Author Topic: post-boil gravity not calculating correctly  (Read 15895 times)

Offline klnosaj

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post-boil gravity not calculating correctly
« on: May 07, 2012, 01:47:21 PM »
I posted this in the support forum but apparently that wasn't the correct space for it.

The past few brews I've done have all come in with low OG's.  My pre-boil volume and gravity are correct and my post-boil volume is correct but the post-boil gravity is always lower than the software predicts.  Here's what I don't get: if I start with 7 gallons of pre-boil wort at 1.033, the post-boil gravity assuming 1 gallon loss to boil and evaporation should be 1.039 (rounded from 1.385) yet the software keeps telling me I should have a post-boil gravity of 1.041.  (33 points per gallon in 7 gallons = 231.  231 points in 6 gal = 1.0385)  No matter what recipe or what the starting gravity is, the software always predicts a post-boil volume that is impossibly high given the pre-boil gravity and the amount of boil-off.   My equipment profile is configured correctly from what I can tell and my post-boil measurements always add up (as in: 7 gals pre-boil - 1 gal boil-off -.5 gal trub loss - 4% cooling shrinkage=5.25 gallons into the fermenter).  What gives?  I'm increasingly annoyed by relying on software that's inducing me to miss my OG.  I'm sure it's user error, but where?

Offline jomebrew

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Re: post-boil gravity not calculating correctly
« Reply #1 on: May 08, 2012, 09:38:02 AM »
What OG are you hitting?  1.039 or 1.041?

Is 1.033 measured or is this calculated?

Is your instrument calibrated?  Adjusting properly for temperature?

I would recommend taking a fermenter sample at 60F and measuring it with a calibrated instrument.  Only then will you know for sure the gravity and can focus on what, if anything in BS2 needs to be adjusted.

 


Offline tom_hampton

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Re: post-boil gravity not calculating correctly
« Reply #2 on: May 08, 2012, 10:37:06 AM »
I think he is saying that BS2 is calculating an incorrect (too high) post-boil gravity based on his boil-off rate. 

Calculated pre-boil   = 1.033 @ 7 gallons (231 points)
Calculated post-boil = 1.041 @ 6 gallons (246 points)...where did the 15 extra points come from?

His actual pre-boil is the same as BS, but actual post-boil is 1.039 @ 6 gallons = 234 points.

I haven't had a chance to play with this, and see if there is a way to force BS2 to create gravity points during the boil.  I'm pretty sure I don't have this issue, but I wouldn't sweat 2 points of SG (that's within my tolerance of good enough).  So, I can't be sure I would take note of a consistent 0.002 low reading.  I WOULD notice 4 points in 70, though.  If it is a linear error...I would certainly catch that.  1 Plato is where I draw the line, and look for a cause. 

I did NOT have this issue with my pliny batch (I was dead on at 1.070 OG) on Sunday.

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Offline klnosaj

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Re: post-boil gravity not calculating correctly
« Reply #3 on: May 08, 2012, 10:52:23 AM »
I think he is saying that BS2 is calculating an incorrect (too high) post-boil gravity based on his boil-off rate. 

Calculated pre-boil   = 1.033 @ 7 gallons (231 points)
Calculated post-boil = 1.041 @ 6 gallons (246 points)...where did the 15 extra points come from?

His actual pre-boil is the same as BS, but actual post-boil is 1.039 @ 6 gallons = 234 points.

I haven't had a chance to play with this, and see if there is a way to force BS2 to create gravity points during the boil.  I'm pretty sure I don't have this issue, but I wouldn't sweat 2 points of SG (that's within my tolerance of good enough).  So, I can't be sure I would take note of a consistent 0.002 low reading.  I WOULD notice 4 points in 70, though.  If it is a linear error...I would certainly catch that.  1 Plato is where I draw the line, and look for a cause. 

I did NOT have this issue with my pliny batch (I was dead on at 1.070 OG) on Sunday.

