Author Topic: Post Boil SG request  (Read 358 times)

Offline gizzygizmo

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Post Boil SG request
« on: June 20, 2019, 05:26:39 PM »
I'd like to request a variable be created to display the post-boil gravity... while the wort is still hot and hasn't shrunk yet. I've been playing with removing shrinkage to see the math that adjust volumes based on temp and I've come to the conclusion that $EST_OG is the gravity reading of the cooled/shrunk wort.  Trying to match this value at post-boil will be off.

Examples...

Shrinkage removed from equipment:
1) post mash/preboil volume 8 and Gravity of 1.039
2) Post boil volume is displayed as 6.5 gallons => 8 gallons - boil off 1.5 and no shrinkage
3) OG of 1.048 is displayed
Math on this is 39 * 8 = 312 points, divided by final volume of 6.5g = 1.048, all is well.

Having a 4% shrinkage in equipment:
1) post mash /preboil volume of 8.27 and Gravity of 1.038 (i suspect because 8 gallons + 3-4% shrinkage at 150degF is 8.27 and will make the Gravity be (39*8)/8.27 => 37.72 and it rounds up to display 37.27
2) Post boil volume is displayed as 6.77 = 8.27 pre boil, minus boil off 1.5, minus shrinkage of .22
3) OG of 1.048 is displayed
Math on this is 38 * 8.27 = 312.26, divided by final volume of 6.5 = 1.048

My issue is that I like to check the gravity right toward the end of boil.  So when the post boil volume is reached around 6.77 gallons i'm reading 1.046 etc, and trying to figure out why i'm short. My instinct is to do the math of "preboil OG * preboil volume / current volume" to get what my SG should be.  At the end of boil I would assume i should get the OG.  But the OG displayed is for chilled/cooled/shrunk wort not hot 212degF expanded wort.

My custom brewsteps currently  display "measure post boil volume: " and suggest $POST_BOIL_VOLUME.
Then I have a line for "measure post boil gravity: " and i'm suggesting $EST_EG. 

The $EST_EG  is usually off by a bit, in one case i'm getting 1.046.  Initially I thought i was off and would think i need to adjust with some DME to 1.048.  If that's done then it'll chill/shrink and be at 1.049.  These numbers don't seem to bad,  and i never really noticed until i started doing some batches of RIS in which the values are exaggerated.  A 8.27gallon volume preboil at 1.084 gives me 6.77gallons at 1.102, chilled it is 1.106.   I don't swat 1 or 2 gravity points, but i want to know why i'm off by 4-5 points and the only thing I did was boil.

I would like to be able to just display the value here.. or if I could do some math somehow and display my own values that would be nice.  I'd like to multiply the batch volume * gravity and divide by pre-boil volume to display a target "post boil gravity" that I can check before the wort is chilled.  If any numbers are wrong "now" i can throw DME in or water and it will be sanitized by heat.  If i wait until its chilled it's too late.

For now i'm just scribbling in a number, and i can do math if the volumes are off (larger/smaller preboil or post-boil than planned means I need to change the numbers anyway)

Offline gizzygizmo

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Re: Post Boil SG request
« Reply #1 on: June 21, 2019, 12:04:29 PM »
While researching I found the boil off tool actually has this data... and someone else with the same questions:

https://imgur.com/a/uiAMQ

This tool shows 1.103 Post Boil Gravity at 6.75gal.  Then when i check my recipe i have 1.106 at 6.5gallons.

I just want to be able to print $POST_BOIL_GRAVITY in custom reports.

Offline gizzygizmo

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Re: Post Boil SG request
« Reply #2 on: June 21, 2019, 12:07:10 PM »
I think the real question would be that if we are cooling our sample down to 60-68degF is the sample shrinking 4% as well and therefore the PreBoil and PostBoil SG calcs should not include shrinkage/dilution. 

I always cool my samples down, but i'm using a hydrometer so it doesn't take long.  The SG does go up from the initial measurement, but I would have taken any readings and assume that my mash eff was a function of my preboil SG vs what's technically possible.

Offline Oginme

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Re: Post Boil SG request
« Reply #3 on: June 21, 2019, 01:23:04 PM »
As in your other post, I think you misunderstanding the gravity measurements.  The gravity measurement should always be taken at the calibration temperature of your hydrometer and this figure should be marked on the scale of the hydrometer.  What the program does not do accurately enough is correct the volume to standard conditions for purposes of completing the sugar balance equation.  This error is equivalent to the actual thermal expansion when the volume measurement is taken and recorded.  If you use a thermal expansion coefficient of 4% (212F/100C) and measure the volume at 154F/68C, then the thermal expansion of water is around 2.3%.  This means that the sugar balance error is around 1.7%, which is typically around most brewer's measurement error for volume. 

At hot temperatures or cold temperatures the amount of sugar and the contribution to density is the same.  It is the thermal expansion of the water which is different from room temperature to hot temperatures.
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Offline gizzygizmo

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Re: Post Boil SG request
« Reply #4 on: June 22, 2019, 09:47:20 AM »
So are you measuring the gravity of your wort at the preboil temp of 170degF (mash out) and then again post boil at 212degF?  Or do you take the sample and chill it to 68degF and measure? 

My older hydrometer has corrections for temp, so technically I guess you could take it at 170 or 212 degF and adjust for temp.  But I've since moved on to a refractometer and hanna which both require the sample to cool (which happens in a matter of minutes).  This is very similar to pH readings, which are always displayed and talked about at room temp, nobody takes pH readings at mash temp ... and if they do they have to correct for temp back to down to room temp.

The original request I had was to be able to display the "POST BOIL GRAVITY' which would be the gravity at 212degF before cooling.  The program currently only displays the EST_OG which is the SG at room/pitching temp.  This way I could add/remove DME while the wort was still hot enough to sanitize it the adjuncts.    What i'm finding now is that I'm going to want the numbers corrected to room temp volumes because that's the temp i'm having to-do readings.   I'm not taking the SG readings at preboil/boil temps and then trying to correct the SG value for temp; i'm actually cooling the sample and taking the readings at room temp like I do with pH.

It may make more sense to print the total gravity points, then the corrected SG reading with the room temp, volume in parenthesis in case you're doing math on total points, and then preboil volume.

Example, maybe allow access to these variables so I can pick which one sto display.. or display them all:

PreBoil:
8.27 gallons (8 gallons room temp)
1.084 SG at 170degF (1.086 SG room temp)

Final Volume:
6.77 gallons post boil (6.5 gallons room temp)
1.103 SG at 212degF (1.106 SG at room temp)

The value i thought I needed was 1.103 "post boil OG" but what i'm finding I really want is "pre boil sg at room temp".

Offline dtapke

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Re: Post Boil SG request
« Reply #5 on: August 12, 2019, 08:16:43 AM »
I was trying to answer some questions to a new user of the software about this on a recent brew day. I would love to see the addition of perhaps @TEMP in the pre-boil, post-boil, etc measurements. this would reduce the need to worry about thermal expansion and adjusting the rate in the equipment profile.
'
this way one could input "pre-boil volume" as X.XX units @ XXX degrees (F/C), as well as into fermenter at X.XX units @ XX Degrees (F/C)
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