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[Apologies: my comment placed at the beginning of the "Standardising Sensors made it appear as if I (Bruce) had originated it: it was in fact a post from (Tom A Reynolds) who put forward the idea.] Gregg Dill, P.Eng. writes Bruce; In response to your initiative, "There seems to be some lack of attention to the notion that sensors must be standardized. By this I mean, a benchmarking methodology which isolates sensor performance."... I think I know where you are coming from (it is only one point in the Universe). This does not solve the real problem of determining true volumetric moisture content which is required to calibrate any of our fancy/expensive instrumentation. There is no shortcut for field calibration if your objective is to determine actual soil moisture. I realize this is a different topic than you raised, but its related and close enough! A far more important issue is how to measure bulk density accurately to convert gravametric moisture to volumetric moisture in the process of calibrating any instrumentation. The next issue, relating to using instrumentation for irrigation management, is the determination of field capacity and to a lesser degree, permanent wilting point. Everyone knows there are no lab methods to satisfactorily determine this number. It takes a lot of work to determine it in the field, especially if several sites are involved, but in my mind it is the only alternative. The real question is 'does basic neutron theory support using it to determine absolute soil moisture content to the accuracy required for research?' It seems we spend as much time trying to justify the numbers we collect as we spend on researching the stated objectives. We have done field calibration based on textural analysis. To date we have chosen to select the most appropriate of three calibration curves based on the percent of sand in the soil. For soil moisture monitoring we characterize a site, select the appropriate curve, collect Neutron Probe data, Ratios, in the field and bring it back to the office for analysis. This approach eliminates the potential of selecting the wrong calibration curve, in the field, for a particular site. Our next foray may be to investigate the sensitivity of Neutron Probe and TDR readings to bulk density! Gregg Dill, P.Eng. At 06:15 PM 3/12/98 GMT, you wrote: >There seems to be some lack of attention to the notion that sensors must >be standardized. By this I mean, a benchmarking methodology which >isolates sensor performance. Example: Take 500 liters of # 50 quartz >sand, add 300 liters of deionized water, set the sensor under evaluation >in the center of this rounded or squared container full of media, and >take readings for five days, twice a day. Do this with all methods, most >notably the neutron probe, which has been standardized in a 500 liter >barrel of tap water. If the neutron probe is not determined to be >anappropriate umpire (benchmarking) methods, then show me the data sets! >Nobody is showing anybody any of their standardizing data sets. Why? > _____________________________________________ > Gregg Dill, P.Eng., Irrigation Specialist > Alberta Agriculture, Food and Rural Development > Bag 3014, Agriculture Centre, > Lethbridge, Alberta, Canada T1J 4C7 > Telephone +1 403-381-5136 FAX +1 403-382-4526 >____________________________________________