Best time to read OG

[wells11]

A few friends and I have been discussing the best time to take OG readings during the brewing process.

 

[andrewmaixner]

I measure:
1) out of the mash (@150~, adjust for temperature, just for a rough number)
2) before pitching yeast (between 50F-70F, it will be within +/-0.001 of the calibrated 60F measurement)

 

[midfielder5]

I do it twice too. for all grain batches, measure from full volume in the bk. I chill sample in freezer, while bk comes to boil. Adjust hopping if necessary.
2) after getting my volume in the fermenter, I pour out the last hops and trub bits into a large pitcher. it settles out and I use the top to make my reading.

 

[Yooper]

I do it twice too. for all grain batches, measure from full volume in the bk. I chill sample in freezer, while bk comes to boil. Adjust hopping if necessary.
2) after getting my volume in the fermenter, I pour out the last hops and trub bits into a large pitcher. it settles out and I use the top to make my reading.

Yep, me too.

Those are the readings that matter. The preboil gravity lets you know if you've hit your efficiency numbers, so you can adjust the hopping (bittering) if necessary. The postboil is the actual OG of the batch, and is done just before pitching the yeast.

 

[joshesmusica]

depends if it's 1) all-grain or partial-mash, or 2) extract full-boil, or 3) extract top-up.

1) after the mash to make sure you hit your pre-boil gravity. then after the boil and cooling, and right before you pitch your yeast.

2) after the boil and cooling, and right before you pitch your yeast.

3) after the boil and cooling, but before top-up (depending on how much you're topping up with.) once you get the gravity reading, then get a dilution calculator like the one found in beersmith, and calculate how much water you need to top up with in order to hit your OG.

 

[Qhrumphf]

depends if it's 1) all-grain or partial-mash, or 2) extract full-boil, or 3) extract top-up.

1) after the mash to make sure you hit your pre-boil gravity. then after the boil and cooling, and right before you pitch your yeast.

2) after the boil and cooling, and right before you pitch your yeast.

3) after the boil and cooling, but before top-up (depending on how much you're topping up with.) once you get the gravity reading, then get a dilution calculator like the one found in beersmith, and calculate how much water you need to top up with in order to hit your OG.

I think that's a big trick for extract brewers. So many questions about incorrect gravities in extract beers on this forum that can simply be explained by insufficient mixing of wort and top off water. By taking the reading BEFORE you top off, and then calculating the diluted OG, you avoid that issue, and if by some off chance you're off with your gravity (which you pretty much will never be by more than point or two with extract beers unless you messed up somewhere), you can adjust top off to the right gravity.

As far as measuring batch gravity for all grain or partial mash beers, preboil gravity and post-boil gravity are the big ones, but I also monitor sparge runnings gravity (typically the first mash runnings followed by last sparge runnings) to make sure A) there wasn't a mash problem and B) I'm not over-sparging. However, those aren't really your OG per se. But I also, when doing particularly long boils, will monitor the gravity as I go to adjust boil time as needed.

 

[joshesmusica]

I think that's a big trick for extract brewers. So many questions about incorrect gravities in extract beers on this forum that can simply be explained by insufficient mixing of wort and top off water. By taking the reading BEFORE you top off, and then calculating the diluted OG, you avoid that issue, and if by some off chance you're off with your gravity (which you pretty much will never be by more than point or two with extract beers unless you messed up somewhere), you can adjust top off to the right gravity.

As far as measuring batch gravity for all grain or partial mash beers, preboil gravity and post-boil gravity are the big ones, but I also monitor sparge runnings gravity (typically the first mash runnings followed by last sparge runnings) to make sure A) there wasn't a mash problem and B) I'm not over-sparging. However, those aren't really your OG per se. But I also, when doing particularly long boils, will monitor the gravity as I go to adjust boil time as needed.

oh yeah, failed to mention that I'm doing BIAB. The one time that I've brewed using a sparge we measured the runnings as well. We got such good efficiency that we went with a slightly lower OG than we planned for, and then used the rest to make a session brew.

 

[doug293cz]

If there is a mash involved, you should also take a gravity reading of the first runnings. This reading, along with your grain bill and strike water volume, can be used to calculate your conversion efficiency (actual sugar produced / max potential sugar in grain.) If doing a vorlauf, the gravity sample should be taken after the vorlauf, as wort under a false bottom may have a lower gravity than wort in the bulk of the mash.

So, three gravity readings should be taken:

1. First running SG
2. Pre-boil SG
3. Post-boil SG (this is your OG)

If using a hydrometer, all gravity samples should be cooled to (near) the calibration temp of your hydrometer before reading, as the temp correction calculators are less accurate the farther you are from calibration temp. If using a refractometer, allow the sample to come to equilibrium with the refractometer temperature before reading (30 seconds is usually sufficient due to the small sample size.)

You should also take temperature corrected volume readings pre-boil and post-boil. Either one alone, along with its corresponding gravity reading, can be used to calculate your mash efficiency. If you have all four readings (2 volume & 2 SG) you can do a sanity check on your readings.

Pre-boil_volume * (Pre-boil_SG -1) should = Post-boil_volume * (Post-boil_SG - 1)​

If they are not close, then at least one of your measurements has significant error. If this is the case, you should carefully remeasure the post-boil volume and SG to resolve the source of the error. Of course if you lost significant volume during the boil (e.g. to a boil over) then the check is likely to fail. Also, if you added sugars during the boil, you need to correct for those additions prior to doing the check calculation.

Brew on

 

[dkevinb]

If I had a refractometer I'd do it more often - Pre-boil gravity just after mash and OG after chilling as I'm transferring to the fermenter. Since I'm still using a hydrometer I generally only take pre-boil gravity when I'm making a significant change in my process. I always take the OG as it goes into the fermenter. As doug293cz says you can always calculate your PBG backwards from your OG, although it is too late to do anything about it at that point.