Additional Notes - UMS HYPROP Operation Manual

Additional notes

Influences on the measuring range
Three factors limit or extend the measuring range of the tensio shafts:
the air entry point
•
the water vapor pressure (boiling point)
•
the boiling delay.
•
The air entry point of the tensio shaft
This value is specific for a porous hydrophilic structure and depending on the
contact angle and the pore size. The air entry point of the UMS tensio shafts is
about 8.8 bars so it does not limit the measuring range.
Water vapor pressure
At a temperature of 20°C the vapour pressure of water is 2.3 kPa above vacu-
um. This means: If the atmospheric pressure is 100 kPa at 20°C the water will
start to boil or vaporize as soon as the pressure drops below 2.3 kPa (= 97.7 kPa
pressure difference). At this point the measuring range of the tensio shafts ends.
Please note that the atmospheric pressures announced by meteorological
services are always related to sea level. However the true atmospheric pres-
sure at an elevation of 500 meters above sea level is for example only 94.2 kPa
(although 100 kPa are announced). In this case the measuring range at 20°C
is limited to -91.9 kPa. Even if the soil gets drier and drier the tension shown by
the readout will remain at this value. But as soon as the bubble point is rea-
ched a spontaneous compensation with the atmospheric pressure occurs.
Then air enters the tensio shaft and the readout will rapidly drop to zero.
Osmotic effect
The ceramic has a pore size of r = 0.3 µm and therefore cannot block ions.
Thus, an influence of osmosis on the measurements is negligible. If the tensio
shaft is dipped into a saturated NaCl solution the readout will show 1 kPa for a
short moment, then it will drop to 0 kPa again.
82 | Theory