Throw them away!
As we begin to prepare for the up coming irrigation season here are some things to consider. Watermark soil moisture sensors are probably the most widely used sensor here in the Delta. We are now into the fifth or six year utilizing this technology to trigger irrigation scheduling. Watermark sensors have a useful life of approximately five years per Irrometer’s web page. Considering the time and effort it takes to replace a bad sensor after installation, dated sensors should be discarded and replaced with new sensors. The MSU Irrigation Team routinely inscribes with a permanent marker the date the sensors are put into service. This allows us the opportunity to discard old watermark sensors before they become a problem.

Pre-conditioning of the Watermark sensors is a must!
Watermark sensors must be pre-conditioned prior to installation. To pre-condition, slowly wet the sensors by partially submerging them (no more than half way) in water for 30 minutes in the morning, let dry until evening, wet for 30 minutes, let dry overnight, wet again for 30 minutes the next morning and let dry again until evening. Soak over the next night and install WET. Sensors must be installed wet. Pre-conditioning the sensors ensures quick response to changing soil moisture conditions. If a sensor is only soaked and then installed, several irrigation cycles must pass before it will respond accurately. Link to Complete Article