From: Trevor Finch (rsne) To: sowacs@aqua.ccwr.ac.za Date: Saturday, June 22, 1996 7:00 am Subject: SOWACS - cheap soil moisture sensors Marty Grogan asked about the relative importance of price andaccuracy. A very cheap 'switching' type sensor has a role in automaticcontroller systems (although the switch point would have to be adjustable),but I think that a sensor for commercial irrigation scheduling needs to have arange of outputs. It does not need to be linear, but it does need... 1. No hysteresis (same reading whether it is getting wetter, orgetting drier) 2. A calibration that does not involve site-specific calibration. The equation could be different depending on soil type, although thiswill give interpretation problems with duplex soils. It should be possibleto install the sensor(s) and take a measurement, with no site calibration 3. The slope of the equation is far more important than anyconstant term. Commercial users are looking for *changes*, not absolute. 4. The calibration should not change with time, water quality,soil structure or anything else, over an installed life of severalyears. It should be just sensitive to changes in soil moisture. 4. It should be able to be installed reasonably easily intoundisturbed soil. 5. It should be able to be installed and measure at variousdepths. 6. It should measure a sufficient volume of soil that readingsare not unduly influenced by preferential drainage paths caused by sensor installation (eg water running down tubes or cables), or by localroot activity. An analogue (that is, not 'switching') sensor that had acalibration that varied at each site would be acceptable if it was cheap. As longas the output didn't change with time, each site could have a 'full' and'refill' point identified that might not mean much in absolute terms, butwould allow for scheduling. Actually a very large cost is data handling and interpretation,which is often forgotten when costing. Excellent data without regularinterpretation is obviously a waste. ---- Trevor Finch Research Services New England 8/16 Nicholson St, Balmain NSW 2041 Australia email: rsne@mpx.com.au tel: +61 (2) 810 3563 fax: +61 (2) 810 3323