Author Topic: Cooling shrinkage volume and OG issue  (Read 1434 times)

Offline MC1980

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Cooling shrinkage volume and OG issue
« on: December 17, 2018, 07:53:13 PM »
First post here on this forum.

I've been using BS2 for some times now, but just recently for a new 5hL brewery. I've managed to set up my equipment profile to get quite reliable predictions overall, except for my OG.

Take for example my last batch stats:
Est. preboil G: 15.07 P
Meas. preboil G: 15.10 P
Est. OG: 16.96 P (which I thought was too high according to previous batches and a rough estimate based on my boil-off rate)
Meas. OG: 16.35
All my volumes (pre- and post-boil, evaporation rate, trub and chiller losses, batch size) where spot on.

I suspected a boil-off calculation error so I went on to play with the Boil off tool. With the shrinkage rate set at 4%, I get the same estimated OG (16.99 P), but the post-boil volume includes this "loss," contrary to the Equipment profile's calculated Post boil volume. If I set the cooling shrinkage to 0, I get the exact measured OG I got on this batch (16.35 P) and the tool's post boil volume matches the equipment profile's one.

So, it is obvious that BS2 counts the cooling shrinkage as a water loss the same way as evaporation, but it's not. Shrinkage is just that: shrinkage. It has no effect on gravity (when temperature corrections are accurately made).

Has anyone had this problem and what are the possible solutions?

Any help will be appreciated.

PS. Using version 2.3.12


EDIT:

In fact, i did find a way around but it's at the expanse of my setting's accuracy. First, my equipment profile was already set up as described in this post: http://www.beersmith.com/forum/index.php/topic,8579.0.html

Now, if I go to my recipe's equipment profile, set the cooling shrinkage (4% or 22 L in my case), to 0 and add this 22 L to trub and chiller losses instead, I end up with the same post-boil and batch volumes, which are correct. But, the est. OG is still off at 16.96, and the est. pre-boil G is now off at 15.66.

To make these estimates match my readings, I have to change my BH efficiency according to the BH measured efficiency. In other words, at this point, I don't mind the est. and measured Mash efficiencies as described in the above mentioned post; anyway they don't change and are still equal with this adjustment.

On screen, it works. I will test it tomorrow. But as I said, it's a way around, certainly not how it should be.
« Last Edit: December 17, 2018, 08:31:37 PM by MC1980 »

Offline Oginme

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Re: Cooling shrinkage volume and OG issue
« Reply #1 on: December 18, 2018, 05:51:54 AM »
This is an estimation in the software, confirmed by Brad in an email to me.  When the calculations are made, it uses the thermally expanded volume post mash/pre-boil and the gravity reading (at standard conditions) for the gravity points calculation.  It calculates the gravity points at the fermenter and loss to trub and chiller, both volumes which are measured at room or standard temperatures.  You can follow the volume calculations on the 'vols' tab in any recipe.  This leads to an inaccurate total of gravity points, in which the error is equal to the thermal expansion coefficient.  Since the program does not ask for nor record the actual post boil volume, relying instead on the calculated value, it is up to the user to make note and perform the boil off rate calculation outside of the software.   Personally, I feel that this is an easy measurement and calculation that would enhance the ability of the user to 'dial in' the equipment profile to match the system. 

As you have discovered there are several ways around this issue.  Since my volume measurement is taken right after the mash with the wort temperature well below boiling, I set my thermal expansion to 2% (for water at around 150F) so that my measurement of volume at this point and the software will coincide.  Using this figure and the post boil volume, I can offset the thermal expansion to some extent by hiding it in the boil off rate.  For my standard process, yielding around 11 liters (Room temperature) at the end of boil and with a measurement accuracy of around 0.2 liters, the measurement error is pretty close to the expansion rate of 2%, so that is about as accurate as I can get to begin with. 

Brad's comment to me was that the standard measurement error for most brewers would be close to that of the thermal expansion which for my case is pretty accurate.  It does not mean that that error in calculation does not exist, but more that the user needs to be aware of this shortcut when doing the calculations for the equipment profile parameters and 'bake in' the error in the profile to balance out the calculations.  This will also reflect in an inflated mash efficiency number being calculated by the program, but, since program utilizes the total or brewhouse efficiency, it is only an internal value which does not affect the outcome.

If you follow your process pretty closely and from your notes it appears you do, you will still be able to accurately model your process in terms of volumes throughout and that the error would appear in gravity prediction pre-boil which would be pretty much match the measurement capability of most brewers using a standard hydrometer.  For your measurements in Plato to the hundredth place, it would certainly show up for the measurements taken using thermally expanded volumes.  In the end however, the final gravity would work out to target. 
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Offline brewfun

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Re: Cooling shrinkage volume and OG issue
« Reply #2 on: December 18, 2018, 06:24:41 AM »
I agree 100% with what you're saying. I've found that at commercial volumes, the density of water has a big impact on apparent volume. I've brought up that the BeerSmith shrinkage function changes (dilutes) the expected gravity quite a while ago, but Brad publicly refuted it saying it doesn't. I believe he doesn't have a direct link, but somewhere in the vast number of calculations, it gets linked.

