Tensiometer - Quickdraw

ABOUT THE QUICKDRAW

The Model 2900F Soilmoisture Probe is a tensiometer-type instrument that reads soil suction directly. The “soil suction” reading is a direct measure of the availability of moisture for plant growth, and the standard unit of measurement is the “bar”. The bar* is a unit of pressure in the metric system and is used to define positive pressure (above atmospheric pressure), or negative pressure or vacuum (below atmospheric pressure). The gauge on the Probe is calibrated in hundredths of a bar (or centibars) of vacuum, and is graduated from zero to 100. In scientific work, it is becoming customary to express pressures and vacuums in a unit of measure called a “Pascal”, and a “Kilopascal” which is 1000 times as large as a Pascal. A “centibar”, as used above, is exactly equal to a Kilopascal. Therefore, the dial gauge on the Probe also reads in Kilopascals and is graduated from zero to 100 Kilopascals (KPa).

Soil suction is actually created by the attraction that each soil particle has for the water in the soil. Because of this attraction, water forms a film around each particle of soil and collects in the capillary spaces between the soil particles. As the soil becomes drier, these films become thinner and the attraction or soil suction increases. The plant root has to over-come this soil suction, or attraction force, in order to withdraw moisture from the soil. The measurement of soil suction then gives a direct indication of the amount of work the plant root must do to get water from the soil. The only moisture measuring instruments that accurately measure soil suction are those using the tensiometer principle. These instruments read centibars of soil suction directly without calibration for soil type, salinity, or temperature.

When the Probe is inserted into the cored hole, there are various effects associated with the movement of the porous ceramic sensing tip through the soil. The soil surrounding the tip is slightly compacted and the wiping action of the porous ceramic through the soil causes small thermal effects. It takes a few moments for these disturbances to disperse, and this is the reason that it is not desirable to move the Null Knob for the first minute after insertion of the Probe.

In order to obtain a soil suction reading, it is necessary for a small amount of water to transfer between the sensing tip of the Probe and the soil. When the Null Knob is turned clockwise, water is forced out of the Probe sensing tip and into the surrounding soil. When the Null Knob is turned counter-clockwise, a vacuum is created within the Probe, which causes moisture to move from the soil through the ceramic sensing tip and into the Probe. In order to obtain an accurate reading within the minimum amount of time, one must be careful not to disturb the moisture conditions surrounding the Sensing Tip.