I'm converting from BIAB to a 2 vessel continuous sparge system. I understand that I want to sparge at a rate equal to lautering to keep a few inches of water on top of the grain bed. Should I keep a submerged grain bed even when approaching the target pre-boil volume? Example: assuming I mash with 9.5 gallons of water (approx. 1.5qt/# of grain) and my target pre-boil volume is 14 gallons: QUESTION: Do I continue to sparge to keep a completely submerged grain bed until a full 14 gallons are collected leaving 9.5 gallons of water in the mash tun? (seems like a waste of water....) Or do I stop adding sparge water and drain water through the grain bed so that there is minimal water left in the mash tun by the time 14 gallons are collected in the boil kettle?
You would sparge with enough water to end up with whatever you need for your boil volume. No more, and no less, ideally.
#2 You will have to figure the the calculations for your system. Take into account dead space in mash tun, grain absorption (usually 0.1 - 0.15 gal/lb), expansion for heat, boil off volume. If you overshoot water you will lose efficiency. Not the most important thing but if you want best practice...
You should take things like grain absorption, deadspace in the mashtun, the amount of wort you'll have from first runnings, into account. Then add the missing variable of added sparge water. Ideally, this would equal your preboil volume. I have a hunch that in a real-world scenario for you that you don't want to wait for 3 hours to get every last drop of water through your mashtun. Be flexible your first time and have extra sparge water on hand in case you come up shy on your preboil volume.
The latter option, you'll have to estimate when to stop the first few times, but you will get the feel for it after a few batches.
It´s a bit hard for me to say clearly what I want to say, my idiom barrier plays against me. There are many variables you need to be able to handle in regards continuos sparging: 1. You need to know (aprox.) how much is your evaporating rate when boiling. 2. Knowing the above number you will need to collect more wort accordingly to compensate evaporation while boiling. 3. This number also helps to indicate the place in the mash tun where you can make a level mark indicating a check point of runnings. Using your example of 14 gals of pre-boil volumen you can use a level mark at 12.6 gals.This check point is very useful to take a gravity read to avoid runnings with a gravity lower than 1008 and/or you can take a Ph read to avoid runnings with a Ph higher than 6( both issues could lead to tannins extraction).Once you know you are sparging safely you can continue a bit more then make a second read and so on until you match 1008 gravity or Ph 6 whatever occurs first, then stop runnings. Now is the time you must stir your collected wort to uniform first runnings with last ones and then take a gravity read.You also need to know how much wort you have, 2 possible scenarios : A- Your efficiency was lower than your spected one. Then you decide to keep going this way or you can add malt extract to hit your desired pre-boil OG. B- Your efficiency was higher, then you decide to keep going this way or you can dilute this wort with boiled water to hit your desired pre-boil OG 4. It is important you always keep 1 inch of water atop your grain bed to get good efficiency. 5. It takes time to master these variables, but in a few batches you are going to make it. Tebuken
