I use a formula that takes into account the following:
the grain bill
the grain temperature
the volume of water
the required mash temperature
I use a chiller box which is small by comparison to your behemoth of a mash tun, but the calculation will take all of this into account.
The method I use is to stuff a thermometer into the grain, leaving it for several minutes to stabilize.
I pour the water into the mash tun/coolbox at a temperature somewhat higher than required, close the lid, and let it stabilize. The formula I use to calculate the correct temperature is this:
Strike Temperature = (Tm * (Vw + (0.4 * Wm)) - (0.4 * Wm * Tm))/Vw
Where
Tm = required mash temperature (deg C)
Vw = the volume of water in the mash tun (litres)
Wm = the weight of grain (Kg)
by the way, the 0.4 is the specific heat of malt.
I know it looks long-winded, and it is, but I use a spreadsheet to record all these figures and to write the formula, which I can then copy down to all the other cells in the column. This removes the pita element from it.
so I do this calculation in the spreadsheet, which gives me the strike temperature - in my case this is around 72 C. So I put my 12 litres of water at around 75 C into the mash tun/cool box, and let it stabilize. If it settles above the required strike temperature, I simply open the lid for 5 minutes, and check again. If it's too low, I take out say, 2 litres of water, and replace it with 2 litres from the boiler, stir it in, close the lid, and check after a couple of minutes.
I know your operating on a completely different scale, but this works well, regardless of size. It also removes the need to work out the thermal mass of the mash tun, since this is dealt with in an empirical fashion. I'm sure you could also calculate it's thermal mass and factor this into the equation, too.
Good luck with it, and please let me know if you use this method, and if you do, how it pans out.
On the basis of an 80 litre mash tun, I would guess that you might use 40 or 50 litres of mash liquor, and assuming about 3 litres of water for 1 kg of malt, I'm guessing that you'll be using 17 to 20 kg malt, which in turn suggests about 200pints - wow! How often do you do that?!!**%$£. Do you have lots of good friends, or do you need to see someone about it, or could it be a brew pub? Mmmm.....
Do well, do well
Mike
