A gypsum block sensor is a very simple and low cost device. Basically it consists of two electrically separated electrodes cast in a block of gypsum (Plaster of Paris). You can buy them on-line or make them yourself using the guidelines below. Because of their low cost, you can install many as you like in the vineyard.
The electrical resistance between the electrodes is a measure of soil moisture level, or better: the soil water potential. The gypsum acts as a buffer for saline solutions making the measured electrical resistance of the sensor less responsive to salt in the soil.
- Limited life, the gypsum decomposes and the sensor last only a few seasons
- Hysteresis, the sensor quickly responds to a moisture increase, but is slow to reach equilibrium with the surrounding soil under drying conditions
- Temperature dependency. The resistance varies over temperature, however this effect is not as strong as resistance variation over soil moisture.
- Resistance can only be measured using AC voltage. There have been efforts to measure DC resistance with multi-meters, but this fails because DC current causes electrochemical effects in the sensor. The description of a simple to build sensor reader will follow in a later posting.
The pictures below show the simple setup: use stainless steel machine screws (use #6-32 x 2") to prevent iron oxide penetrating the gypsum. Adding a piece of plastic window screen mesh helps improve the structural integrity of the sensor, extending its useful life. The electrodes are fixed before pouring the Plaster of Paris by plastic spreaders. The dimensions of the sensor are not critical, 1” diameter cylindrical by 2” length, but using our example dimensions will help get comparable resistance results. Isolate the terminals with liquid tape of something alike to avoid an electical path outside the sensor after connection of wires to the sensor terminals.
Quick coarse calibration can be done by soaking the sensor in distilled water, and let it air dry while measuring the sensor weight with an accurate scale and noting the resistance values over time.
In practice, soil moisture is good when measuring below 1 kΩ. Values well above 1 kΩ tell you it’s time to open the drip irrigation valves.
The deeper the sensors are installed, the longer the response time after irrigation start. At 30" depth, there is no measurable response after one full night of irrigation. Therefore we now measure once a week.
Wildlife destroyed our electric sensor cables within one week (!!) after installation. We now have PVC tubes to protect the wires. So far that seems effective.
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