High gravity brewing: How to hit my OG without knowing my efficiency?

[skibb]

This is quite a technical question...

Background on my system (if it matters)
I fly sparge with my system and I regularly get 90% efficiency with up to 12.5# of grain (with most grain bills that's around 1.070 OG) for a 6 gallon batch. My water/grist ratio is 1.25 qt/lb, and my boil off is 1 gallon per hour - if I do a 90 minute boil I can usually eke out 2% more in my efficiency via the extra sparge water.

I'm going to attempt to brew up a blonde barley wine with an OG of 1.100 - In the past I would just mash/sparge up to 12.5# of specialty malts and base malt and make up the remaining gravity units with DME - but at this point in my brewing I would like to be able to move past this technique while making bigger beers.

My main question is: When do I stop sparging so that I know by the end of the boil I will have a 1.100 OG beer, though it won't necessarily be the normal 6 gallon quantity?

I have a refractometer, and I can easily measure my runoff. I'm planning on doing a 90 minute boil in order to allow myself time to adjust/scale all my hopping additions to this mystery amount of beer. I really just want to know the math involved with figuring out my desired cutoff point in the sparge. My mash tun is a keggle, so I will have no problem fitting 20-30# of grain in it.

Cheers!

 

[dbrewski]

well, if you want to end up at 1.100 and six gallons, that's 100*6 = 600 gravity points. You'll need to sparge until you get that in the pot. For example if you have 9 gallons of wort at 1.066 preboil specific gravity and boil off 3 gallons, you get roughly 1.100 final gravity.

 

[skibb]

Sorry I didn't really explain - I am aware I can always sparge and boil down to my desired gravity. I do not want to do a boil longer than 90 minutes. It is not my goal to have 6 gallons necessarily, but to have a beer with a 1.100 OG after normal boiling times.

For examples sake - the barley wine recipe I will attempt all this on is...
18# Pale Ale malt (Briess)
1# White Wheat malt (Briess)
1# Carastan (Bairds)

...which at 75% efficiency has an OG of 1.100

 

[feinbera]

Without knowing more details of your system -- copious notes on what volumes and OG you're getting out of first runnings and sparges of various weights and grist:water ratios -- it'll be hard for us to tell you how much grain and sparging you're going to have to do.

If you know you wanna do a 90-minute boil, and you know your boil-off rate, you can do the math like so:

(pre-boil OG) x (6.0 gallons + gallons boiled off in 90 min) = 600 points

(pre-boil OG) = 600 / (6.0 gallons + gallons boiled off in 90 min)

If you know how much you boil off in 90 minutes, you can solve for the pre-boil OG, so, you'll know you want to get so-many gallons at such-and-such OG. How you hit that target volume and OG is a matter between you and your system.

If getting it up to 1.100 is the key thing, and you don't mind spending an extra couple bucks on base malt, I'd say, model really crappy efficiency -- say, 60% -- and sparge conservatively until you "sneak up" on those 600 points. It's easier to pull out less than 100% of the sugar than it is to pull out more than 100%, and if you end up with a small volume of really thick pre-boil wort, say, 5 gallons of 1.120, you can always top off your kettle before you boil.

 

[skibb]

Without knowing more details of your system -- copious notes on what volumes and OG you're getting out of first runnings and sparges of various weights and grist:water ratios -- it'll be hard for us to tell you how much grain and sparging you're going to have to do.

I understand - I would have to pretty much do the math required on the fly after I measure the very first runnings coming out of the mash tun.

If getting it up to 1.100 is the key thing, and you don't mind spending an extra couple bucks on base malt, I'd say, model really crappy efficiency -- say, 60% -- and sparge conservatively until you "sneak up" on those 600 points. It's easier to pull out less than 100% of the sugar than it is to pull out more than 100%, and if you end up with a small volume of really thick pre-boil wort, say, 5 gallons of 1.120, you can always top off your kettle before you boil.

Yes! That's why my planned recipe is 20 lb total and modeled for 75% efficiency. I don't care if my final amount post-boil is 3 gallons and not 6 - I just want to know when my run off has reached the point to where after a full 90 minute boil I will have a 1.100 OG. The "sneaking up" part is where I'm still foggy - how will I know when I've snuck up to it. I'm also a bit confused b/c I doubt the gravity's dropping off at a constant rate through the lautering/sparging - my initial 1st runnings (similar to a party-gyle) would be dramatically more saturated in sugar than the first bits of my sparge water after filtering through the grain bed. So, I guess I would have to take multiple measurements every set amount of minutes to get a better picture of the rate and thus how many gravity units are already in the kettle so I know when I'm approaching the point to stop. Oh I think I'm just confusing myself now...

 

[dbrewski]

Yes, stir the wort to insure consistency then measure it, then do the above calculation, rinse, and repeat until you hit it. Regardless of the fact that you don't care about your end volume, volume is a critical part of the equation. That's why in my example I used six gallons at 1.100 as the goal. Then as fienbara states you do the math from there. You need to know your boiloff rate. Ideally, you need to know your typical kettle lossage too but since you don't care if your a bit short ignore that. Then do the math as he shows (more clearly than I did).

 

[billl]

So, you are basically looking to calculate gravities on the fly since you don't know your efficiency?

Basic algorithm:

1) Use a grain bill you think will get you in the ballpark. eg 70% efficiency
2) Draw off the first runnings as usual.
3) Take a gravity reading and multiple by the volume collected to get your points. eg 1.150 @ 3 gallons = 450 points.
4) Start sparging and collect 2 more gallons. Take another reading from the steam as well as your 2 gallons. Recalculate the total points you have.

Now you need to start making some decision.

If you are relatively close to 600 points, you can make a plan to creep up on it. ie take the points of sparge coming out at the end of that 2 gallon sparge to get an approximation of what running off a another gallon will get you. Repeat in half gallon increments. When you hit your points, just top off with enough water to account for evaporation.

If you aren't close, you'll be adjusting you final volume down and lowering the total points needed. eg down to 5 gallons and 500 points.

 

[skibb]

Ahh thanks guys it's become much more clear for me - hah sorry math definitely isn't my strong suit but I think with this knowledge in hand I can get a good enough grasp and attempt brew this big boy up and land an OG near or at 1.100 - As said before I can always top off so I would rather be higher than lower. Thanks again!