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.
This shows the back of the Coolview, with its overall 3 3/4" length and backnut with silicone O ring. To adjust the calibration, the nut at the base of the stem is turned with a small wrench. Installs in any 5 or 10 gallon insulated cooler once you drill a 5/16" hole.
You can be the first to ask a questions.
seems accurate, and leak free so far
Installed this on my 10 gallon Rubbermaid MLT and have done 3 batches so far. No leaks, although it reads about 2-3 degrees high compared to my calibrated thermometer. No biggie. I know its adjustable, but I haven't taken the time to remove, adjust, and reinstall.... Eventually I will, but for bow I just use it to monitor any temp decrease over the period of my mash.
It's okay
The good: it's very easy to install. The bad: it's not especially accurate and it has a limited range. So for calculating the temp of your mash tun before putting in water (necessary for knowing the right temp to add to hit your mash temp) it's not that helpful. Calibrating is not easy-- you have to guess how much your adjustment is moving the needle and it has to be done with the thermometer out of the cooler.
Mash tun thermometer
I was tired of using a thermometer inside the mash tun and it was not reliable. I purchased this item and installed it in my igloo cooler. It was easy to install and keeps an accurate temperature during the mash.
Coolview Thermometer
Very easy to install ( I have two, one for the "hot liquid" tank-sparge H2O and the mash tun, both are orange H2O coolers-5 gal/10 gal respectfully ). Works quite nice. No leaking .
Nice idea that was a long time coming
Purchased this after always wanting something specifically designed for coolers. Didn't work out for me but I can recommend it for others and I can always recommend the great folks at William's.