They are thermocouple units that run off the Pu decay heat. Zack Clayton Ohio EPA - DERR email: zack.clayton@epa.state.oh.us voice: 614-644-3066 fax: 614-460-8249 ************************************************************************ The RADSAFE Frequently Asked Questions list, archives and subscription information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html