BeerSmith™ Home Brewing Forum
BeerSmith Software => BeerSmith 2 Bugs/Support => Topic started by: sbowler on March 06, 2017, 09:52:26 PM

Hi,
First off, love the software. It's an invaluable tool for preparing and running a successful brew day.
I'm wondering if the calculation that BeerSmithuses to predict postboil OG may be slightly off?
My understanding is that the formula used to calculate the predicted OG is:
(PreBoil Vol x Pre Boil Gravity Points) / Post Boil Vol = Post Boil Gravity Points
At the moment, it appears that BeerSmith uses the noncooled volume (i.e. thermal expanded volume) for the preboil volume. If the cooled volume is used for the preboil volume, there is a difference of several gravity points.
An illustrated example:
PreBoil Vol: 63L (volume at 77C)
PreBoil Vol factoring shrinkage (Assume 3% @ 77C): 61.16L
PreBoil Gravity: 1.075
Post Boil Vol (@25C): 55L
Formula 1: Raw preboil volume
Calculation: (PreBoil Vol x Pre Boil Gravity Points) / Post Boil Vol = Post Boil Gravity Points
Calculation: (63*75)/55=85.9091
=> Post Boil OG: 1.086
Formula 2: Equivalent PreBoil Volume factoring shrinkage
Calculation: (PreBoil Vol factoring shrinkage x Pre Boil Gravity Points) / Post Boil Vol = Post Boil Gravity Points
Calculation: (61.16*75)/55=83.4
=> Post Boil OG: 1.083
Formula 1 appears to match the numbers I get out of BeerSmith, but my feeling is that Formula 2 is the way the OG should be calculated?
Is this a bug, or have I misunderstood/miscalculated something above?
Simon

I have also been thinking of this. I think that Beersmith calculates its specific gravities based on cold liquid.
If so you would have to, as you examples suggest, add water to the boil volume to compensate for the higher SG, hense the shrinkage.
Having said that, the difference is so little, that it makes no sense for me to adjust. I'm not saying that Beersmith shouldn't take it into account, but only as comfort it doesn't change much for me. As a homebrewer my equipment is so imprecise anyway that Beersmith predictions will always be off. For instance, the way I read the volume of the wort in my fermentation bucket. I'm not so sure that the sticker on the outside of the bucket, indicating the volume is that accurate. And on top of that comes my reading. Lying there on the floor trying to get a leveled measure. And lastly my hydrometer readings are also not that precise. I look at it at guestimate it, probably with 1 SG error margin.

It's a fair point that there are degrees of measurement error across the process. I would say that I spend more time than is probably useful trying to ensure the accuracy of my measurements :D. I suppose that I was keen to understand what value/formula was used for the calculation.
If so you would have to, as you examples suggest, add water to the boil volume to compensate for the higher SG, hence the shrinkage.
To provide a bit more clarity of my examples above, the volumes are the same volume in both Scenario 1 and 2, it just depends on whether you use the Hot volume which assumes 3% expansion or cold volume. i.e. 61.16L at 15C, measures at around 63L at 77C, even though no water has been added (water is less dense at the warmer temperature).
The problem I'm seeing (I believe) is that BeerSmith calculates the gravity based on Scenario 1, but Scenario 2 are the results I see in real life. i.e. I hit the target preboil gravity, hit the boil off rate, but the measured post boil gravity is 0.03 points lower than predicted.

You are correct the preboil volumes do not account for expansion. I'm not aware of any programs that do but I've made a note of this to try to correct this in the future.
Brad

Brad,
If the preboil volume is not adjusted for thermal expansion, then why does the volume target (preboil) change when the thermal expansion value is changed in the equipment profile?

Hi Brad,
Thanks very much for your response and appreciate you adding it to your backlog.
Like Oginme I had a similar question around what the thermal expansion value is used for. I'm guessing you're meaning that the preboil volumes used in the OG calculation formula don't account for expansion, whereas the batch size calculation (and scaling function) each consider the thermal expansion value in the equipment profile?
Simon

It is not only the preboil volume target that changes with setting the thermal expansion to zero. The calculated volume of strike water changes also, indicating that this volume has the thermal expansion component calculated into it.
FWIW, this is the way I had always figured that the program worked. When I measure water for a batch, I scale the target from BeerSmith back to account for the thermal expansion as I am measuring cold water into my kettle for BIAB. The volumes as a result have always worked out very closely based upon this practice. My calculated mash efficiency has always been close to the mash efficiency calculated by BeerSmith (accounting for rounding errors and my ability to measure provides an error band which is well in excess of any difference that I get.)

Hi Brad  Just wondering if there was any timeline on this being included in a future version of beersmith?

I concur that the OG calculations in beersmith are incorrect. This is very apparent when you nochill as you're just going off the hot measurements when measuring OG. I always end up about 2% higher on my mash efficiency to hit the intended BH efficiency to get to the right ABV and volume. Correcting the preboil volume for cooling or using an uncorrected volume for the finishing volume (same effect) seems what's required.

Actually, the OG target calculation is correct. It is the intermediate target for volume and gravity (preboil) which does not take into account the thermal expansion of the water and correct back to standard conditions (20C) when calculating mash efficiency.

Hi Brad  Is this fixed in BeerSmith 3?

Still having similar issues in BS3... either the preboil volume is off (setting shrinkage to 0%) or your preboil SG is off (shrinkage at 4% but volume is used to calc preboil SG)

Hi @Beersmith  Just wondering if there is any update on this? I haven't noticed anything in the release notes that suggested that this has been fixed  but given the accuracy that carries across in the rest of BeerSmith, it would be great to have this consistent too.

With the launch of BeerSmith 3 last year, all development on the BeerSmith 2.x version has ceased.

Yep  Sorry i figured that. I was intending to ask if this has been fixed/intending to be fixed in Beersmith 3.