Brigadoon - An Occasional Corner on the Internet
Pipe Selection for the Sensor Array
Designed by Mark Little
Pipe Selection
If you are going to go for the 50cm sensor, then a piece of pipe about 60cm long should be ideal.  If you are going for 100cm, the 120cm riser is suitable.
A pipe diameter of about 20~25mm is ideal as it provides enough room to slide in the sensor array, but not too large that it is difficult to put into the ground. However that may ultimately be determined by the size of the connector mounted on the upper end cap.

If the installation of a connector becomes too difficult, it is possible to run the cable though a hole into the pipe, but the enrty point must be well sealed and the cable must be moisture proof and UV-stabilised so that the sensor array does not need to be pulled out to repair a cable degraded by the elements.

If you can get a piece with threads at both ends, this will make sealing the pipe with end caps much easier later on. A suitable pipe for this task is Garden Rain™ 1200mm or 600mm x 20mm Poly Rural Irrigation Riser, available from Bunnings (for Australian constructors).
Two end caps will be needed later to seal the pipe when it has been installed. For threaded end caps, plumbers tape on the threads will help seal the pipe when the end caps are attached. Additionally, a sealant such as Silastic™  Can be used to provide extra sealing where the pipe is screwed into the end caps.

  Unless you pick a weatherproof connector and socket, they will also need to be made weatherproof, after testing, to ensure that moisture cannot get in to corrode the connector, socket or the sensors.
In both cases a single piece allows enough depth underground, while leaving the upper end cap out of the ground. This is important, since later on, a socket will be mounted on this end cap to allow the cable ton the Raspberry Pi to be connected to the sensor array.