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NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za with the body of the message containing the line: unsubscribe sowacs >The access tubes had to be plastic as metal tubes would interfere with >the electromagnetic equipment we use at the site. I am seeking advice on >calibration techniques. I am looking at moisture changes in short time >period: the leak is relatively high magnitude and so tests only last ~ >2-4 hours because the ground becomes saturated). Therefore I wondered >what people thought about the importance of calibration when the >difference in control and leak data is of primary concern to my work. A check on the stability of a neutron probe can be done by taking readings in the same tubes installed in a 200 l litre drum of water. The average of 8*16 sec counts with a CPN 503 should not vary by more than 3-400 counts between tests. The water drum count also allows cross-correlation between different tubes and different probes. An equation that works for many soils, and shows reasonable agreement with measured amounts of rainfall and irrigation, is... Volumetric Soil Moisture = (Count/WaterDrumCount) * 0.68 The slope (0.68) is dependant on soil type, and published research show it varying from 0.65->0.8 There will also be a 'constant', but if you are looking for changes this will not be as important ------------ Trevor Finch Research Services New England 8/16 Nicholson Street Balmain, NSW 2041 Australia T: +61 (2) 9810 3563 F: +61 (2) 9810 3323 mailto:support@rsne.com.au http://www.rsne.com.au --------------------------