HI,
A lot of people are using these in Aquaponic, Aquarium and other applications:
http://arduino-info.wikispaces.com/Brick-Temperature-DS18B20#TS
These use dual-layer heatshrink sealing and are good at reasonable temperatures (say < 75C)..
The supplier says:
- High-quality stainless steel anti-rust waterproof package (Exact SS Type is not known)
- Stainless steel housing (6 * 30mm), overall length to end of wire 100cm
- Exposed straight round Stainless Steel section about 19mm before rounded end start
This is about .236 inches diameter and one user is putting these into a 1/4 inch compression fitting screwed into a tank. I don't have results from him yet.
DISCLAIMER: Mentioned stuff available in my own shop...