Anticipating Og

[Julez]

Hi all,

in order to work out your target OG, according to Daniels, you use this formula:

GU(beginning of boil) x Wort Volume(beginning of boil) = OG in fermenter
Wort Volume(end of boil)

However, this can't be right! The formula is based on the idea the total gravity doesn't change from the beginning of the boil to the end of the boil. In terms of evap, this is correct, because evap has no impact on the volume of sugars in the wort. But when the boil is finished, there is kettle trub and remaining wort, which means remaining sugars!

So am I correct in assuming then, that to work out your target gravity correctly, you actually need to calculate your wort volume at the end of the boil, not as the final volume in the fermenter, but as the final volume in the kettle, LESS losses in trub?

Does this make sense?? :huh:

Cheers.

 

[Julez]

Actually, the real question is, how do you work out how much total GU you lose in trub? Think I just worked it out. If I have 1 gallon of trub, at a specific gravity of 1.050, then I have 50GU of total gravity in trub (volume x GU = TTL gravity).

So this GRAVITY loss has to be allowed for when calculating your target OG at the start of the boil.

Does this sound right?

 

[rich_lamb]

Oh yeah - absolutely correct! You calculate your final gravity at the end of the boil in the kettle. The gravity in the fermenter will be the same, but there will be less volume as you left some behind.

That's why so many of us are so desperate to filter out the last drops in the kettle - there's a lot of work gone into makin' that

Of course, if you then dilute again in your fermenter you have to run a second ratio...

 

[Julez]

Oh yeah - absolutely correct! You calculate your final gravity at the end of the boil in the kettle. The gravity in the fermenter will be the same, but there will be less volume as you left some behind.

That's why so many of us are so desperate to filter out the last drops in the kettle - there's a lot of work gone into makin' that

Of course, if you then dilute again in your fermenter you have to run a second ratio...

Yep just played around with the numbers and confirmed it.

So his formula should be:

GU(beginning of boil) x Wort Volume(beginning of boil) = OG in fermenter
Final kettle Volume including trub

Based on a 5 gallon batch with half a gallon lost to trub, this formula will give an OG that is 5GU less than Daniels' formula, with an end result of 1.046 instead of 1.051. So it's a pretty big mistake!