Author Topic: Several problems with OG calculations  (Read 12071 times)

Offline rosenjm

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Several problems with OG calculations
« on: July 25, 2011, 05:44:48 PM »
OK, so I've noticed a few things.  Someone help me out if I'm totally off base here.

1.  I already brought this up in this thread: http://www.beersmith.com/forum/index.php/topic,5124.0.html but to sum it up...

I have about 1.25 gal of losses between my boil kettle and the fermentor.  If I select a batch size of 10.5 gal into the fermentor, BeerSmith2 gives me a PRE-boil volume of 14.3 gallons and a POST-boil volume of 12.2 gal (which works, 2.1 gal in evaporation; 14.3 - 2.1 = 12.2).

For a recipe with a OG of 1.045 with 100% pale malt (36 GU/lb), BeerSmith2 calculates 17.5 lbs of malt.  However, if you do the math 17.5 lbs of pale malt will yield an OG of 1.045 for a 10.5 gal POST-boil volume, not a 12.2 gal POST-boil volume.

Assume 75% efficiency:

45 GU * 10.5 gal = 472.5 total GU
472.5 /36/.75 = 17.5 lbs pale malt

45 GU * 12.2 gal = 549 total GU
549/36/.75 = 20.3 lbs pale malt

In order to hit an OG of 1.045 with a POST-Boil volume of 12.2 (in order to get a batch volume in the fermentor of 10.5 gal) I need 20.3 lbs of malt.  BeerSmith2 is telling me I need 17.5 lbs.

2. Changing the boil time has no effect on OG.  It will change the POST-boil volume, but regardless of the boil time, the OG remains the same.  I'm not sure what boil time it is using to determine the OG.  Perhaps the default boil time in my equipment profile?

Offline BeerSmith

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Re: Several problems with OG calculations
« Reply #1 on: July 26, 2011, 04:33:54 PM »
Hi,
  I've covered this quite a bit - the brewhouse efficiency includes losses (its not the mash efficiency):

  http://www.beersmith.com/blog/2008/10/26/brewhouse-efficiency-for-all-grain-beer-brewing/

Cheers,

Brad
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Offline rosenjm

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Re: Several problems with OG calculations
« Reply #2 on: July 26, 2011, 06:02:59 PM »
I'm really confused.  I'm not having a problem with efficiency.  I'm having a problem with the fact that the software is (appears) using the wrong volume to calculate OG. 
Quoting from your article:

“(potential_pts) = (grain_pts) * (weight lbs) / volume_gals
Each grain has a dry grain potential, which you can find from our grain listing or from the malter’s web site.  The grain_pts is calculated from the grain potential by subtracting 1.000 and multiplying by 1000.  For example, a grain with a potential of 1.035 becomes simply 35 points.  5 pounds of this grain in a 5 gallon batch would add 35*5/5 = 35 potential points to the beer.  If we sum all of the potential points from the various grain additions we can get the overall potential.  If we had no losses in the system, the 35 points above would give an ideal starting gravity for our beer of 1.035.
I mentioned that the potential points represents the gravity under ideal conditions.  In practice one gets much less than this, usually around 70-80% for brewhouse efficiency overall.  Therefore the actual original gravity is determined by the potential points times the gravity:
(batch_pts) = (potential_pts) * (brewhouse efficiency)
So if we consider a recipe with 40 potential points, and a 75% brewhouse efficiency we get 30 batch points or an original gravity of 1.030.  This is how original gravity is estimated.”

Let’s do some math:

Using my example of a recipe with 100% pale malt with a (grain_pts)=36 and a efficiency of 75% with a target OG of 1.045.

If I put this recipe into Beer Smith, it tells me I need 17.5# of grain for a batch size of 10.5 gal in the fermentor.  Under the water profile, it tells me, based on my equipment profile that my post boil volume will be 12.2 gal.  The difference between the 12.2 gal post boil volume and the 10.5 gal fermentor volume is based on a 4% shrinkage (.49 gal) and 1.25 gal trub and chiller losses.  These losses are due to a loss of volume and will have no affect on the gravity of the wort. (If I take a pot with 12.2 gallons of wort at 1.045 and dump 1.74 gallons of that wort on the ground, the gravity of the 10.46 gallons of wort left will still be 1.045).

