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Harry Whittle asked about reading gypsum blocks using 24 vac or other AC voltage. We have used gypsum blocks and stainless steel probes with the Turf Tech "WaterWatcher" control unit (not to be confused with the WaterWatcher that uses infrared technology). We use 24vac. One should not use DC to read gypsum blocks, as the direct current can cause migration of cations toward one of the electrodes, similar to the problem Harry discussed earlier with his stainless probes. This causes "polarization" and coating of the electrode and eventual malfunction (manifested as an infinite resistance) of the block. Even when using 24 vac, there can be some polarization effect if there are any ground voltage gradients between the sensor and the control/excitation unit. One can usually place a capacitor in series with the ground leg to overcome this. We've had some gypsum blocks in place that are read continuously using 24 vac, with no ill effects, provided they are protected from ground currents. The benefit of gypsum blocks over stainless probes is that they are not as sensitive to additions of fertilizer in coarse soils (medium and fine soils are not so sensitive). The disadvantage of blocks is that they may not respond very quickly to drying in coarse soils, and they do dissolve over time. Also, in freezing soils, blocks may suffer some cracking and premature aging. We have found that (on medium/fine soils at least), that the stainless probes provide the best response, with only a minor drift in readings with changing soil temperature and fertility (EC) of soil. This is not a problem if the "target" or threshold point for irrigation is somewhere (say half-way) between the "stress" point and field capacity. This way, even though the target point may drift some over time, as long as the water added each irrigation is less than the difference between field capacity and the target point, the rooz zone will hold all of the water added. Stainless probes, in freezing soils, need to be removed and reseated every two to three years or so, if the freezing action creates a void or air pocket between the probe and the soil. This is not a problem if the probes are inserted directly from the soil surface, as we do ours. Rick Allen Biological and Irrigation Engineering Utah State Univ. Logan, UT AllenRic@cc.usu.edu > >I am interested in the effect of applying AC to the gyp blocks on a >continous basis and am wondering what sort of range of resistances are >obtained. Are the blocks in the process of breaking down while the >current is applied such that they cannot be energized continuously? I >am also looking for a commercial source of the blocks. I would run my >own tests but I'm generally lazy and don't want to needlessly repeat >what others I'm sure have already done. >Harry Whittle