General Informations

Tensiometers measure the so-called suction tension.  In dry soil, the permeable ceramic tip of the tensiometer transports water outwards, creating a subpressure in the sealed tensiometer tube. This subpressure is an indication of the moisture level and can be determined as a particular value or used directly to activate a switch. The traditional unit of measurement is hPa (hectopascal): 1 hPa = 1 mbar = 1 cm water column.
The suction tension is the force with which the water is retained in the soil or is available for plants. The fine pores and capillaries in the soil play a critical role in this factor. A tensiometer measures this important soil characteristic for plants directly. It does not need to be calibrated as a measuring instrument. This is a particular advantage in contrast to electrical measuring devices.
The suction tension increases as long as the surrounding environment is drier and the substrate is able to continue diffusing water. When the surrounding moisture level has increased to a certain point, the process reverses.
An extremely close contact between the tensiometer and the substrate is a prerequisite for a quick reaction time and for the recording of values typical for particular soil and substrate types. However, a tensiometer can also function in coarse or very loose substrate. The minimal contact surface and the larger proportion of hollow spaces also produce suction tension values specific to this situation. Suction tension measurements are largely independent of soil salt content. False results can be caused by large temperature differences, in particular when the air volume in the tensiometer is too high.

Tensiometers use up very small amounts of water during the measuring process, especially when measuring in dry ranges, because 100% of the water does not re-enter the tube. Continuous measurement in dry substrates, therefore, results in a gradual depletion of water from the tensiometer tube. For this purpose, tensiometers with larger water volumes are available to provide a longer standing time, requiring only minimal refilling. Small tensiometers are predestined for use in moist ranges, where maintenance is minimal. The measuring range of the tensiometer is from 0 to about -900 hPa. Suction tension values typical for beginning irrigation in various substrates and soil types lie in the range between 100 - 500 hPa.
Tensiometer measurements are principally a punctual measurement. The ascertained value is applicable only for a particular measuring location. The usefulness of measuring suction tension is shown when measurements made at many locations are compared, and when series of measurements are depicted over the course time. Single measurements often have little predictive value. Also, controlling irrigation using suction tension values requires continual measuring, preferably from one typical, reference location.
The new maintenance-free sensor, TensioMark, enables suction tension measurements to be made beyond the range of a tensiometer. Each suction tension value is determined exactly by using the thermal capacity of a reference body (here, the ceramic tip) that is heated through pulsation. The TensioMark is not influenced by external soil factors and functions year round.