Mulv is correct, the
oil temp, water temp use a closed copper capillary tube with a bulb at the end that is in direct contact with the
oil or the water. The fluid inside the sealed capillary expands and contracts with the rise and fall of the temp of the
oil or water. I bought mine from:
Nisonger Instruments
http://www.nisonger.com
225 Hoyt Avenue
Mamaroneck, NY 10543
Phone: 914.381.1952
Fax: 914.381.1435