Fly Sparging - why 1" above grain bed?
 

From what I have read, when you fly sparge, you should adjust the infeed of sparge water so as to maintain about 1" (or something like that) of water above the grain bed.

Why not just introduce sparge water as fast as possible (without disturbing the grain bed). Doing so would surely prevent channeling and also mix quickly with existing water in the bed to give a higher yield.

I must be missing something - please advise.

Cheers!

You want to have at least 1" of water above the grainbed. More is fine, and I normally have about 3", give or take, as I find that's best for me as to not disturb the grainbed and avoid channeling.

The reason for not putting it in fast is because the extraction of the sugars works by the property of diffusion- higher SG traveling to the lower SG (water). You don't want to mix with the existing water for that reason.

You

Yooper,
Many thanks for the quick reply!
So, you need to maintain a water level above the grain bed but you don't want to introduce the sparge water too fast because it takes time for the sparge water to extract sugars from the grain.
Have I got that right?
Cheers!

Sort of. You want the level above the grainbed, so you don't have channeling where the water flows through the path of least resistance.

You also want to add the water to the MLT for the sparge at about the same rate you remove it to the boil kettle- so as the water flows slowly through the grainbed it "pulls" out the sugars. You stop when your BK is full, or when you the SG of your sparge runnings is +/- 1.010- because that's when you can begin to extract tannins due to a raised pH.

Oops!! Yooper meant you can begin to extract tannins due to a increase in PH. i.e, above 6.0, not a lower one.Also, don't let the Temp. of mash get above 168F as well. Cheers!!!!

Tannins are not very soluble below a PH of 6.0, or a temp. below 168F.

Yep, total typo. Don't listen to me! You don't want a RAISED pH. Thanks for correcting me! I'll fix my post right now.

Hang on a minute.
I've been re-thinking this.
Why can't you let all the sparge water into the mash (but without disturbing the grain bed), so it is say 12 inches above the grain bed.
Then draining slowly. The only thing governing how fast it drains is the tap at the bottom of the tun.

The advantages, as I see it, to letting in all the sparge water at once are:
1. Less chance of channeling
2. More mixing of the waters.
3. Freeing up the sparge kettle earlier so you can use it as the boil kettle

Obviously I'm missing something because nobody does this.

There are a couple of brewers I know of that do it.

But the things you listed as "advantages" aren't. You won't have less chance of channeling, because when you sparge you sprinkle the water (or put it slowly in with a tube) into the MLT as to not disturb the grainbed at all. "More mixing up of the waters" would NOT be an advantage, and instead a disadvantages because the whole concept works by diffusion (and you don't want the water to "mix").

And sure, you could free up the boil kettle- but is your MLT big enough to hold all of the sparge water AND the mash volume? Mine isn't, but it's only a 15 gallon MLT.