A tensiometer is a device used to determine matric water potential Psi_m (soil moisture tension) in the vadose zone. The tensiometer consists of a glass or plastic tube with a porous ceramic cup, and is filled with water. The top of the tube has either a built-in vacuum gauge or a rubber cap used with a portable puncture tensiometer instrument, which uses a hypodermic needle to measure the pressure inside the tensiometer. The tensiometer is buried in the soil, and a hand pump is used to pull a p… (More on Tensiometer)