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Add Thermometer Probe to HLT MLT Cooler

Sunday, February 15th, 2009

Instructions for adding a thermometer to your hot liquor tank (HLT) or mash lauter tun (MLT). This makes a handy addition, it is cheap to do (~$16), and improves temperature accuracy.

Lets face it, brewing is an art and a science.  Science in itself requires certain controls and measurements to be executed effectively. This is no different with all-grain brewing. To the all-grain brewer temperature measurement and control is of the utmost importance. The following is a simple, efficient and effective way to accurately measure your HLT and more importantly MLT temperatures. The art of building quality brewing equipment is to make it simple, effective and efficient.

This DIY project is targeted at the all-grain brewer who chooses to utilize coolers, whether they be beverage coolers or “cube” coolers, that desire a simple way to measure temperatures inside their vessels without opening them.

Outside view:
thermometer probe brewing beer

Inside view:
thermometer probe brewing beer

Parts you will need:

#5.5 SOLID gum stopper or silicone stopper (Qty 1) – $1

Digital (or dial) probe thermometer with a range of at least 100F – 200F (Qty 1).  Anywhere from $11-$16 on (search for TruTemp thermometer or CDN thermometer).

Tools you will need:

3/8” chuck power drill
1” hole saw
3/16” drill bit
Utility knife or file.


Step 1)
Calibrate the thermometer before installing it.

Step 2)
Locate your hole for the thermometer in a place where you can easily read it. There are a few considerations for placement. Make sure it will be submerged below your water level in your HLT and the strike water level in your MLT. Determine the smallest volume of strike water you will use when mashing in, and place the MLT hole below this level. Why? When you pour your strike water into your MLT, you can monitor the temperature as the cooler absorbs some of the heat from the water and when it reaches your strike temperature, mash in. For a 5 gallon all-grain brew you are looking at 2.5-4 gallons of strike water, for a 10 gallon all- grain you will double that amount to 5-8 gallons. Be sure your thermometer will be below these levels!

Drill your 1” hole here through the wall of the cooler. Once the hole is drilled, be sure to remove any rough edges or hanging plastic splinters with a utility knife or file.

Step 3)
You will need to “drill” a small hole through the center of the solid stopper in order to accommodate the probe of the thermometer of your choice. Utilize the 3/16” drill bit to do this, you will not see much of a hole due to the stoppers ability to seal itself off, but it will provide an opening for the thermometer probe.

Step 4)
Place the stopper into the 1” hole that you drilled in the cooler, from the INSIDE. While holding the stopper in the hole, carefully press the thermometer probe into the hole in the center of the stopper from the outside. Sometimes using a food safe lubricant like keg lube can help slide the probe into place.

Step 5)
Test that the fitting is water tight with warm but not boiling water. As always when doing any modifications with your equipment, test it before you brew again! The worst time to realize that you have a leak is when you are in the middle of a brew session with 150-180F water in all of your vessels!

Congratulations, your installation is complete and you are ready to brew!

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