Campbell Scientific recommends pn 9922, 20 AWG 1 Twisted Pair Santoprene Cable. Santoprene stays pliable over the environmental temperature range, making it easy to work with in the field. It is also resistant to ultraviolet radiation and will not degrade over time.
Campbell Scientific cable pn 9922 also fits properly in the Heyco bulkhead fittings on the sample intake enclosures. The Heyco bulkhead fittings serve as strain relief for the heater power cable.