Error somewhere in my gravity measurements

[codysorgenfrey]

Can some one help point me in the right direction?

The last three brews I’ve done I’ve thought I nailed my pre-boil gravity and volume, then after my boil my OG is low but my volume is correct.

I understand it’s not possible to “loose gravity”. Gravity and volume are a ratio and by boiling you’re simply concentrating the sugar. This leads me to believe there’s an error in my process somewhere.

I am very consistent and confident in my volume and gravity readings. But maybe I’m missing something with boil off or cooling shrinkage? Any help would be great.

Heres my typical process and today’s brews measurements:

1. Mash for 60 minutes. Today I mashed at 150°F with a grist ratio of 1.25 qt/lbs and a pH of 5.4.
2. Mash out by adding 10 qts of 212°F water and let rise for 10 minutes.
3. Vorlauf ~4 qts till clear.
4. Collect estimated pre-boil volume (estimated by BeerSmith). 7.5 gallons via fly sparging on a 3 tier gravity system.
5. Take pre-boil gravity. After temp adjustment it was 1.054. It was estimated to be 1.057. I decide to compensate by boiling an extra 20 minutes to get the OG I need (1.069).
6. Boil for 80 minutes, adding various hop additions.
7. Dunk immersion chiller and chill to 70°F.
8. Take gravity reading adjust for temp. I got 1.064 with 5.8 gallons. WTF?! It was supposed to be 1.069.

 

[Noob_Brewer]

Couple of things:

• #5 from your post suggests that if your volume was correct but your gravity was low (1.054), Id first look at grain crush.
• What is your boil off rate for a 60minute boil suppose to be? You end up with 7.5gallons for pre-boil volume and then after 80minutes of boil and chilling you have 5.8gallons.
• Volume accuracy - How are you determining/measuring this? I initially used a pyrex quart glass to estimate the volumes in my anvil foundry - 10.5g and make marks on my measuring stick. When I did this, I noticed the stick measures were a little higher than the kettle markings. It wasn't until I had several brews where my gravity was low and I "thought" my volumes were correct, that I decided to revisit my stick measurements. This time, rather than rely on the quart marker on the pyrex glass, I filled the kettle by weight and remarked my stick. As it turns out, the pyrex quart markings were incorrect (slightly over one quart). Ever since then, my volumes have been better and my gravities were corrected. So, this is why I am asking you now how you have measured your volumes.

 

[Qhrumphf]

Reading volumes at different temps can definitely screw up the numbers because of thermal expansion/shrinkage. I would suggest taking your pre and post boil volume measurements at the same temperature- immediately prior to and immediately after the boil is how I've done it. You may not be boiling off as much as you think you are (and that's not necessarily a bad thing, most homebrewer boiloff rates are egregious, you just need to plan for it).

That said, occams razor says your volume markings are probably off as said above.

 

[codysorgenfrey]

Couple of things:

• #5 from your post suggests that if your volume was correct but your gravity was low (1.054), Id first look at grain crush.
• What is your boil off rate for a 60minute boil suppose to be? You end up with 7.5gallons for pre-boil volume and then after 80minutes of boil and chilling you have 5.8gallons.
• Volume accuracy - How are you determining/measuring this? I initially used a pyrex quart glass to estimate the volumes in my anvil foundry - 10.5g and make marks on my measuring stick. When I did this, I noticed the stick measures were a little higher than the kettle markings. It wasn't until I had several brews where my gravity was low and I "thought" my volumes were correct, that I decided to revisit my stick measurements. This time, rather than rely on the quart marker on the pyrex glass, I filled the kettle by weight and remarked my stick. As it turns out, the pyrex quart markings were incorrect (slightly over one quart). Ever since then, my volumes have been better and my gravities were corrected. So, this is why I am asking you now how you have measured your volumes.

Ahhhh great insight. I do the same thing. I made markings on my kettle based off water measured with a Pyrex measuring cup. That’s probably what’s happening.

I’m not too concerned about the mash being a few points under. I had my brew house efficiency set higher in BeerSmith than I actually get on accident.

As far as boil off goes it’s usually around a gallon an hour but with incorrect volume markings who knows.

 

[grampamark]

Five gravity points are hardly worth getting excited about. Nobody’s gonna taste your beer and say “Dude-your OG was 5 points low”.

That said, I would bet that small errors throughout your process, especially liquid volume measurements as noted above, are the likely culprit. Absolute accuracy isn’t as important as consistency. Measure all your volumes, in every vessel, using the same method. A stainless steel ruler is cheap and more accurate than kettle markings.

 

[codysorgenfrey]

Five gravity points are hardly worth getting excited about. Nobody’s gonna taste your beer and say “Dude-your OG was 5 points low”.

That said, I would bet that small errors throughout your process, especially liquid volume measurements as noted above, are the likely culprit. Absolute accuracy isn’t as important as consistency. Measure all your volumes, in every vessel, using the same method. A stainless steel ruler is cheap and more accurate than kettle markings.

^^ So true . It’s not much difference, but I enjoy geeking out about the process. Thanks for the tip!

 

[jrgtr42]

Personally, I feel that a .003 (OG) difference in gravity is in the margin of error, so I don't worry about it.
It'll have to be something like .010 difference in estimated versus actual for me to take notice.
Also, was the recipe meant for 5.8 gallons? My seat of the pants math says that if it was around 5.5 you'd be pretty much in the area of the expected.
But yeah, one thing to check is that your volume markers are actually accurate.
Personally, I have one stick (oak dowel) that I marked all my volumes on for various kettles, and compared that to markings on my fermenters. So, if it was off, it was consistantly off. I do and did have one quart-size p|yrex measuring cup that I use for all the measuring.
As it happenned, I got a new kettle that has the markers pre-etched in, and the 5 gallon mark on the new one matches to 5 gallons on my stick, and therefore my other kettle. So if my measurements back then were off, at least the new one is off too.

 

[Qhrumphf]

Also, if you sight your volume markings with room temp water, that won't translate to hot wort either. So that can also muck with the math.

 

[doug293cz]

One thing that I didn't see mentioned: SG measurements prior to boil can be very unreliable. If the wort is not completely homogenized prior to taking a sample, the SG reading can be either too high or too low, depending on where the sample was taken. If you sparge, stir the wort aggressively before you take an SG measurement.

Brew on

 

[Vale71]

Assuming mash-out temp was around 170°F you'd multiply your pre-boil volume measurement by the correction factor 0.98. That would give you a room temp volume of 7.5*0.98=7.35g
Post boil volume was measured at room temp so no correction needed. If volume is measured at flame-out then the correction factor 0.96 needs to be applied.
We now divide pre-boil volume by post-boil 7.35/5.8=1.267.
We now multiply pre-boil OG-1 by this factor and add back 1 to the result to get the expected post-boil OG.


(1.054-1)*1.267+1=1.0684

You appear to be off by 0.0044 gravity points so either your gravity measurement was off (stratification?) or you volume measurements are still inaccurate despite being corrected for temperature. Hope I've geeked this out enough for you.

 

[jerrylotto]

Boil does more than just concentrate a sugar solution. You are driving off volatiles, precipitating out protein-tannin complexes, dissolving and isomerizing hop oils, and oxidizing beta acids like lupulone. One thing you may want to change about your process is your mash out water temp. Too hot and you pull extra nasties out of the grist. I would recommend 172F max for anything that touches grain - that temp will deactivate both alpha and beta amylase activity.