I have a digital thermometer with a probe. The probe is stainless steel but the cable and probe housing are apparently made of PVC. In my last brew I just submerged the whole probe in the boiler ...
I'm making a late hopped extract beer later tonight. (Jamil's Evil Twin as posted http://www.mrmalty.com/late_hopping.htm) He suggests at 90 minute boil with the first hop addition coming 20 minutes ...