You are correct: my problem is with BeerSmith calculations and not with my measurements.  The error does not appear to be linear within a given equipment profile but the error does change depending on the profile selected.  With my current equipment profile beersmith is always 2 or 3 points over what math tells me the number should be.  I'd really like to be able to dial this software in more precisely but there appears to be some blackbox calculating going on and I would love it explained.

Offline jomebrew

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Re: post-boil gravity not calculating correctly
« Reply #4 on: May 08, 2012, 11:10:57 AM »
Software is never going to be precise.  Your process has too many variables.  Software can tell your the ballpark. 

You are rather fortunate to have no problems with your process and your measurements tools are perfectly calibrated and your process for measuring is flawless. 

I would guess there is a problem with your boil volumes.  are you topping off with water that is not in your equipment profile?

Offline klnosaj

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Re: post-boil gravity not calculating correctly
« Reply #5 on: May 08, 2012, 12:14:27 PM »
Software is never going to be precise.  Your process has too many variables.  Software can tell your the ballpark. 

You are rather fortunate to have no problems with your process and your measurements tools are perfectly calibrated and your process for measuring is flawless. 

I would guess there is a problem with your boil volumes.  are you topping off with water that is not in your equipment profile?

Your sarcasm is misplaced and unappreciated and does not reflect well on this forum.

I am not at all talking about my measurements.  Measurement has exactly nothing to do with my question.  I am simply trying to figure out how the software calculates the OG.  Given a pre-boil volume of x with y gravity points, the garvity points in x' are easy to determine.  Yet the software doesn't seem to me to come up with the correct number.  I'm trying to figure out why and what I'm misunderstanding. 

Offline tom_hampton

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Re: post-boil gravity not calculating correctly
« Reply #6 on: May 08, 2012, 12:31:45 PM »
The error does not appear to be linear within a given equipment profile but the error does change depending on the profile selected.

This is interesting.  can you make some simple recipes with just say 5, 10, and 20 lbs of 2-row...and use that to test each equipment profile?  Then can you post the different equipment profiles and what their errors are?

Somewhere, I think beersmith is thinking some "extra" wort is appearing from the mash.  Its like it thinks your preboil volume is higher by 1/4 gallon or so. 
R.I.P.:Belgian Blonde
On Tap: Apfelwein, Kolsch(v2), Pumpkin Ale, Belgian Specialty 
Aging/Storing: Coffee Porter, Chocolate Porter, Flanders Red, English Barlywine
Fermenting: Maggie's Altbier
Next Up: PtE(1.1), Belgian Dubbel?

Working thru all BCS recipes

Offline klnosaj

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Re: post-boil gravity not calculating correctly
« Reply #7 on: May 08, 2012, 03:10:40 PM »
The error does not appear to be linear within a given equipment profile but the error does change depending on the profile selected.

This is interesting.  can you make some simple recipes with just say 5, 10, and 20 lbs of 2-row...and use that to test each equipment profile?  Then can you post the different equipment profiles and what their errors are?

Somewhere, I think beersmith is thinking some "extra" wort is appearing from the mash.  Its like it thinks your preboil volume is higher by 1/4 gallon or so.

I have to correct myself.  After following your suggestion it turns out the error is linear.  See attached.

   

Offline MaltLicker

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Re: post-boil gravity not calculating correctly
« Reply #8 on: May 09, 2012, 04:18:02 PM »
I find the quickest way to resolve equipment profile questions is to post the equipment profile so people can look at exactly what is going on. 

There are lots of little check boxes and options buried on multiple screens, etc.

I'm sure some experienced user will try to help or Brad might, but providing the setup will certainly help.

Offline klnosaj

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Re: post-boil gravity not calculating correctly
« Reply #9 on: May 10, 2012, 06:41:37 PM »
I find the quickest way to resolve equipment profile questions is to post the equipment profile so people can look at exactly what is going on. 

There are lots of little check boxes and options buried on multiple screens, etc.

I'm sure some experienced user will try to help or Brad might, but providing the setup will certainly help.

