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Continuous Sparge Question

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?

I usually stop my sparge water and try to run out of runnings as soon as the pre-boil volume is reached. Otherwise, I might be diluting the runnings further.

Technically, yes, you should keep the grain bed submerged until pre-boil volume has been reached.

When fly sparging the less dense water sits on top of the denser sugar water pushing the sugar water down and out of mash tun and at the same time washing (picking up) the remaining sugars of the grain.

It’s important to maintain this relationship until the wort being washed out of the mash is no lower than about 1.008 - 1.010, the pH starts to rise or pre-boil volume is reached.

Having said that, most homebrewers do what Greg does and only sparge with enough water to reach pre-boil volume. Unless you’re looking to devoid the grain of every last sugar and starch molecule it’s probably not worth the extra time, effort, resources, money, etc…

The pro brewers I’ve worked with all calculate their sparge water volume to allow the mashtun to drain dry just as the pre-boil volume is achieved in the kettle - doing otherwise is a waste of water and the energy to heat it plus the weak wort becomes wastewater which is an added expense.

This is my strategy on the homebrew level. I want to empty the tun as much as I can at the end of collecting wort. I usually have some weak wort left in the tun, as it this is hard to get perfect, but I get pretty close.

I apologize for taking this slightly off-topic, but to the OP, if you are in process of designing your system, have you considered batch-sparge instead?

I switched from continuous to batch several years ago, and am glad I did.

It’s easier, more forgiving (don’t care about channeling or stuck sparge), a little bit quicker, and darn near same efficiency (I get 83%).

And, to your original question, with batch sparge you always drain the bed dry.

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