Author Topic: Temperature Correction for Post Mash & Preboil Gravity  (Read 1928 times)

Offline brewfun

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Temperature Correction for Post Mash & Preboil Gravity
« on: September 24, 2016, 08:43:17 PM »
The fields for Post Mash and Preboil gravity seem to be responding to cooling shrinkage in the equipment profile as if it were a form of dilution. With all other equipment fields at zero, these two estimated gravities will rise and fall with the expansion/shrinkage. Since the gravity is a room temperature measurement, it should probably be independent of that.
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Offline gizzygizmo

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Re: Temperature Correction for Post Mash & Preboil Gravity
« Reply #1 on: June 21, 2019, 12:09:14 PM »
Popping this one to the top... I noticed that preboil SG is adjusted by shrinkage, and in the "boil tool" the post-boil is also calculated with shrinkage/dilution.   Then the EST_OG is calc'd on the final volume cooled.

I'm not sure if the sample cooling means the SG displayed should not include shrinkage but i'm inclined to think so.  Most recipes the diff is only 1-2 gravity points at preboil so people may have not been noticing.  I noticed on some larger beers where the diff was significantly more.

Offline gizzygizmo

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Re: Temperature Correction for Post Mash & Preboil Gravity
« Reply #2 on: June 21, 2019, 12:10:33 PM »
i opened a request to be able to display post-boil-gravity, but depending on how these threads go I may just want to be able to display pre-boil-gravity without shrinkage adjustments.

http://www.beersmith.com/forum/index.php/topic,20782.0.html

Offline Oginme

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Re: Temperature Correction for Post Mash & Preboil Gravity
« Reply #3 on: June 21, 2019, 01:09:50 PM »
Gravity is always measured at standard temperature which is or should be on your hydrometer.  What I think you are looking for is the VOLUME with and without the thermal expansion.
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Offline gizzygizmo

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Re: Temperature Correction for Post Mash & Preboil Gravity
« Reply #4 on: June 22, 2019, 09:03:37 AM »
I am looking for the volume to know how much should be there... but the SG measurement in BS for preboil is accounting for thermal expansion too... which I do not belive is wrong.   If i have not hit my numbers correclty I can't multiply the volume times SG to get total points, because the SG has a temperature correction applied.   If i change my equipment profile to have 0% shrinkage the preboil SG goes "up".  The SG should not go up regardless of shrinkage percentage because it's measured at the same temp  (room temp).   Having 6.7gallons at 212degF should measure the same SG as the the vessel at 68degF.  The total points of SG in the bucket are the same, and you cool to 68degF when you measure.   If you measured at 212degF and temp-adjusted the resulting SG value you could maybe get a different SG value because of thermal expansion.

On of the camps is correct, but in BS we're doing it different in two spots.  You can test this by doing the following:
1 -> the pre-boil SG target changes if you adjust your equipment profiles shrinkage from 4% to 0%.  it goes "up" when there is no shrinkage because the per-boil volume goes down.
2 -> your estimated $EST_OG (final OG) which would be measured at pitching temps does not change, but in the boil tool the "post boil gravity" does change if you adjust shrinkage from 4% to 0%.  If you change shrinkage to 0% it goes "down" because it assumes no shrinkage and your finale "volume" is larger.

I think either

(1) the preboil SG should be actual SG at room temp
or
(2) we should have a second value displayed "post boil OG" which is the OG adjusted for shrinkage

IMO, after researching this a bit I believe that all the SG values should be displayed as measured at room temp which would mean you adjust the volume for shrinkage down to room temp.  To further help we could display some notices/values that say "at room temp" or "at 68degF" to help alleviate confusion. 

I know a lot of people do quick math of SG * volume to get total points, and then divide by your current volume to find your OG.  I do this a lot when I've overshot my target... collected to much or to little preboil volume.  But this math breaks with the current values on larger beers...

Offline gizzygizmo

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Re: Temperature Correction for Post Mash & Preboil Gravity
« Reply #5 on: June 22, 2019, 09:28:02 AM »
This example may help... i think a lot of people do this:

1) RIS target pre boil volume 8.27gallons, 1.084 SG.
2) boil off 1.5gallon, shrinkage .27, trub loss 1gallon => should leave me with 5.5 into a fermenter, 1/2gallon of yeast/trub loss gives me a full 5gallons
3) post boil OG says 5.5gallons at 1.106,  but 84 * 8.27 = 732.48 / 6.5 = 1.112

The math of using 1.084 is fine if you're doing total point calcs... "84 * 8.27 = 694.68".  You can take the 694.68 and divide by your ending volume of 6.5 and get the 1.106 value you are planning on ending up with.  But if you measure that SG of the 8.27gallons it'll be 86 not 84.  That's because your sample at room temperature shrunk...

Someone taking a preboil reading here would assume they were 2 points higher at 1.086 and may choose to boil softer and not loose as much wort due to evaporation to keep a 1.106 OG rather than the now calculated 1.109.

Conversely, someone measuring 1.084 thinks they are "spot on, nailed it" but when they are done they will be 4% off... their final OG will be 1.103 and they'll be left trying to figure out where the other 3 points of gravity went... checking volume markers, was their boil kettle stratified when they took a sample, etc.  No, they had 1.084 at 8.27 gallons... but that's really 8 gallons at room temp and they measured 1.084.  And 84 * 8 = 672, and 672 divided by final volume of 6.5 = 1.103.

For smaller beers this issue isn't as bad.. because we're talking percentages and only 3-4%.  My Amber ale is expected to be 1.042 pre boil in BS (with shrinkage), and displays 1.043 when shrinkage is removed.  I really only see an issue when beers are over 1.050 ... because 4% of 50 is when the SG will be drifting 2 or more based on shrinkage.