Pre-boil gravity vs SG

[Twofox]

Had a question about target gravities. I've brewed the same recipe 4 times now, and every time miss my target grav, and grav before pitching (bad memory here) was the same I believe.

According to Ray Daniels in "Designing great beers", you take a pre-boil grav, then add extract to hit your target. I haven't bothered yet, just boiled, took another reading and pitched.

However, this last time, the pre-boil grav was low (as susptected, 1.046 vs 1.056)), but when I took a reading before pitching yeast, it was up close to target (1.054).

Question is, is that normal for that to happen? If so, what likely caused it to increase?

 

[bob1852]

When you boil, you lose volume but the sugar amount stays the same; hence the increase in gravity.

Was your 1.056 target the pre or post-boil number?

 

[Drunkenfool]

If u use some type of software you can predict post boil grav. You need to have a way of knowing evap rate and volumes in your kettle. I usually get the grave where I want it b4 the boil mark the level and then top off with boiled water to the pre boil level. Even if u don't do that the software will tell you how much to add to achieve desired sg

 

[Twofox]

When you boil, you lose volume but the sugar amount stays the same; hence the increase in gravity.

Was your 1.056 target the pre or post-boil number?

The 1.056 was post-boil. If promash showed that on the recipe am I ok then?

 

[bob1852]

The 1.056 was post-boil. If promash showed that on the recipe am I ok then?

Yes, if your target was 1.056 and you were at 1.054; you're fine.

 

[OLDBREW]

you have to make sure your pe-boil wort has been mixed really well before taking a sample. first and last runnings being collected are very different gravities.

Once it is mixed well, you cool the sample below 100*F before taking a reading. you can then use the correction temperature chart for your hydrometer.

you need accurate volumes and a well stirred accurate gravity readings to get close to your true gravity

 

[bullinachinashop]

Preboil gravity x volume = Postboil gravity x volume

Example...

1.046 x 6.5 = ? x 5.5
46 x 6.5 = ? x 5.5
299 = ? x 5.5
54.36 = ?
1.054 = postboil gravity

Got it?

Bull

 

[Twofox]

Yep, makes sense that less volume, with the same sugars gives a higher volume. Also, temp difference I think I was forgetting.

Ok, thanks for all the replies, I feel lots better about wtf is happening here? lol

:rockin:

 

[92greenyj]

you have to make sure your pe-boil wort has been mixed really well before taking a sample. first and last runnings being collected are very different gravities.

Once it is mixed well, you cool the sample below 100*F before taking a reading. you can then use the correction temperature chart for your hydrometer.

you need accurate volumes and a well stirred accurate gravity readings to get close to your true gravity

what he said. I use Beer Alchemy and the software auto corrects the gravity readings for me by just plugging in the grav reading and the temp it was taken at. It doesn't need to be below 100 either. the software will corect up to boil temp. And the volumes are pretty critical too, at least for efficiency calculations. I found after using my software for a few batches my efficiency was much lower than I thought and couldn't figure out why. Then I checked my calibrations on my sight glass and equipment and found they were all way off. (dont recall what we calibrated them with originally) so I re-calibrated all of my equipment and my efficiency jumped 10%