Drain Water Sensor; Use - Priva Compass Installation Manual

Priva

Drain water sensor

Use

Drain water is water that is not taken up by the plants and the substrate (for instance mineral wool,
coco peat or Perlite). This excess of irrigation water is necessary to prevent the substrate from
becoming excessively saline as a result of the accumulation of fertilisers.
The Priva Drain water sensor accurately measures the electrical conductivity (EC value) of the drain
water and the quantity of drain water from the substrate. The Drain water sensor is connected to
the process computer.
EC measurement
The EC value is an indicator of the fertiliser-water ratio in the drain water: a high EC value points to
a high concentration of fertilisers, while a low EC value points to a low concentration of fertilisers.
The chemical composition of the drain water is determined by the selective uptake of the fertiliser
elements by the plant roots.
The desired EC value of the irrigation water depends on the crop, the substrate and the composition
of the fertilisers.
The EC sensor is located in the flow tube underneath the Drain water sensor. This tube is a trap
construction, so that drain water is always present in the EC sensor.
Quantity measurement
Based on the quantity measurement of the Drain water sensor, the optimum strategy for supplying
irrigation water is determined.
The drain water derived from the substrate is collected in a drain trough (available separately). At
the end of the drain trough, the drain water flows into the tray of the Drain water sensor. The water
then flows, via a dirt filter, into the flow tube underneath the Drain water sensor.
At the end of the tube, the drain water is collected drop by drop on the tipping ladle. When the ladle
is full, it tilts downwards, thus activating a reed relay. The number of times that the tipping ladle tilts
over is a measure of the quantity of drain water.
The Drain water sensor has a maximum measuring capacity of 3.6 litres per hour.
If the Drain water sensor is processing 3.6 litres per hour, the tipping ladle is continuously
active. This is not a desirable situation. There must be a period between two irrigation cycles
in which the tipping ladle is not active, otherwise the cycles run into each other and it is not
possible to correctly measure the quantity per irrigation cycle.
96 Installing Priva Compass - 00.014