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Trevor Finch <support@rsne.com.au> writes Irricrop Technologies in Australia have done some comparisons between Theta probes and neutron probes Steven Laird has the full results (but is at the Cotton Conference in Queensland at the momment...) Contact him next week at <steve@irricrop.com.au> for a full description. One trial used 4 Theta probes installed at different depths down to 800 mm, installed in separate 45 degree access holes, with an adjacent neutron probe in aluminium tube. Using the Theta probe manufacturers calibrations, and the Irricrop Technologies 'standard' neutron probe calibrations, the absolute volumetric soil moisture agreed to about 2% VSW (eg 38% and 40% ) Changes in soil moisture tracked 'exactly' (an offset on the graphic output was required to distinguish the 2 curves) This trial was done in heavy cracking clay. During a drying cycle, as the cray cracked, the Theta probe started to read low. Like other contact gauges, this was possibly caused by air gaps. A similar trial by Xavier Ruis in South Australia ( <xrius@catworld.net> ) on lighter soils, using the same installation techniques, gave similar results, but without the cracking clay errors. A test by Cape Irrigation Consultants in South Africa in flowers, with controlled volumes of applied water, showed 'excellent' results comparing delivered volumes with measured changes in soil moisture - see Graham_Hill<aphill@globalnet.co.uk> for more details. A dedicated logger ( 1 & 2 channel) is available for the Theta probe, enabling a totally self contained system (no solar panels) that will log for about 12 months. ---- Trevor Finch Research Services New England 8/18 Nicholson St, Balmain NSW 2041 Australia email: support@rsne.com.au tel: +61 (2) 9810 3563 fax: +61 (2) 9810 3323 ----