Since the pot is only so big I need to figure out just how much water I can fit in there with my grain bill. Once we know the strike water volume we can figure out the strike temperature based on the grain temperature, grain mass, and desired mash temperature.
One important thing to note... grain does not absorb water instantly. Therefore if theoretically your pot can hold 5 kg of grain + 13L water, if you just dump your grain in it will overflow. Because of this I usually understate the volume of my boil kettle a bit.
I am attaching my python script I wrote that does my calculations. Note that I also use Beersmith 2 but there is no way there to figure out my max water volume based on my BK volume and grain bill. My program does not account for heat loss in the kettle either. That should not matter as it is pre heated. Also, I know from doing math on the diameter and depth of my pot that it holds 18.7L and for every centimetre of depth it holds 0.706 litres. This kind of calibration will only work on a flat walled kettle or pot, not a keg with wavy walls.
Ok, for a grain bill of 5.450 Kg and my 18.7L pot. I am looking for a mash temperature of 154F and the grains are currently 68F. I tell it to use a total volume of 17.5L because it takes a bit for the grain to absorb the water and for the air to bubble out:
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$ ./brew_calc.py --target_temp=154 --grain_temp=68 --grain_mass=5.45 -t F --total_vol=17.5 --vol_factor=0.706
The max strike water you can use for 17.50 L with 5.450 Kg of grain: 13.848 L at 2.541 L/Kg
That's 19.62 cm deep in your brew pot.
The strike water should be heated to 167.54 F for a target temperature of 154.0
Mash In by Hot Pepper Daddy, on Flickr
I cover, lay a towel over the top, and monitor the temperature. On my last brew, even with the reflectix wrap, I had to add heat to keep the temperature up over an hour. No biggie, and I cought it in time.
I stir my mash a couple of times as it's pretty thick and we need as much efficiency out of this method as possible. I did a mash out last time, raising the temperature to 168F while stirring. I lift the bag and allow to drain through a colander over the pot then squeeze the crap out of it while wearing rubber gloves. We need that sweet wort! I'll then do a small rinse sparge of 168F water from my kitchen tea / coffee kettle (only one litre or so) and squeeze some more. I then move the colander and grain bag to drip over another pot. Oh, by this time I've already added my first wort hops.
I now bring it to boil as usual. The water volume is now around 11L. Before boiling I take volume and gravity readings to determine the mash efficiency. MY efficiency here is between 55-60%. My last brew was 60%. I actually add some boiling water from my kitchen kettle as it's coming to boil to increase my volume some. My glass top stove can't handle a full boil.
Before the boil is over I toss in my immersion chiller to sanitize. When the boil is over I move my pot from the burner and Immediately top up the kettle with cold tap water, adding probably 4-5 litres. This gives a nice kick start to cooling the wort. I then finish chilling and drain into my fermenter. I top up the fermenter my pouring a bit more cold tap water through the pot / spigot. Aerate, pitch yeast. Clean.
Please note my program requires a python interpreter to run. I use linux so I have it. You can run under windows if you have python installed. I was thinking about writing a javascript version but my webserver is down. I may do that down the road though.
This could be used to figure out volumes and temp for keggle BIAB batches as well, even for 10 gallon batches:
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$ ./brew_calc.py --target_temp=154 --grain_temp=68 --total_vol=55 --grain_mass=11.2 -t F
The max strike water you can use for 55.00 L with 11.200 Kg of grain: 47.496 L at 4.241 L/Kg
The strike water should be heated to 162.11 F for a target temperature of 154.0
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$ ./brew_calc.py --target_temp=154 --grain_temp=68 --total_vol=55 --grain_mass=11.2 -t F --strike_vol=45.1
Your mash thickness with 45.100 L of water and 11.200 Kg of grain is 4.027 L/Kg
The strike water should be heated to 162.54 F for a target temperature of 154.0