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Irrometer Model Watermark Sensor

Please note:

We recommend the use of two sensors. One sensor at top of root growth and second sensor at base of root growth.


  • Measuring soil moisture tension. Suitable for use with data logging devices.



  • Two concentric electrodes in granular matrix material in a stainless steel case. Matrix material gives buffering the the effect of soil salinity

  • Unaffected by freezing temperatures, does not require removal during winter

  • Can be used for automatic control in irrigation systems (requires interface unit WEM)

  • 0-200 centibars of soil water suction, non-linear, measures full soil water range

  • Electrical resistance type sensor. 500-30,000 ohms resistance

  • Sensor excitation current is 5 vac, 100-120 Hz (square wave) for data logger use only (WDM optional extra)

  • Sensor manufactured from non-corrosive parts. 50mm x 21mm in dimension

  • Built for direct burial. Will not dissolve in soil

  • Can be glued directly to 1.5mm PVC pipe to provide a permanent root depth set point

  • No sensor maintenance. No calibration needed

  • 1.5mm cable lengths 


When and How Much:

Watermark readings reflect soil water tension or suction. The matrix material has been selected to reflect the maximum change of electrical resistance over the growth range of production crops. In operation, soil moisture is constantly being absorbed or released from the sensor. As the soil dried out, the sensor moisture is reduced and the electrical resistance between the electrodes (in the sensor) is increased.


The meter (WDM optional extra) internally converts the electrical resistance reading of the sensors to the tension or suction value. This major physical force of soil water is a direct indicator of how hard the plant root system has to work to extract water from the soil. The drier the soil, the higher the reading, readings are displayed in centibars 0-200 when using the WDM.


Suitable for all soil types




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