Partial Boil

[philired1]

Hi all,
I would like to know what happens to IBU's when it comes to dilution. As i only have a 19l pot i brew at a higher gravity then dilute in the fermenter to my batch size which is normally 13l. I scale recipes to suit 13l's this includes hops. I take the 13l OG and put it into a dilution calculator to get the OG for 11l's. I no chill in an 11l cube. I then re scale the recipe quantities to the new OG including hops. As i normally add 2 litres of water for dilution what is the affect on the ibu. Do i add mor hops to the boil. My boil setup is 15.5ls and my loss is about 2.5 to 3 lts which leaves just over 12ls after boil. I have a spigot in my pot which holds about 800mls below it.

 

[philrob]

I have never done this, as I brew full volume, but if you have brewing software such as BeerSmith3 it will work this out for you.
The prevailing wisdom appears to be that higher gravity boils are less efficient in extracting IBUs, but even after 13 years of brewing, I'm no expert.
Others may have more detailed responses.

 

[MHB]

philrob is pretty much right.
The way to look at is to treat either side of the boil separately.
After the boil diluting the 11L is a simple proportional dilution (i.e. if you doubled the volume you would half everything OG, IBU's, Colour...) so in going from 11L to 13L you can apply the standard dilution equation (which every brewer should know)
C1V1=C2V2 where C is concentration (or condition i.e. temperature) and V is volume (or quantity i.e. kg's). for example if you had 11L at 40 IBU's and diluted to 13L it would be 40IBU*11L = xIBU*13L, (40*11)/13=33.85 IBU's.
The only catch is if you are working with the gravity, the 1.000 part of your OG is what we call non-unitary it isn't a number like Litres its a comparison to the density of water (1.050 is 1.050 times as dense as water) use the 1.000 in the calculation and it will go to shit pretty quick. You need to work in points (i.e. call 1.050 50 points or 0.050) or work in Plato (oP) (given 1.050 is about 12.5oP)
If you had 11L of 1.050 wort and diluted it to 13L
In Points 50*11 = X*13, (50*11)/13 = 42.3 or 1.0423
Alternatively 0.05*11 = X*13, (0.05*11)/13 = 0.0423 or 1.0423
in oP 12.5*11 = X*13, (12.5*11)/13 = 10.577oP (which is) 1.0423 (surprise surprise)

So what you need to do is
Work out what you want after dilution.
Calculate what you need in your cube (before dilution)
Work out what you need to achieve the full volume of the kettle (from OP about 11.8L) at your concentrated condition.

You are really making a small dilution so it isn't going to make all that radical a difference, when people are trying to dilute to get double the volume or so it can be a lot more complex.

Do your bittering calculation based on the end of boil volume and gravity and it should be all good.
Mark

 

[philired1]

philrob is pretty much right.
The way to look at is to treat either side of the boil separately.
After the boil diluting the 11L is a simple proportional dilution (i.e. if you doubled the volume you would half everything OG, IBU's, Colour...) so in going from 11L to 13L you can apply the standard dilution equation (which every brewer should know)
C1V1=C2V2 where C is concentration (or condition i.e. temperature) and V is volume (or quantity i.e. kg's). for example if you had 11L at 40 IBU's and diluted to 13L it would be 40IBU*11L = xIBU*13L, (40*11)/13=33.85 IBU's.
The only catch is if you are working with the gravity, the 1.000 part of your OG is what we call non-unitary it isn't a number like Litres its a comparison to the density of water (1.050 is 1.050 times as dense as water) use the 1.000 in the calculation and it will go to shit pretty quick. You need to work in points (i.e. call 1.050 50 points or 0.050) or work in Plato (oP) (given 1.050 is about 12.5oP)
If you had 11L of 1.050 wort and diluted it to 13L
In Points 50*11 = X*13, (50*11)/13 = 42.3 or 1.0423
Alternatively 0.05*11 = X*13, (0.05*11)/13 = 0.0423 or 1.0423
in oP 12.5*11 = X*13, (12.5*11)/13 = 10.577oP (which is) 1.0423 (surprise surprise)

So what you need to do is
Work out what you want after dilution.
Calculate what you need in your cube (before dilution)
Work out what you need to achieve the full volume of the kettle (from OP about 11.8L) at your concentrated condition.

You are really making a small dilution so it isn't going to make all that radical a difference, when people are trying to dilute to get double the volume or so it can be a lot more complex.

Do your bittering calculation based on the end of boil volume and gravity and it should be all good.
Mark

I am hoping this is correct. The recipe figures i am aiming for are as follows for 13ls. 3.8% ABV, 27 IBU's,1.037 SG, 1.008 FG. Calculating from above for 11l's in cube i should be aiming for 1.044 SG and 32IBU's. These figures are from a dilution calculator.

 

[MHB]

Looks close enough.
It's handy to be able to do basic calculations your self rather than just using an app. its funny but when you have them in your head, mistakes sort of just jump off the page at you, I think it helps to make better brewers.
Mark