Dear Mr. Wizard:

What's the best way to check the temperature of a mash? I use a five-gallon, round picnic cooler (Rubbermaid). The first time I did an all-grain brew, I followed "The New Complete Joy Of Homebrewing" by Charles Papazian (Avon Books, 1991). It says to raise the temperature of the water about 12° to 17° F above the desired mash temperature. I did this and the mash temperature dropped to 144° F; I was targeting a temperature of 152° F. I checked it with a dial thermometer. I also used 170° F sparge water. What is the temperature drop of sparge water?

Gilbert Korrubel
The Hague, Netherlands

Mr. Wizard replies:

The best way to check mash temperature is with a calibrated-dial or alcohol-filled thermometer. The easiest way to calibrate a thermometer is by filling a glass with ice cubes and then filling the glass with water. In a few minutes the water temperature will drop to 32° F. If the thermometer is a dial-type, it most likely has an adjustment screw or the face can be rotated. Simply adjust the thermometer so that it reads 32° F (0° C). Alcohol-filled thermometers cannot be adjusted and are typically more reliable than dial thermometers because they are calibrated when made and don't change over time. The rule of thumb printed in "The New Complete Joy of Homebrewing" works well if you use 0.79 gallons of water per pound of grain (3 liters per kilogram). If the mash tun is at the same temperature as your target mash temperature and the malt is at room temperature, it works out so that 12° F added to the water temperature is perfect. For example, if you want a mash temperature of 150° F and are using 10 pounds of malt at 68° F (20° C), then 7.9 gallons of water at 162° F works.

Your problem could be the result of several factors. To begin with, your thermometer could have been out of calibration. If the malt you used was cooler than 68° F, then the malt will cool the water down more than the 12° F rule allows. The easiest way to avoid this problem is to store your malt at room temperature or bring it into a warm room the day before you plan on brewing. Finally, the cooler also cools the mash down. I pre-heat my mash tun to compensate for that particular problem.

The sparge water cools as it flows from the hot water pot to the sparge device, but it will cool much less than 12° F because it's not being cooled by a large mass of malt. The best way to reduce heat loss in sparge water is to minimize the distance from the water pot to the sparge device. A typical temperature drop is 2° to 5° F.