I use a bigger keg now for my MLT, but when I was doing 5 gallon batches, I used a 10 gallon round cooler (I still have it and use it occasionally).
I would preheat the cooler by adding my strike water at 180 degrees. Any higher, and the cooler warped a little (oops). Let it cool to my strike temperature, about 11 degrees warmer than my desired mash temperature (usually 164 degrees), and stir in the grain.
The cooler holds mash temperature very well (I lost something like 1 degree per hour) doing it this way.
Ditto about preheating and starting approx. 10-15 degrees high. However, I also keep a gallon of cold water on hand. If my mash temp is too high after I dough in, I slowly add cold water, stir and recheck the temperature until I hit my perfect mash temp. I don't worry about the extra volume of water and have never had any problems doing it this way.
I just transfer water at 172-174F over to the cooler MT, let it sit for at least 10 minutes, then dough in. I hit right in the range I want every time; although I'm one of those mavericks who doesn't give a shart if my mash temp lands at 151 or 155.
I missed temps pretty badly a few times, but experience pays off. I keep good records and can usually interpolate the available data to get very close to where I want to be based on grain/water ratio.
I'm not a fan of adjusting with additional water. I usually just go with whatever I get, unless it's way off.
One thing to note, it takes much less volume to drop the temperature than it does to raise it. This is due to the fact that the "cold" water addition will be about 100 degrees colder than the mash temp, whereas the water in the kettle is probably only about 20ish degrees hotter. The smaller temperature differential means you need more to make a given change.
I dumped 3.5 gallons of 180F water into my 10 gallon Igloo cooler and let it sit for 10 minutes.
Checked temp of water and it was at 161.5.
Target mash temp for 8.75 pounds of grain + ~4 cups of rice hulls at ~65F is 152F.
Douhg in grains and stirred well and temp was 145F!
This makes no sense to me.
Any ideas?
I added 1 gallon of boiling water to get temp up to 152.
I think the problem may have been with my thermometer. During the boil it read lower than 212 (i.e. 209) and I calibrated it before the brew. I don't trust it so I ordered a Thermopen.
I use a bigger keg now for my MLT, but when I was doing 5 gallon batches, I used a 10 gallon round cooler (I still have it and use it occasionally).
I would preheat the cooler by adding my strike water at 180 degrees. Any higher, and the cooler warped a little (oops). Let it cool to my strike temperature, about 11 degrees warmer than my desired mash temperature (usually 164 degrees), and stir in the grain.
The cooler holds mash temperature very well (I lost something like 1 degree per hour) doing it this way.