From the HBD Archive
From: "JEFF CASEY / (617)253-0885" <CASEY@NERUS.PFC.MIT.EDU>
Subject: Re: temperature of mash
Date: 1990-03-03 19:30:00 GMT

>>From: Steve Anthony <steveo@Think.COM>
>>A question for the physicists out there.
>>
>>...I mash in a Le Cruset enamaled cast iron pot that hold the heat
>>very well without having to add heat to maintain a certain temperature.
>>The thing I'm noticing is that when I stir the mash with the thermometer
>>in it, I get a lower temperature than if I don't stir. This is with the
>>bulb of the thermometer as clsoe to the center of the mash as possible
>>and with no heat being added to the pot.

As a physicist and a homebrewer, I'll wave my arms on this one -- by the
way, I've noticed it too.
1) No matter how thick the iron of your pot, it still is a major source of
heat loss. The pot will be at a lower temperature than the center of the
mash, since it is in thermal contact with the room air.
2) The thermal conductivity of the mash is pretty poor, thus there will
be a temperature difference between the mash near the wall of the pot and
the mash in the center.
3) When you stir, you mix all this up, and lower the temperature of the
mash at the center.

Moral:
For uniform mash temperature, stir constantly or use an insulated box.

>>My theory is that there is different thermal conductivity of the grains and
>>water and that when stirring, the thermometer is in contact with the water
>>more than the grains and when at rest it's in contact with the grains more
>>than the water.

Regarding your theory: the grains and water will have different thermal
conductivities, but since the grains are small, I would imagine that the
time scales for grains to equilibrate to the nearby water are pretty
small (seconds or less), so they would be at the same temperature regardless.
In addition, I don't see how stirring will affect whether the thermometer
is in contact with the grains or water.

Hope this help you relax etc. Jeff Casey MIT Plasma Fusion Center
casey@nerus.pfc.mit.edu (best net addr)
or casey#jeff%mfe.mfenet@ccc.nmfecc.gov
or jac@pinet.aip.org
or 70411.573@compuserve.com

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