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Watermark Smart Interface

The WATERMARK (granular matrix) sensor is an indirect method of measuring soil water. It is an electrical resistance type sensor, that converts the electrical resistance reading to a calibrated reading of centibars of soil water suction. The Watermark sensor operates on the same principle as the gypsum block, but incorporates a punched stainless steel outer cover over a membrane that is designed to provide greater longevity of the sensor in harsh conditions.

Principle of measurement

The Gypsum Block is a cylindrical block of gypsum (CaSO₄) into which two electrodes are inserted. The gypsum is porous and allows water to move in and out of the block as the soil wets (under irrigation or rainfall), and dries (as the plant draws water away). In the presence of moisture the CaSO₄ goes into solution, allowing ion movement between the two electrodes. When the block is excited electrically, ions move to the respective electrodes, establishing an effective block resistance. A greater amount of water in the block equals more ions and a lower resistance to electrical current flow.

Microprocessor interface

ICT International has designed a Plug & Play Smart Interface specifically for use with the Watermark sensor. The interface consists of a microprocessor that provides a regulated AC voltage to excite the sensor, holds resident in memory the calibration equation to automatically convert the electrical resistance to kPa of soil water suction, and allows up to four Watermark sensors to be logged at a user defined interval.

Flexibility of experimental design

Typically, the Watermark sensor is used with a hand meter, and read manually. This can be time consuming and limit the application of the sensor. By interfacing the sensor with a microprocessor, it can now be continually logged with an SL5 Smart Logger, and incorporated into a wide variety of research projects involving a range of other sensors all on a single logging system.

Related Documents

	Specification Sheet	170K PDF
	List of References

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