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Chris5899 01-10-2013 06:10 AM

Dumb Question about OG
OK, I know I have a target OG of 1.069. I also know that I should stop lautering when the runnings hit about 1.010. The question is, do I add plain water to the wort to hit my desired OG if it is still too high after lautering, and if that is the case, I suppose it is possible not to hit my desired batch of 6 gal if the OG is getting to low before that.

I'm new to all-grain (actually, brewing all together). Just looking for clarification that I can't seem to find.

kallen 01-10-2013 07:20 AM

Your OG will be much higher after boiling then your specific gravity after sparging. So, you will need to know your pre-boil gravity number and your evaporation rate of your kettle/boil time, etc, if you want to get a particular OG for fermentation.

I never try to hit anyone else's gravity numbers, I created my own recipes, so I have never been too worried about that. But, figure that the specific gravity of plain water is 1.000 at 60F, anything higher then that would mean that you are still rising sugars out of the grains. So, if you are trying to get a pre-boil volume or drop a pre-boil gravity, I would prefer to run water through the lauter tun and get more sugars instead of just adding plain water.

dbsmith 01-10-2013 07:34 AM

Here's a little chemistry to help you out. It's a basic formula for concentration which works with any units.

c1 x v1 = c2 x v2

c1: initial concentration
v1: initial volume
c2: final concentration
v2: final volume

The concentration can be in units of gravity (so 1.030 would be 30). Volume is whatever unit you want, but usually gallons. You can use this handy formula to figure out how to manipulate your wort to be what you want it to be. You want 69 gravity points per gallon for your final concentration, so that is your value for c2. Let's say you collect 7 gallons of wort at strength 1.045 (45 gravity points per gallon). Let's do some math:

45 x 7gal = 69 x v2gal

v2 = 4.6 gal

This means you would have to boil your wort down to 4.6 gallons to have the correct gravity you wanted. There are many ways to manipulate your volume, such as boiling times, adding water, etc. Hopefully this will give you an idea. When I'm sparging, I usually collect about 6 gallons, check my gravity, and then decide if I need to sparge more or just add a specific amount of water so that I can boil for the appropriate amount of time and end up with the correct volume and concentration.

You can even manipulate the equation by plugging in the final volume you want, and solving for the amount of gravity points you need initially to achieve the correct final concentration. There are many ways to achieve the desired result.

Chris5899 01-10-2013 02:25 PM

OK, I get that. So for more clarification, OG = Gravity post boil/pre fermentation, NOT post lauter/pre boil?

BrewerinBR 01-10-2013 02:30 PM

OG = Original Gravity ... the specific gravity post boil.

So the answer is YES!

duboman 01-10-2013 02:31 PM


Originally Posted by Chris5899 (Post 4769501)
OK, I get that. So for more clarification, OG = Gravity post boil/pre fermentation, NOT post lauter/pre boil?

Correct! In the AG set up there is also a pre-boil gravity that can be taken which assists in calculating what your post boil OG should be, so to some, it is a worthwhile reading to take. IME, the pre-boil is usually about 8-10 points lower than the post boil OG meaning the boil will concentrate your wort by that amount of points.

By taking a pre-boil you can evaluate the efficiency of your mash/lauter and it also allows you to decide whether you need to increase boil time or add DME to the recipe to arrive at your desired post boil OG if you run a little short or if you are too high to add some water to dilute.

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