The filter sand performs three functions.
First, the filter sand allows fine particles to pass through into the float reservoir, but holds back particles that might block the steel mesh on the base piece.
Second, the sand acts as a wick to draw water out of the soil in the funnel down towards the float reservoir.
Third, the sand helps to empty the float reservoir by wicking water up out of the neck of the funnel and back into the soil.
The stainless steel mesh around the base piece is rated 100 microns (0.1 mm), so particles smaller than this can pass through the mesh and into the float reservoir. For this reason the filter sand should have a minimum diameter greater than 100 microns.
However, the sand particles must be small enough so they can still act as a wick. The strength of the 'wicking effect depends on the size of the pores between the filter sand particles.
The smaller the particles, the stronger the wicking effect. This is important both for pulling water down to the float reservoir during wetting and pulling water up from the float reservoir when drying.
The filter sand should mostly fit into the size range 100 to 400 microns (0.1 to 0.4 mm diameter), with 20 to 50 % of the particles in the 100 to 200 micron range.
You can test whether sand has the right characteristics. Fill a glass or Perspex tube (about 5 cm in diameter and 30 cm long) with dry filter sand and stand upright in a tray. Add about 1 cm depth of water to the tray. The wetting front will be sucked up the filter sand column. It should take less than 30 minutes for the wetting front to be sucked 20 to 25 cm above the water level.
If the wetting front takes a longer time to reach a height of 20 cm, then the filter sand is too coarse.
This experiment can be seen in the photos below. The filter material in the left cylinder in each photo has only 5% in the 0.1 to 0.2 mm range. The filter sand in the right hand cylinder has 20% in the 0.1 to 0.2 mm size range.
FullStops originally were supplied with sand similar to that in the left hand cylinder. Switching to a finer grade filter sand should improve the sensitivity.
Another method is to fill a FullStop with your filter sand to about 1 cm above the lock-up ring. Add water to the funnel until the float pops up. Now add a couple of handfuls of dry sand to the funnel. The dry sand should be able to wick the water back out of the funnel. It should be possible to reset the detector in about 10 minutes.
Because of difficulties with the import and export of sand we are experimenting with different filter materials. For example we have used diatomaceous earth (often used for swimming pool filters) in place of the sand.
Diatomaceous earth has worked well, and may have advantages over sand because it can increase the sensitivity of the FullStop. Diatomaceous earth has a strong ability to pull water from other soils, so can quickly pull water out of the wet soil in the funnel and transmit it to the float reservoir.
We do not know if diatomaceous earth might have more problems with blockages in the long term, since it filters very small particles. It may also have an impact on the composition of the soil water.