Inside the hypodermic needle are a thermocouple junction and a resistance heater. A rigid plastic body attaches the hypodermic needle to the ceramic matrix and secures the thermocouple and heater wiring. A sketch of the sensor is shown in the following Figure. The sensor operates on a heat dissipation principle. A heat pulse is generated by supplying an electrical current for a short time period to the resistance heater inside the sensor.
Heat is dissipated by water held in the pore spaces.
The temperature rise resulting from the heat pulse and subsequent dissipation of some of the heat is measured with the thermocouple junction inside the sensor. If the pore spaces contain much water, a large amount of heat can be dissipated and the temperature rise will be small. If the pore spaces contain little water and mostly air, the air acts as an insulator, little heat is dissipated, and a large temperature rise results. A calibration curve relates water potential to temperature rise. The calibration curve returns a water potential value based on the temperature rise reported by the sensor.