Calculating offset on partial boils ?

[BxBrewer]

My boils are about 3 1/2 gal. Is there a way to calculate what the OG/Brix should be to hit the target OG after adding the rest of the water in ?

 

[GingerFlock]

Just for clarification, do you do any calculations prior to starting (i.e., theoretical OG, FG)? What's your recipe/grain bill?

 

[brewkinger]

My boils are about 3 1/2 gal. Is there a way to calculate what the OG/Brix should be to hit the target OG after adding the rest of the water in ?

There most certainly is! I am fairly new at this, so others SHOULD check my math, but is essentially goes like this:

Lets say that total volume is going to be 5 gallons.

If your expected OG (after topping off with water) is say 1.050 then the calculations are basic.
50 points (another way of reading 1.050) dissolved in 5 gallons of water is 250 total points (250 points / 5 gal = 1.050)

Here is where it gets a little more technical. If you start with 3.5 of wort and then boil for an hour, you will lose water BUT sugars stay in the wort.

So depending on how much water evaporates (you have to figure this out if you have not already), your volume after boiling will vary.

But, like stated before, if you want 5 gallons of 1.050 wort then it translates down like this:
4 gallons of wort (with same sugars in it) has gravity of 250/4=1.063
3.5 gallons 250/3.5 = 1.071
3 gallons = 250/3 = 1.083
2.5 gallons = 250/2.5 = 1.100 (this is probably where you will end up post boil)
2 gallons = 250/2 = 1.125

Does this make sense?

 

[GingerFlock]

Brewkinger has the gist, here's the calculations...

My boils are about 3 1/2 gal. Is there a way to calculate what the OG/Brix should be to hit the target OG after adding the rest of the water in ?

3.5 gal is your starting boil I'm assuming. If you want to get really technical, you can assume a 20% evaporation rate (if you boil for 60 min). This means that post-boil, your volume will be 2.8 gal. At this point, take a gravity and temperature reading, and adjust for temperature (I believe +1 for every 7F above 60F). For sake of illustration, assume your SG is 60 points. If you want to get it to 40 points, that means we need to dilute 20 points "out". The math is as follows:

60 pt/gal * 2.8 gal = 168 pts

We thus have a total of 168 points that we need to get to a rate of 40 pt/gal (or 1.040)

168 pts / 40 pts/gal = 4.2 gal

We need a TOTAL of 4.2 gal to bring it to our desired OG. To find our top-off:

4.2 gal - 2.8 gal = 1.4 gal top off

Hope that helps!

 

[brewkinger]

Brewkinger has the gist, here's the calculations...

3.5 gal is your starting boil I'm assuming. If you want to get really technical, you can assume a 20% evaporation rate (if you boil for 60 min). This means that post-boil, your volume will be 2.8 gal. At this point, take a gravity and temperature reading, and adjust for temperature (I believe +1 for every 7F above 60F). For sake of illustration, assume your SG is 60 points. If you want to get it to 40 points, that means we need to dilute 20 points "out". The math is as follows:

60 pt/gal * 2.8 gal = 168 pts

We thus have a total of 168 points that we need to get to a rate of 40 pt/gal (or 1.040)

168 pts / 40 pts/gal = 4.2 gal

We need a TOTAL of 4.2 gal to bring it to our desired OG. To find our top-off:

4.2 gal - 2.8 gal = 1.4 gal top off

Hope that helps!

^this