Good morning-
Shoot, almost anything can be used. I would suggest
PVC sprinker pipe, and/or schedule 125. There are
threaded pipe adaptors with end caps that can be easily
used for this purpose. The circuit board in question is
placed inside the pipe, one end of the the pipe is glue
capped, the other with the treaded adaptor. The wires
through the treaded end cap with silicon water barrier.
Screw the whole thing together and stuff it in the
ground! There might be a time lag between the surrounding
soil and the temperature probe, but not much. Any
temperature rise between the existing electronics and
the outside soil should be negliglable, however, if not,
then the probe outside the pipe.
A quick trip to the hardware store and a tad bit of work
with a drill press or hand drill and you are ready to
rock and roll. Or, perhaps soil and roll?
Cheers,
Rich S.