The Stevens pF Sensor measures the "matric potential" of the soil. Also call "soil water potential", a soil's matric potential is represents the force with which water is held within the soil and how much energy is needed to overcome this force to extract water from the soil.
The matric potential of a soil is the pressure it takes to pull water out of soil and increases as the soil gets drier. The matric potential changes with the soil water content and is highly variable from one soil type to the next. Matric potential is usually expressed in units of pressure. The Stevens pF Sensor gives readings in “pF units”. The unit of pF is the log of the pressure in Hectopascals (hPa). For example, the log of 10,000 hPa is equal to 4 pF (1 hPa = 100 Pa).
The Stevens pF Sensor can be used in conjunction with the Stevens Hydra Probe II soil sensor to measure a wide variety of soil parameters at one time.
The pF Sensor is a heat capacitance probe. The soil moisture equilibrates with the ceramic tip that contains a heating element and temperature sensors. The sensor calculates the matric potential from the heat capacity of the ceramic tip.
A pF unit is the log of the soil tension in hector Pascals (hPa). The Stevens pF Sensor accurately measures soil matric potential thermodynamically, not directly by pressure. This means the pF sensor can provide high-accuracy measurements in extremely dry or highly saline soils.
Stevens pF Sensor Specifications (see datasheet for full specifications)
Measurement range: 7 pF to 0 pF (10,000,000 hPa to 0 hPa)
Measurement Range Resolution: 0.01 pF
Power Requirements: 7-13 VDC
Current Draw: Active: 55mA (5 seconds per reading), Quiescent: 2mA