I couldn't figure out how to format correctly so it is herewith attached as a .pdf (note the only fudge factor is where I set trub loss to .53 gals so that total boil volume added up to a nice and neat 7 gals total.  My measured trub loss is usually ~2qts so the error isn't much of an exaggeration at all.)  I hope someone can help me demystify this.  I'm surprised to learn I'm the only one for whom this problem seems to manifest.  Makes me more convinced it's something I'm doing wrong.  Now if only I/we could figure out what that is...
« Last Edit: May 10, 2012, 06:43:58 PM by klnosaj »

Offline MaltLicker

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Re: post-boil gravity not calculating correctly
« Reply #10 on: May 11, 2012, 12:33:43 PM »
Go to Profiles \ Equipment, highlight yours, right-click, export as, name it, then post it.  Here's mine.

Offline MaltLicker

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Re: post-boil gravity not calculating correctly
« Reply #11 on: May 11, 2012, 12:52:21 PM »
By looking at my equipment profile, I noticed a check box checked on yours and not mine:  "Adjust Mash Vol for Deadspace" which seems to force BSmith to add mash volume to compensate for that loss, which would put "whatever fraction" that gets out of mash tun in the boiler, boosting gravity? 

Your 0.13 lauter tun deadspace is about 16 oz so you might check math to see if that amount would create the 1-2 points extra gravity you're trying to explain. 

It also takes that amount directly out of sparge water, replacing water/wort mix with pure wort?  That's my best guess. 

Offline klnosaj

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Re: post-boil gravity not calculating correctly
« Reply #12 on: May 14, 2012, 06:39:01 PM »
By looking at my equipment profile, I noticed a check box checked on yours and not mine:  "Adjust Mash Vol for Deadspace" which seems to force BSmith to add mash volume to compensate for that loss, which would put "whatever fraction" that gets out of mash tun in the boiler, boosting gravity? 

Your 0.13 lauter tun deadspace is about 16 oz so you might check math to see if that amount would create the 1-2 points extra gravity you're trying to explain. 

It also takes that amount directly out of sparge water, replacing water/wort mix with pure wort?  That's my best guess.

I don't think that's it either, though I really appreciate you looking so closely at my profile.  The water I'm putting into the kettle is the right amount at the right gravity so that extra pint (.13 gallons in bsmith) is already accounted for.

My thought has turned to boil-off rate.  What assumptions is the equipment profile dialog box making when it asks for "Boil Off?"  I calculated by measuring the total volume remaining in the kettle--that's wort, trub, break, hops and all--at the time of flame out.  Is that when/how I'm expected to do it?

Offline tom_hampton

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Re: post-boil gravity not calculating correctly
« Reply #13 on: May 14, 2012, 09:18:59 PM »
Cooling shrinkage.

41*5.75 = 235 points.  That's close enough for round off error to account for the rest.

R.I.P.:Belgian Blonde
On Tap: Apfelwein, Kolsch(v2), Pumpkin Ale, Belgian Specialty 
Aging/Storing: Coffee Porter, Chocolate Porter, Flanders Red, English Barlywine
Fermenting: Maggie's Altbier
Next Up: PtE(1.1), Belgian Dubbel?

Working thru all BCS recipes

Offline klnosaj

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Re: post-boil gravity not calculating correctly
« Reply #14 on: May 16, 2012, 03:00:33 PM »
Cooling shrinkage.

41*5.75 = 235 points.  That's close enough for round off error to account for the rest.

By golly, I think you may be right.  I re-ran all the test batches using 96% of post-boil volume and the numbers got a lot closer.  I still can't figure out why my pre-boil gravity is always correct and my post-boil gravity always low even though my post-boil volume is as close to correct as I can measure.  There's an extra quart appearing somewhere.  Now the fun really starts as I try to figure out where (/sarcasm)

I'm still unsure that I'm measuring post-boil volume the way bsmith expects me to (please see my previous post in this thread).  Can anyone help me with that?