A Different Efficiency Issue

Hi all,

Longtime software user, first time poster.

The past couple of brews, I've been using the 15 gallon Boilermaker equipment profile (that's the equipment I have, except for a 20 gal boil kettle), and fly sparging with the recommended volumes that the software puts out there for me.

Today, I checked the runoff gravity when I had collected 12 gallons, and it was already at 1.005, when I was supposed to still collect 2.5 more gallons of pre-boil wort! The pre-boil gravity was much higher than estimated, and after I diluted with 2.5 gallons of filtered water, the pre-boil volumes and gravities were where they should be. However, there was still probably about 2 gallons of sparging water in the mash tun, but overly diluted, and with a drastically dropped pH. I'm trying to avoid astringency, of course.

Topping up works fine, and I'm getting the expected mash efficiency of about 80%, but it's annoying to have to do it this way.

Is there  way to correct for this? I've had similar issues for the past couple of brew days, and am about to just give up on fly sparging altogether and start batch sparging.

I'm sparging about 1.5 quarts per minute, which is about as slow as I can manage. I crush my own grain, with mill gap set to 0.037"

Thanks, all.

What was the grist weight and target pre boil gravity?

This is not an uncommon occurrence in lower gravity beers, due to the reduced grist height.

17.5 lb total grist, target pre-boil gravity 1.035. Fairly light (a dry stout with rye), but the grain bed was a good 5" deep after all drained.

Looking around the internet, maybe I had the estimated efficiency too low, and that had something to do with it? Estimated brewhouse efficiency was 72%, mash efficiency 75.3%, measured brewhouse 86.8%, mash 85.3%.

I still don't quite see how that makes the difference, but...

Sugar diffuses from grain into water until the two hit equilibrium. Generally, first runnings represent 50% of the wort sugar for this very reason.

With less grain but the same amount of water, the diffusion during sparge is more efficient, or faster. Plus, the head pressure on the false bottom is lower, so velocity can be increased.

If you were measuring the pressure differential between the mash and under the false bottom, you'd use the column inches as the unit of pressure. Much more than 3 inches of pressure difference and you begin to compact the bed, which is inconsequential in batch sparging, but a disaster for fly sparging.

With larger grists, your grain bed height increases enough that that column pressure begins to matter for lautering speed. So, you slow down to equalize that pressure.

Basically, it means you can sparge faster on lower gravity beers. Maybe as high as 1.8 quarts/min. or 20% faster.