# Pre-boil gravity problems

Discussion in 'Brew Sessions' started by Mark Farrall, Sep 8, 2020.

1. ### Mark Farrall Well-Known Member

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#1
I'm having problems with pre-boil gravity (and possibly volume) predictions. Most recent was using this recipe on the weekend - https://www.brewersfriend.com/homebrew/recipe/view/1045967/bnp.

I was spooked when I entered the pre-boil numbers, as it seemed I'd missed the pre-boil prediction by 12 points. The recipe predicts 1.060 and I'd hit 1.048. Though the efficiency numbers on both conversion and pre-boil were completely in line with expectations.

So I ignored it at the time and started the boil. At the end of the boil I find I've hit my OG of 1.068 and the correct volume of 7 litres.

So it's looking like the pre-boil prediction is off. When I entered the pre-boil log entry I noticed that it was expecting 8 litres. Which explains the the 1.060 number, as that's what this recipe would be with 8 litres of volume and the 75% efficiency.

Though I've no idea how it's decided that the volume would be at 8 litres at pre-boil. It should be at 10 litres as I'll boil off 3 litres for the final 7 litres of the batch size. And 10 litres for this recipe at 75% efficiency should be more like the 1.048 I actually achieved. It seems like the pre-boil gravity prediction is assuming I'm only doing a 20 minute boil.

2. ### Pricelessbrewing QA Software Tester Staff Member

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#2
Thanks for the report, I'll take a look this morning.

Just to confirm:
Recipe Preboil volume = 8L
Recipe preboil SG = 1.060
Recipe postboil = 7L
Recipe OG = 1.068

Measured Preboil volume = Not measured?
Measured preboil SG = 1.048
Measured postboil = 7L
Measured OG = 1.068

If so, the actual preboil volume should be (68 * 7 ) / 48 = 9.92L +- 0.2L given the instrumental uncertainty of gravity readings.

I copied your BnP recipe in your account, and named it BnP Pricelessbrewing.

All I did was open it up, and click that "calc" button next to the preboil volume, which calculates the boil volumes based on the batch size and the equipment profile. Since you have it set to batch target = kettle, this also rounded the batch size to 6.99.

The recipe pre-boil size is by default a user input.

3. ### Mark Farrall Well-Known Member

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#3
Recipe preboil volume = 10 L
Recipe preboil SG = 1.060
Pre-boil log entry expected volume = 8 L
Actual preboil volume = 10 L
Actual preboil SG = 1.048

Recipe postboil = 7 L
Recipe postboil SG = 1.068
Actual postboil volume = 7 L
Actual postboil SG = 1.068

Your batch stats in the copy look like the correct numbers. Mine are

4. ### Pricelessbrewing QA Software Tester Staff Member

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#4
Okay thanks for confirming, I'm not sure how the recipe initially ended up with a preboil volume of 8L, but it is a user input so it'll be fixed if you change it to 10L or click the calc button and it'll change to 10L automatically according to your equipment profile.

5. ### Mark Farrall Well-Known Member

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#5
Ah, got it now. I just had to click the calc button to get it to reset the numbers. Thanks.

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