Pre-boil vs Final Gravity

[uatuba]

Topic should say: Pre-boil vs Original Gravity

How can my pre-boil gravity be too low according to Beer Alchemy, but my OG be a point higher than predicted? Volumes were the same as predicted--I didn't boil off more than planned and I actually ended up with slightly less wort due to grain absorption.

Keep in mind, my pre-boil gravity was measured at 131F and my OG was measured at 84F. I read somewhere a few years ago that temperature corrections for hydrometer readings didn't work well outside of certain ranges. Could this be the issue?

I've never checked gravity pre-boil because I've always hit or exceeded my OG. I wanted to make sure I wasn't too high this time (due to my efficiency creeping higher and higher) and make the necessary adjustments if I was, but when I saw that it was actually "low," I didn't think it was right so I proceeded as normal. I'm glad I did.

 

[ajf]

Temperature is one possible cause. I never rely on temperature corrections if the temp is > 100F.
Another possible cause is stratification. You pre-boil wort needs to be mixed really well before taking a gravity sample, otherwise the wort on the bottom will be much denser than the wort at the top.

-a.

 

[duboman]

Temperature is one possible cause. I never rely on temperature corrections if the temp is > 100F.
Another possible cause is stratification. You pre-boil wort needs to be mixed really well before taking a gravity sample, otherwise the wort on the bottom will be much denser than the wort at the top.

-a.

+1 to this plus gravity will dependent upon your volumes as well. You could have had low efficiency in your mash resulting in a lower pre-boil gravity but if you then boiled off more than expected your resulting post boil gravity could have been higher than expected.

 

[uatuba]

I thought about stratification, but I mixed like a mofo before I checked. Then I mixed again, and checked again. I'm guessing it's the temperature. So...I need to get a refractometer I suppose.

+1 to this plus gravity will dependent upon your volumes as well. You could have had low efficiency in your mash resulting in a lower pre-boil gravity but if you then boiled off more than expected your resulting post boil gravity could have been higher than expected.

Did you read the post? All volumes were spot on except for LESS wort, which would actually increase the gravity if the quart difference was that big a deal.

 

[duboman]

I did....

You stated you got less wort due to absorption so then how were your volumes spot on?

If the volume in your primary was correct then you perhaps boiled off less? Did you top off?

Are you judging your volumes based solely on bucket markings? I've found several buckets to not be properly calibrated....never assume these things

In the end I'm just trying to help you figure this out, no need to be snarky about it

Also regarding calibration, when was the last time you checked your measuring devices like hydrometer and thermometer? And one other thing, never trust a temp correction over 100F, they are hugely inaccurate


Sent from the Commune

 

[uatuba]

I did....

You stated you got less wort due to absorption so then how were your volumes spot on?

If the volume in your primary was correct then you perhaps boiled off less? Did you top off?

Are you judging your volumes based solely on bucket markings? I've found several buckets to not be properly calibrated....never assume these things

In the end I'm just trying to help you figure this out, no need to be snarky about it

Also regarding calibration, when was the last time you checked your measuring devices like hydrometer and thermometer? And one other thing, never trust a temp correction over 100F, they are hugely inaccurate


Sent from the Commune

I appreciate the help and didn't mean to come across as snarky. I said "slightly" less...which, in my experience, should mean a higher gravity if all else were equal. We are talking about 1 or 1.5 quarts difference. I am judging the volumes based on bucket markings, but they're my own markings...I checked them twice before marking by using a 1 gallon pitcher that was tested both by weight as well as by a pre-marked Erlenmeyer flask (the pitcher was easier to fill and pour).

The volume at pitching (when I measured the original gravity) was spot on, without topping up. The measured original gravity was higher, not lower, than predicted by 1 point (after correction). My hydrometer has been calibrated (well...not really, as it measured distilled water correctly at 1.000). My thermometer is accurate at boiling and freezing, so I have to hope it is correct at mash temps (I've always hit my original gravity as predicted or slightly higher--I just never measured pre-boil gravity). The only reason I chose to measure this time is that I have noticed a slight increase in original gravity that last few times, and I wanted to adjust pre-boil if necessary.

It's good to confirm that the temperature correction formula isn't correct for temps at extremes and I am willing to accept that as the difference in what was expected using my standard process. It wouldn't make sense to have a perfect volume and near perfect gravity at pitching if the pre-boil gravity were considerably low with a slightly lesser amount of wort. My question now becomes...how does everyone else measure pre-boil gravity? Mix like hell and chill the sample?


Sent from my iPad using Home Brew

 

[duboman]

No worries...

Yes, mix the wort well and chill the sample or use a refractometer with ATC. I personally do the former, don't have the latter.


Sent from the Commune

 

[MikeInMKE]

BTW, I've found that the ATC on my refractometer doesn't work as well as you'd expect after sitting on a cold table in a cold garage during a Wisconsin winter.

My testing supplies and tools (refractometer, hydrometer, pH meter and buffers) now stay inside at room temperature during brew day.

 

[duboman]

BTW, I've found that the ATC on my refractometer doesn't work as well as you'd expect after sitting on a cold table in a cold garage during a Wisconsin winter.

My testing supplies and tools (refractometer, hydrometer, pH meter and buffers) now stay inside at room temperature during brew day.

True, while I don't have an refractometer I have since been informed that the ATC pertains to the temp of the device, not the sample itself, so yes, if your device is too cold or too hot the readings will be off.