TyTanium

Recipes 

Ahh, yes, this is very important if you're fly sparging. You face diminishing returns, so you get less sugar/volume as you go - after a certain point your cost of sparging (tannins, etc) + cost to boil down > benefit of more sugars from sparge, so you stop.

RUNningonbrew

Recipes 

So say you hit your target OG based on your calculation before boil at 5 gallons instead of 6.5 would you continue to collect or adjust somehow or just stop sparging?

TyTanium

Recipes 

Up to you - if you sparge more, you collect more sugars. So you'll either have a higher gravity beer (if same volume), or more beer (if same gravity)

If you stop, you'll have less beer than you originally planned at your target gravity, or your planned amount of beer at a lower gravity.

Remember, specific gravity is a ratio, sugar/volume, so you have 2 variables. I always multiply by volume so I can compare total sugars to target sugars, independent of volume, throughout the process. So in your case, 5g at target gravity will obviously have less total sugars than 6.5g at your target gravity.

RUNningonbrew

Recipes 

So really you can calculate what your before boil gravity should be based on your OG...and based on that as you are collecting your wort you can take gravity readings to hit the mark? Do you have a real world example of this too? Sorry I need to see an example to completely understand it

TyTanium

Recipes 

Yup. Do you have a recipe in mind? I can help run through your numbers with you.

RUNningonbrew

Recipes 

(Two Hearted Clone)
10 Lbs Briess Two Row Pale Malt
2.83 lbs Briess Pale Malt
.5 lbs Crystal 40L


Hops (total boil time 75 minutes)
1.2 Oz Centennial 45 Minutes
1.2 Oz Centennial 35 Minutes
4.5 Oz Centennial Dry Hop after 7 Days

Target OG 1.063

TyTanium

Recipes 

Cool.

First, you need to know the total gravity available from your grains.
10lbs x 37pts/lb = 370 from two row
2.83 x 37pts/lb = 105 from pale malt
0.5 x 34pts/lb = 17 from crystal
Add 'em all up and you get 492 total gravity points. But that's at 100% efficiency - multiply by your efficiency to get your target total gravity. Let's say 70%.
492 x 70% = 344 total target gravity points. These are the total sugars you expect to get from your system.
Now divide 344 by your preboil volume, say 6.5 gallons, 344/6.5 = 53 gravity points, or 1.053 is your target preboil gravity.
Divide by your target postboil volume of 5.5 gallons, 344/5.5 = 63 gravity points, or 1.063 target OG.

Now lets say you take a preboil gravity reading. It's 1.049 instead of 1.053. And you have 6.75 gallons instead of 6.5. How'd you do? 49 gravity points x 6.75 gallons = 331 gravity points, a little lower than your 344 target. (331 / 492 total = 67% efficiency). So how do you compensate?
Two ways - either have a lower OG (331 / 5.5 = 60, so 1.060 OG) or boil longer to reduce volume to hit target OG (331 / 63 target = 5.25 gallons instead of your original 5.5)

EDIT: I highly recommend walking through the math on your own. It's confusing to read, but really simple once you work through it once or twice.
EDIT 2: You also lose ~4% of volume when cooling. 5.75 gallons at boiling ~ 5.5 gallons cooled. I left it out for simplicity, but it makes a difference. I usually stop my boil at 5.75 gallons. (though I know my boiloff rate of 1qt every 20mins, so I can time my hop additions right)

RUNningonbrew

Recipes 

Thanks! That helps a lot, so as i'm collecting my wort, and I take a reading at say 4 gallons, I would take 344/4 = 86 which is 1.086 at 4 gallons, i know i'm on the right track to hit my pre boil gravity of 1.053 at 6.5 gallons?...which would put me on track to hit my post boil gravity of 1.063?

TyTanium

Recipes 

Not exactly, since as you're collecting you'd still be collecting more sugars. And as sugars are rinsed, the gravity of what comes out will drop. But what you CAN do is take measurements as you go so you can determine when to stop sparging. For example, say at 4 gallons you had 1.075 wort. 75 x 4 = 300 gravity points, then you know you're shooting for 44 more points. You could take another reading at 5 gallons, 6 gallons, etc.

That said, I don't fly sparge, so I can't give you "best practice" for sparge math on the fly.

Fennis

Recipes 

I get it. I am new to all-grain brewing, granted I do not have an elaborate setup as I just do brew in a bag, but I messed up my first batch because I added additional water before I boiled in fear that I sucked up too much water in my grains and was going to be off, but in reality that was a big mistake as if I had just left it alone, I probably would have been okay. I didn't add water on the second attempt, and I hit my target gravity exactly as expected. So, you are saying taking gravity readings before my boil could have prevented me from being off on my first batch. Good to know!