Post boil gravity

[Jethro Choicebro]

Hi,
Hoping someone has a theory in to why i might be missing on the post boil gravity readings.

Last brew:
Est post mash/pre boil = 1.045
Actual post mash/pre boil = 1.043

boil volume = 7.3gal (as expected)
post boil volume = 5.8 gal (as expected)

Est post boil = 1.059
Actual post boil = 1.049 (??)

Has been fairly consistent on my set up that post boil gravity are low despite being on or close to the pre boil gravity (not usually as bad as above but always 3-5points low), and cannot think of any reason why.

60min boils
Boil off rate is correct.
I'm at ~4500ft so boil at approx 195f - but would assume that the boil off amount is the key number

Using both a refractometer and hydrometer and getting similar (+/- 1point) readings so don't think its a measurement issue

PH mash values are a little high (soon to be rectified) but again assumed that for pre and post boil its all about the boil off?

Typically see FG after fermenting a couple points lower than recipe expected - in line with the OG being lower.

Using beersmith.

Am I missing something?
Beer is coming out great so no problem there, but obviously would like to understand what is happening.

Many thanks in advance.

 

[day_trippr]

At what point are you drawing your OG samples? Some time after filling your fermentor(s)?
I would suspect insufficient homogenization of the wort and you've been drawing your samples from the lighter fraction at the top of the vessel...

Cheers!

 

[Jethro Choicebro]

Taking the samples at the end of the boil - before transferring to fermentor.

In the example I posted we did a reading with about 5 mins to go and then a flame out

 

[RPh_Guy]

I'm assuming that second set of est and actual SGs you listed is supposed to read post-boil.

Are you cooling the samples to the appropriate temperature for your hydrometer calibration?

Also the math doesn't work. 43 gravity points going from 7.3 gal to 5.8 gal gives:
43 * 7.3/5.8 = 54, an expected post-boil gravity of 1.054.
Maybe something in the equipment profile is off, the volume measurements are off, or your gravity readings are off. I'm not sure how to troubleshoot, but I'd start by calibrating the measuring instruments. At least a 2-point calibration of the hydrometer and refractometer.

Don't forget to account for thermal expansion when measuring volume.
Mash temp water is roughly 2% larger than room temp (~68°F) and boiling is 4% larger.

HTH

 

[Jethro Choicebro]

I'm assuming that second set of est and actual SGs you listed is supposed to read post-boil.

Yes, right you are - thanks. Have edited

Are you cooling the samples to the appropriate temperature for your hydrometer calibration?

Did allow sample to cool for a bit before taking reading, and then used a temp adj calculator. I dont recall the exact temp but think it was probably still a little above 100F though so will look at this next brew and get it cooler. Will also re look at calibrating the hydrometer and refractometer.

Also the math doesn't work. 43 gravity points going from 7.3 gal to 5.8 gal gives:
43 * 7.3/5.8 = 54, an expected post-boil gravity of 1.054.
Maybe something in the equipment profile is off, the volume measurements are off, or your gravity readings are off. I'm not sure how to troubleshoot, but I'd start by calibrating the measuring instruments. At least a 2-point calibration of the hydrometer and refractometer.

Don't forget to account for thermal expansion when measuring volume.
Mash temp water is roughly 2% larger than room temp (~68°F) and boiling is 4% larger.

Think you may be on to something here. In my profile I have "loss to chiller and trub" set at 0.3 gal - which is probably true for trub, but i use a CFC and am quite sure that it 'holds' considerably more than that. So when i say the boil off rate is 1.5 gal/hr it really isn't, instead its maybe 1 gal/hr while the other 0.5gal is lost in the chiller - therefore would make sense that the OG is coming lower than expected.

Brew again on Thursday so will make some adjustments...

Thanks for the help!