I used to have 3.25% as my density value (the difference between 90oC and 20oC), which made the post-boil numbers line up. The preboil gravity estimate is always off by just enough to be annoying, but I can live with it.

What I've found to be the most important thing to do is to make sure I measure at the same point in the process. I measure volume and preboil gravity at 5 minutes into the boil, to make sure it's fully mixed. I measure batch volume and gravity AFTER WP and stand, to account for all evaporation up to that point. The WP & stand can be another 10+ gallons.
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Offline MC1980

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Re: Cooling shrinkage volume and OG issue
« Reply #3 on: December 18, 2018, 07:19:04 AM »
Oginme, thanks for your reply.

I see from it that, although with no great consequences, this is a real issue. We both have found ways around it but, again, it still is the wrong way to calculate things and make predictions. Volumes are correct, it's what is made of one of them (shrinkage) that is wrong.

The way I see it, it's pretty simple: BS takes my estimated pre-boil G, takes out water as evaporation (OK) and cooling shrinkage (wrong), calculate an estimated OG (wrong because of accounted shrinkage "loss"), and then remove the trub and chiller losses to calculate the batch volume. What it should do is take the estimated pre-boil G, take out evaporation water, calculate OG, and calculate batch volume from cooling shrinkage and trub and chiller losses.

As you said, it's quite easy when you know your system and you keep notes on your brewing sessions to repeatably get the results you want. I myself got to make BS make predictions reliable enough for my system for most data, which I think is great considering it's a homebrew software. Reliable, except for this last one data. As you did, I found a way around, but something is wrong along the way.

I'm aware that a lot of probrewer work with BS, at least for some time at the beginning of their career. The small measurement error equivalent to this programming error can mean a lot on our scale, not only moneywise. Just think about packaging for example: on a homebrew scale, I didn't mind small errors in my OG which translated to small error in my ABV; but now I would certainly mind about mislabelling my ABV.

I will post a link to this thread in the Suggestions forum.

Anyway, thanks again for your reply. I've been reading a lot on this forum and you are are always of great help.

Offline MC1980

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Re: Cooling shrinkage volume and OG issue
« Reply #4 on: December 18, 2018, 07:23:39 AM »
Brewfun, I was typing while you replied.

I'm "glad" to see I'm not the only one with this issue.


Offline Oginme

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Re: Cooling shrinkage volume and OG issue
« Reply #5 on: December 18, 2018, 07:36:43 AM »
MC1980,

I agree that it is a real issue, which is so easily fixed as all the numbers needed are there to fix the calculation.  On large scale brewing, it can be a problem when you are relying on the in-process calculations to verify that you are hitting your targets. 

I've spent many years doing process engineering and process control engineering in the paper industry.  We typically broke down key measurements and variables by category: critical, major, minor, informational and applied time to each measurement and prediction to ensure that the critical and major data fields were rock solid and tuned for tight control bands.  Minor data fields were predicted and controlled as long as they did not clash with the critical control loops and cause instabilities in the process.  From my perspective, the pre-boil volume and gravity points are minor data to the home brewer, but are elevated to major for larger scale brewing where key decisions are made based upon those results to ensure consistency in the end product.
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Offline gizzygizmo

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Re: Cooling shrinkage volume and OG issue
« Reply #6 on: December 24, 2019, 08:09:42 AM »
FYI, I've seen this same issue... though I looked at it backwards from what you did.  My main concern was never hitting preboil OG.  If i did, then my postboil OG was off by a few points.  The problem is the preboil SG is calculated at thermally "expanded" volume of the mash temp around 150degF (103%).   We don't measure SG at 150degF, we cool the sample when we measure.  So this number is artificially low due to the extra volume of wort.   If you set shrinkage to 0% you'll get a valid preboil SG, since the gravity should be measured at room temp.  You can multiply the preboil SG times volume and divide by final volume and get the OG to match up.   

I think the real issue is how BS is calculating preboil SG... my solutions have involved either setting shrinkage to 0% and accounting for 4% loss volume on final volume or back-calculating the preboil SG by reworking the volume/sg formula (Either multiply OG times post boil volume and divide by preboil room temp volume, or multiply preboil OG by .97 to get a rough estimate).   

A lot of people will simply go back after brewday and update their mash eff so that their preboil SG matches what BS predicted.  Then they're usually off by 2-3 gravity points on OG but chalk that up to user/math error.  The issue becomes more pronounced with larger batches (Imperial recipes).

 

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