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Solution for Temp Monitoring inside MLT

I have finally found a great solution for monitoring mash temp inside my 10 gal Rubbermaid MLT, and I thought I would post here for anyone who has a similar problem.

Background: Originally I used a standard BBQ thermometer with an L shaped probe. I dangled the probe inside the cooler and closed the lid on top of it. My temp readings were unreliable, the mashing process was very frustrating and I ended up with underattenuated brews that I find unpleasant. I believe the two main causes were: the probe didn’t always end up in the grist after I spun the lid closed and I’m pretty sure these are not waterproof.

I wanted a solution that met the following criteria:

  • Ability to read temp without removing the lid
  • No drilling into the side of the cooler (a leak waiting to happen)
  • The hole used for probe insertion can be easily sealed to prevent heat loss
  • Absolute assurance that I know where the probe tip is in the grain bed
  • The probe can be easily removed even during the mash if needed
  • Waterproof probe
  • Reliable accuracy
  • Digital display
  • Can use the thermometer for other purposes

Here is what I cam up with:

Parts list:


  • Drill a hole into the stopper for the rod and probe wire
  • Drill a hole into the cooler lid
  • Cut the rod to the desired length (I wanted it to reach the bottom of the cooler and extend a couple inches above the lid)
  • Insert the probe wire through the hole
  • Insert the rod through the hole
  • Adjust the position of the probe tip in relation to the bottom of the rod
  • Snake the wire around the rod a couple times

I find that the stopper and snaking of the wire keeps the probe tip in place.

Edited to fix link to photos

I should add…this clearly could have been done for less money, but I liked the idea of the Therma K because it came with a free probe that I could use for grilling. The probe is as fast as the highly acclaimed Thermapen, so this was a no-brainer for me.

In a 5 gallon MLT or in a rectangular cooler, this could be done with a 12" dial thermometer. Although there is no way it measures as precisely, it should get you into the right neighborhood.

Since using this configuration, I have achieved 80% and 90% mash efficiency (according to Beersmith) with absolutely no hassle! I was in the mid 60’s before and not having a lot of fun. More importantly, I’ve gotten full attenuation.

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