When installing access tubes, air gaps between the outside of the tube and the soil must be minimised for accurate readings.
The most important thing you can do to ensure accurate readings from the Profile Probe is to minimise air gaps around the access tube. Be careful when augering holes not to create a funnel-shaped hole by waggling the top of the auger, and be prepared to start again if you encounter avoidable stones or roots.
Air gaps can create serious errors in the readings. The Profile Probe is sensitive to the water content of the soil around its pairs of stainless steel rings. This sensitivity is biased towards the soil closest to the rings. Air gaps immediately around the access tube reduce the accuracy of the reading in two ways – firstly the sensors will measure some air instead of moist soil, but also the air gap reduces the penetration of the electromagnetic field into the surrounding soil.
It is virtually impossible to quantify the magnitude of errors from air gaps.
If air gaps are suspected, they can sometimes be diagnosed if the soil moisture content at a particular depth is anomalously low. Also, the reading at that depth may show a diminished range of variation in time, in relation to a known wetting event. However, if saturation and flooding of the soil around the tube occurs, the water content signal may exceed that at neighbouring depths.
After installation of the access tube, air gaps might be expected to close up with natural settlement of the loosened soil. However it is not easy to say how soon, or to what extent, this may occur.