Do you want to monitor the temperature of your 5 or 10 gallon
Igloo or Rubbermaid (and Home Depot) cooler while you are mashing or
storing sparge water? The Coolview™ thermometer from Brewer's Edge®,
features a cooler-specific 100° to 200° F. Range, and a sliding red
reference pointer so you can set it at mash or sparge water
temperature. The 2" dial features a calibration nut for fine tuning the
temperature if desired, as well as a unique 1 3/4" long threaded
stainless barrel that is 5/16" thick, so it can be installed in thick
cooler walls by just drilling a 5/16" hole. Click on the More Info link
above to see a photo of the back of this thermometer.
To prevent leaks into the cooler wall foam insulation, a
specially designed mounting nut features a machined recess to hold a
small silicone O ring. This recess holds the O ring tight in the nut
when you tighten the backnut, preventing it from spreading and causing
leaks. A great aid if you have a cooler mashtun or sparge water tank.
Click 
here to view Coolview installation instructions.