Features needed

• weatherproof
• precise to about +-1°C
• easy to handle
• temperature probe mounted to a > 1m lance

What I built

I didn't have time to create a PCB, solder everything etc. so I bought a off the shelf thermometer with a 3m cable DS18B20 probe. The probe is glued into a Ø10mm stainless tube, so it can be pushed right into the compost and stay there. The glue itself creates a nice chamfer. The stainless steel probe has no metal to metal contact to the outer tube. Thats important, as the huge mass of the tube would need much longer to get to the same temperature as the compost, thus delaying measurements a lot.

The case is a IP65 case with a clear top part. I glued the display right into the clear top, added a switch to the bottom part (with a waterproof cap).

The switch does simply connect the unit to a 9V battery (mounted with velcro glued to the case), no fancy circuitry. Mounted to the lance it is easily operated by your right index finger, while grabbing the case. The sensor cable goes out through a threaded waterproof connection.

In total cost was about 45€ for all parts. I could've bought a cheaper thermometer unit, but this one had a long enough cable and the right precision. Anyway, altogether this unit is fully waterproof for a price about 200-250€ below a commercially available one!