Therefore:  OG of 1.045 SHOULD be based on the 12.2 gallon post boil volume.

Using your equations:
(batch_pts)=[ (grain_pts) * (weight lbs) / volume_gals]* (brewhouse efficiency)
(batch_pts)=[(36*17.5lbs/12.2gal)*.75] = 1.039

Using the conventional method I used in my earlier post, I came up with 20.3 lbs of grain.  Back to your equations:

(batch_pts)=[ (grain_pts) * (weight lbs) / volume_gals]* (brewhouse efficiency)
(batch_pts)=[(36*20.3bs/12.2gal)*.75] = 1.045

As far as the second point goes, I now see that the program adjust the pre boil volume to account for the longer boil.  That makes sense.  The point still remains that if I use Beer Smith 2, it will tell me that to get a gravity of 1.045 at 12.2 gal I need 17.5 lbs of Pale Malt.  See the attached screen shot.  Based on your own article and equations, this simply doesn’t work. 

Offline rosenjm

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Re: Several problems with OG calculations
« Reply #3 on: July 30, 2011, 04:50:45 PM »
Really? Nothing?  OK then, can someone please help me figure out how I have BS2 incorrectly configured?  I would really like to use this software, but I don't want to have to calculate my grain bill by hand.  That is kinda the reason I'm using software.  Anyone?

Offline makisupa

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Re: Several problems with OG calculations
« Reply #4 on: August 01, 2011, 08:58:08 AM »
FWIW, I was originally using the old BeerSmith on a Mac running Parallels. I upgraded to BeerSmith 2 for Mac and my OG calculations are also off. My last two batches fell short by about 15 points. For fun, I entered identical Imperial IPA recipes using identical equipment profiles into both programs to see the resulting "Est. Boil Vol."

Old BeerSmith
Total Grain: 15.03 lbs
Batch Size: 5.25 gal
Efficiency: 75%
Boil Time: 60 min
Est OG: 1.085
Est Boil Vol: 7.02

New BeerSmith
Total Grain: 15.03 lbs
Batch Size: 5.25 gal
Efficiency: 75%
Boil Time: 60 min
Est OG: 1.085
Est Boil Vol: 8.71


Offline makisupa

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Re: Several problems with OG calculations
« Reply #5 on: August 01, 2011, 07:17:19 PM »
I did some research later this afternoon and learned about the inconstant evaporation rate and was able to fix the problem; however, I found another bug that the boil off amount (which I use for the work-around) doesn't remain saved even after pressing OK. :(

Offline MaltLicker

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Re: Several problems with OG calculations
« Reply #6 on: August 01, 2011, 08:04:51 PM »
Really? Nothing?  OK then, can someone please help me figure out how I have BS2 incorrectly configured?  I would really like to use this software, but I don't want to have to calculate my grain bill by hand.  That is kinda the reason I'm using software.  Anyone?

Can you post the recipe file? 

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Re: Several problems with OG calculations
« Reply #7 on: August 01, 2011, 09:19:53 PM »
Hi,
  I'm going to review this all one more time this week - if you have a particular recipe causing problems, please forward it on to me (at my main email which is beersmith at my beersmith.com domain).  Often I find it is actually a problem with the equipment profile and not the program calculations, but please forward your examples so I can go through them.

Brad
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Offline rosenjm

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Re: Several problems with OG calculations
« Reply #8 on: August 02, 2011, 04:51:13 AM »
Can you post the recipe file?  

Here is the recipe file.  This shows a batch volume of 10.5 gal with 17.4 lbs of grain.
« Last Edit: August 02, 2011, 05:04:37 AM by rosenjm »

Offline rosenjm

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Re: Several problems with OG calculations
« Reply #9 on: August 02, 2011, 05:03:19 AM »
Hi,
  I'm going to review this all one more time this week - if you have a particular recipe causing problems, please forward it on to me (at my main email which is beersmith at my beersmith.com domain).  Often I find it is actually a problem with the equipment profile and not the program calculations, but please forward your examples so I can go through them.

Brad

Thank you.  I sent you an e-mail with the recipe file and my equipment profile.

Offline MaltLicker

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Re: Several problems with OG calculations
« Reply #10 on: August 02, 2011, 09:06:01 PM »
Some equipment things that stood out to me..............

