Mario Mendes wrote:

> Thanks to all that replied to this.  Of special note was the observation
> that the system behaves like an RC circuit.  Very nicely put, Harold.

Many systems can be put into a U/I/R/C/L analogy, just as other analogies
(water) often are used to describe the behavior of electrical circuits.
You'll find more information about thermal equivalents in some datasheets
and app notes for power semiconductors; sometimes they give the electrical
analogy of the thermal behavior of the chip in the case, to help with the
calculations of what happens thermally when driving the chip with short
pulses (as opposed to thermal equilibrium, which is what you use when you
make those W/K heat sink calculations).


> After doing some research, which led me to finding out what a joule is
> (and watt-hour, calories, erg, BTU, etc)

Stay with joules and watts -- makes your life a lot easier. There's no need
for different energy and power units. 


>  |   |      |   |
>  |---|------|---|
>  |   |      |   |
>  |   | milk |   | 
>  |   |______|   |
>  |              |
>  |    water     |
>  |______________|
>   ^^^^^^^^^^^^^  <- heat
> 
> 1) heat water to the temperature I want milk to reach and then keep
> water's temperature constant until milk reaches the same temperature
> 2) heat water to a higher temperature than I want milk to heat and then
> let it cool down while milk heats up with the heat given off by water
> until eventually both reach the same temperature
> 
> For my purposes, it is important that the center of the milk reaches
> temperature X without any of its mass going above X, and method 1 does
> just that.  With method 2, if not carefully monitored and controlled,
> the mass surrounding the center of milk can raise above X before the
> center reaches X.  

With method 2, if you heat /all/ the water (I assume the water is stirred)
to a temperature above X, the milk /will/ go above X at the borders. Maybe
only a little, but it will.

> Another complexity that I found during my research is that the specific
> heat of the milk will change as time goes by because it will solidify
> (will change from milk into cheese).  This also adds the small problem
> of not being able to keep stirring the milk while it heats up as I can
> do with the water to maximize the heat transfer.

Should've said you want to make cheese in the first place -- that's
different from heating milk. It's different in that it presumably takes
hours (?), therefore the longer time of method 1 may not be a problem. 

If possible, you could think of stirring the milk during the heating phase,
to get up to temp quicker, and then just let it sit, with the water just
providing the necessary power to maintain the temp as the cheese thickens.

Gerhard

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist