# Update to the boil size, target volume, and batch size

Discussion in 'Brewer's Friend Announcements & Updates' started by Yooper, Aug 16, 2019.

1. ### Yooper Administrator Staff Member

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#1
Our way of calculating efficiency has changed, and this is noticed on the Recipe Builder page. You can choose your boil size, and the amount of wort at the end by choosing the volume going into the fermenter, or the volume at the end of the boil. Those choices do NOT impact your efficiency calculation, but are there to assist you in ensuring your volumes are correct for your brewing.

To understand the efficiency calculation, an example is best.

Let’s say your boil starts at 6 gallons. You have 120 points of fermentables (e.g. 5 pounds of 30 ppg at 80% efficiency). You boil down to 5 gallons. In this example, your system may lose 1 gallon to kettle dead space, hop absorption, losses to the way you transfer wort, wort left in the chiller, and so on. In this case, you up with 4 gallons in the fermenter.

In the old method:

You’d select the fermenter target, and enter 6 gallons as your boil size. Four gallons would be your batch size, since that is the volume going into the fermenter.

The software assumed that the difference from the 6 gallons at the beginning reducing to 4 gallons as the batch size was all boil-off.

As a result, the OG was calculated as: 1 + ( 120 / 4 ) * 0.001 = 1.030

That is clearly wrong because it indicates that all fermentable sugar was condensed into 4 gallons of wort. The reality is that it was condensed into 5 gallons of wort, and some of that was simply thrown away. The loss of wort does not increase specific gravity of the remaining wort, of course. The loss would have the same SG as the wort going into the fermenter.

So in our updated calculation, you would select the batch size of 4 gallons as the target into the fermenter, enter 6 gallons as pre-boil size, and 5 gallons as post-boil size.

You can also let the system calculate from your equipment profile’s boil-off rate.

Now the OG can be calculated correctly:

1 + ( 120 / 5 ) * 0.001 = 1.024

You will notice that it is a 20% difference. That seems small to some, but we wanted to give the most accurate OG predictions possible for our users, both homebrewers and professionals.

AHarper and Pricelessbrewing like this.
2. ### AHarper Active Member

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#2
In the example above what volume is used to generate (calculate) the IBU values?

The only reason I ask is that I have been playing with my current brew - and using the new Boil size values as added (I was wondering when these were added to the screen) and noticed that the IBU value doesn't change when you alter the PRE boil size (up or down) but does when you adjust the Post boil size. How does this work? Sorry if this is a stupid question.

I would have thought that adjusting the INPUT volume would affect the concentration of the IBU Post boil - since you boil off water (and hops volitiles) and not just the final amount - or am I missing some subtle adjustment here?
The reason I was playing about as I was trying to work out why my OG values were way off when, previously (ie.prior to update) they were pretty close - probably due to the small volumes I brew so errors would, naturally, be reduced.

3. ### Yooper Administrator Staff Member

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#3
It's not a stupid question at all! The boil SG is used to determine the utilization in all formulas. The smaller volume has a higher SG so the utilization changes as the wort gets higher in SG. Does that make sense?

I know there are other things that help determine hops utilization (Palmer mentioned that more than 10 years ago- it's not really the SG but instead the amount of break material and suspended solids) but it's the best we have in the formulas currently.

4. ### AHarper Active Member

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#4
[It's not a stupid question at all! The boil SG is used to determine the utilization in all formulas. The smaller volume has a higher SG so the utilization changes as the wort gets higher in SG. Does that make sense?]

Yes this makes sense BUT Palmer talks about Initial Boil Volume - from How To Brew...
"But, since we are only boiling 3 of the 5 gallons due to of the size of the pot, we need to take into account the higher gravity of the boil. The boil gravity becomes 6 x 40 / 3 = 80 or 1.080

It is the gravity of the boil (1.080) that is used in figuring the Utilization. As you will see in the next section, hop utilization decreases with increasing wort gravity. The higher concentration of sugars makes it more difficult for the isomerized alpha acids to dissolve. I use the initial boil gravity in my utilization calculation; others have suggested that the average boil gravity should be used. (The average being a function of how much volume will be boiled away during the boiling time.) This gets rather complicated with multiple additions, so I just use the initial boil gravity to be conservative. The difference is small—overestimating the total bitterness by 1-3 IBUs."

So why does BF Post Boil?

Currently - I'm trying to play with the Tinseth Equasions and the info in How to Brew and equate them to the calculations in the recipe builder AND the IBU Calculator tool. So far NOTHING equates - but I am working on it... I'll let you know the results but at the moment I have some puzzling values. Please see my other post re: IBU values as it displays my concerns. Alan

5. ### Yooper Administrator Staff Member

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#5
The gravity increases as the wort volume drops, and the IBU utilization changes (see Tinseth's equation and info on that for me).
In the case of Palmer's example you mentioned, he's talking about partial boils where wort dilution (by adding 0 IBU water at about 50% of the volume post boil) so it's not apples-to-apples there.

6. ### Pricelessbrewing QA Software Tester Staff Member

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#6
Technically the original tinseth formula uses the average wort gravity, not preboil or post boil.

http://realbeer.com/hops/research.html

"The Bigness factor accounts for reduced utilization due to higher wort gravities. Use an average gravity value for the entire boil to account for changes in the wort volume.

Bigness factor = 1.65 * 0.000125^(wort gravity - 1)"

7. ### BOB357 Well-Known Member

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