Estimated mash efficiency says 84% and measured (per BS) is only 67%, for a 17 point difference.  Since both those appear calculated, it seems to not like the numbers entered.

It is an infusion mash-out but has no volume of water indicated to raise it from 155F to 168F.  When I clicked in there, it plugged in a negative infusion volume and error'ed out.  I changed it to Temperature but didn't see any change.


Offline rosenjm

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Re: Several problems with OG calculations
« Reply #11 on: August 03, 2011, 04:57:17 AM »
The values entered for actual volume and actual OG are not "real numbers".  They are the values BS2 default enters.  This was just a sample recipe that I used to bounce my calculations off.  That would be why the measured efficiency is so far off, I think.

Again, for the mash profile, I didn't really change it from the default as I just wanted to see how BS2 calculated the grain bill.  I use a HERMS system, so there is no water addition to change mash temperature.  I dough in with my strike volume and temperature is controlled via the HERMS coil.

Regardless, BS2 should make the grain, IBU, color, etc. calculations based on the efficiency value that is entered on the recipe creation page, right?  I have calculated my efficiency by hand and know that 75% is a pretty good number.  The way I understand it, the estimated mash efficiency that BS calculates is based on the brewhouse efficiency that the user enters when creating the recipe. 

Offline rosenjm

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Re: Several problems with OG calculations
« Reply #12 on: August 07, 2011, 03:12:56 PM »
OK, another data point.  I ran this whole thing by the owner of my LHBS and he agrees that the calculations are't right.  I made a copy of my equipment profile and set all the boil kettle losses to zero and set the batch volume to my desired post boil volume (fermentor volume + 1.25 gal trub/chiller + 4% shrinkage) of 7.54 gal and then adjust my recipe to my desired OG of 11.86 P.  This should have bypassed my suspected problem that BS2 is using fermentor volume vice post boil volume to make its caluclations.

I brewed this recipe yestarday and hit my pre and post boil gravities and volumes dead on...for the first time since I've used Beer Smith to calculate a recipe.

Offline mmmbeeer

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Re: Several problems with OG calculations
« Reply #13 on: September 14, 2011, 12:32:31 PM »
Hi Brad,

First let me say thank you for your brewing software.  I have used it for many years now, and it has been a great tool.

I wanted to bring this thread back to your attention, along with this one ... http://www.beersmith.com/forum/index.php/topic,5124.0.html ... since the issue of the software accurately estimating OG has been an ongoing problem not just with Beersmith 2.0, but also with 1.4. 

The specifics along with the calculations are already outlined in these two threads (and perhaps others), but the bottom line is that Beersmith is estimating OG from the "Batch Volume", instead of the "Post Boil" volume.  The difference here being that "Batch Volume" includes "Trub and Chiller losses" that have no impact on total gravity.

To work around this, one must set "Trub and Chiller losses" to zero, thus equalizing the "Batch and "Postboil" volumes.  By doing this, it seems that part of the reason for using brewing software is negated.

Would you mind taking another look at this, and maybe provide your loyal users a more permanent fix?

Thanks again!

Offline rosenjm

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Re: Several problems with OG calculations
« Reply #14 on: September 14, 2011, 02:03:17 PM »
I've been fighting this battle for awhile now.  The answer you're going to get is that BS used an overall "brewhouse efficiency" vice the more commonly used mash efficiency.  From what I can tell, the software extrapolates a mash efficiency from the value you enter as a brewhouse efficiency.  It uses this extrapolated mash efficiency to determine the grain bill.  The brewhouse efficiency takes the efficiency of the entire process into account, including the losses to trub and chiller.

The problem is most people don't have an accurate way to calculate overall brewhouse efficiency leading to having to shoot in the dark until you narrow in on it.  IF there is a way, other than trial and error over several batches, please let me know.

For example, I know my mash efficiency is 75%, but if I enter 75% as my brewhouse efficiency BS is going to extrapolate a value of mash efficiency of something like 68%.  This is why my grainbill comes in so short.  It makes sense mathematically, but doesn't seem to work very well practically.

I would rather be able to enter a value for mash efficiency and have the software make its calculations based on that and the post boil volume and use the water losses to account for how much water is needed.