel will be replaced with electronic meters that address these issues as well as improve on-farm efficiency by reducing head loss. This will ensure better command and therefore reduce water use.
Paragraph 88 of the National Water Initiative (NWI) requires the development of water meter specifications, water meter installation standards and standards for ancillary data systems. A Metering Expert Group has been established to coordinate this project. The Expert Group provides essential input from the states and territories to ensure that their expertise is included in the specification and standards development. Industry associations, representing both water users and water suppliers, are also members of the Expert Group.
The new standards will place strict requirements on the testing of meters and their emplacements, before and after installation, to ensure accuracy. This is something the Australian irrigation industry has never been required to do. Upon investigation it was recognised that there are no suitable facilities or equipment available to accurately test meters, before installation or during operation in open channel systems.
AWMA felt it was important that these issues be addressed so that new metering standards can be developed, allowing the industry to move forward with wide scale implementation of new meters that improve accountability, customer service and water savings.
As a result AWMA would like to introduce the MagRig.
Designed as a self-contained system, the MagRig is a mobile unit that can quickly, efficiently and precisely measure the volumetric accuracy of metered outlets at open channel delivery points.
The unit is truck-mounted, with a generator to power variable speed pumps. A temporary block bank is established downstream of the metered outlet to be tested. Water levels in the channel are maintained using Ultrasonic Sensors to ensure the test is carried out under true operating conditions. Water is then diverted through calibrated magnetic flow meters, providing a accurate recording of the volume of water flowing through the irrigation outlet.
The MagRig can measure flows from 0.5 - 30ML per day with better than 1% accuracy.