Author Topic: Inconsitency in Pre-Boil Gravity/Volume and Post-Boil Gravity/Volume  (Read 4210 times)

Offline lparsons

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Looking at the "Session" tab of a recipe, the pre-boil volume is calculated assuming the water has expanded due to heating (by adding cooling expansion factor). The post-boil volume (batch size) assumes the water has cooled. However, the est pre-boil gravity and post-boil gravity (est original gravity) seem to assume that those volumes are both unheated. For example:

est pre boil volume = 6.73
est pre boil gravity = 1.052

boil off = 1 gallon
batch size = 5.5 gallons
est original gravity = 1.064

6.73 gallons * 52 gravity points = 349.96  (using expanded volume)
349.96 / 5.5 gallons = 63.63 ~ 64 gravity points

However, the cooled volume pre-boil is actually 6.47 gallons.
6.47 gallons * 52 gravity points = 336.44
336.44 / 5.5 gallons = 61.17 ~ 61 gravity points

So, either the pre-boil estimated gravity is incorrect, or (I believe) the final gravity estimate is incorrect.
Or, perhaps I've just misunderstood something here?

BTW, this appears to be the case with the most recent BeerSmith2 as well.

Can someone  confirm this is indeed a bug, or perhaps help me to understand what I've got wrong here? Thanks.

Offline Oginme

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Re: Inconsitency in Pre-Boil Gravity/Volume and Post-Boil Gravity/Volume
« Reply #1 on: June 19, 2018, 05:43:00 AM »
It is not so much a 'bug' as the simplification of the calculations which Brad has acknowledged.  You are correct in your math. Since BeerSmith starts with the ending volume (into the fermenter) and amount of sugars that reach that volume (brew house efficiency), then the post boil gravity target and volume should be correct.  This puts the calculation of volume and gravity at the beginning of the boil in question with, since the thermal expansion is not taken into consideration in the mix. 

In the end, it is a minor issue for the majority of brewers who can not/do not even measure to the accuracy to which the program can calculate [at 4%, 6.73 is 6.47 cold volume]. The only purpose in applying the thermal expansion is really the test of the volume versus capacity.  Since most of my readings are taken either cold (at end of process) or at mash out temperatures, and since I measure my water cold going into the process, I've set my thermal expansion to 0 and that works for me.  I use a separate spreadsheet to track my process outputs vs inputs which does take into account the actual measurements and volume corrections in order to monitor process changes.  In the end, the program works well for my purposes and I've always been pretty dead on in my EOB volumes and gravity.

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Offline Briandead

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Re: Inconsitency in Pre-Boil Gravity/Volume and Post-Boil Gravity/Volume
« Reply #2 on: September 19, 2019, 09:41:38 AM »
I think this causes slightly more of an issue if you forget about volumes and think about gravity.

With the cooling shrinkage set to 4%, my stout recipe shows a final gravity of 1.049 which is what I wanted. However, in reality, it came out 1.046 and when I work back through the gravity point calculations then 1.046 is exactly what it should have got,  given the pre-boil volume and gravity given in BS3. I even took into account the fact that shrinkage occurred after the cooler as the wort went into the fermenter.

So why does BS3 give me 1.049 when I calculate 1.046.

The clue came when I set the shrinkage to 0%. Then the pre boil volume decreased, pre boil gravity went up gravity point calculations worked just fine. Obviously this time my calculations ignored shrinkage also.

Here's what I conclude - I may or may not be correct.

BS3 works backwards from the target batch volume and gravity.
It then adds on the loss volume from trub, cooler etc. Gravity remains constant.
Then it applies the cooling shrinkage value to increase that volume and does not adjust the gravity so the calculation the gravity points is unchanged.
Then boil off is factored in and a pre boil volume and gravity are generated with too few gravity points.

The shrinkage calculation should be applied to the volume in the fermenter as all the volumes prior to that are hot. i.e. a 23l batch should be 23.9l pre shrinkage.

Sorry if this is a bit bit garbled but the bottom line is that if you have 4% shrinkage set, you will hit your volume but you will be low on gravity if you follow the BS3 calculation. If you set 0% shrinkage you will hit you gravity but be a bit low on volume.

From now on I'll use 0% and then increase the cooler trub losses by 4% of the boil volume. I've also found I have to drop BH Efficiency a few points to get this to work correctly
« Last Edit: September 19, 2019, 09:49:56 AM by Briandead »

Offline enkamania

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Re: Inconsitency in Pre-Boil Gravity/Volume and Post-Boil Gravity/Volume
« Reply #3 on: September 19, 2019, 11:14:25 AM »


Sorry if this is a bit bit garbled but the bottom line is that if you have 4% shrinkage set, you will hit your volume but you will be low on gravity if you follow the BS3 calculation. If you set 0% shrinkage you will hit you gravity but be a bit low on volume.

From now on I'll use 0% and then increase the cooler trub losses by 4% of the boil volume. I've also found I have to drop BH Efficiency a few points to get this to work correctly

Thanks for investigating this.  For me, I'm more concerned about gravity than volume.

Offline Oginme

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Re: Inconsitency in Pre-Boil Gravity/Volume and Post-Boil Gravity/Volume
« Reply #4 on: September 19, 2019, 12:13:46 PM »
Let me tackle this one item at a time.

First, your gravity in the fermenter is calculated by the program strictly through the Brew House Efficiency (BHE) specified in the equipment profile (which should be customized to reflect your process capabilities and losses).  The program takes the potential available sugars in the mash and multiplies that by the BHE to determine the amount of sugar in the fermenter.  It then uses the fermenter volume to determine the specific gravity.

So your statements:

With the cooling shrinkage set to 4%, my stout recipe shows a final gravity of 1.049 which is what I wanted. However, in reality, it came out 1.046
So why does BS3 give me 1.049 when I calculate 1.046.


Really indicates that your BHE was incorrect in your equipment profile.

Next, the program uses the volume losses to calculate the gravity obtained from the mash and pre-boil gravity. 

You can follow the calculations on the volumes tab, but I think you have the process figured out:

BS3 works backwards from the target batch volume and gravity.
It then adds on the loss volume from trub, cooler etc. Gravity remains constant.
Then it applies the cooling shrinkage value to increase that volume and does not adjust the gravity so the calculation the gravity points is unchanged.
Then boil off is factored in and a pre boil volume and gravity are generated with too few gravity points.


With the exception of your comment in red you are following the calculations correctly.  The program does not change the gravity, so the calculation of gravity points DOES change because your volume now contains the thermal expansion coefficient (shrinkage factor).  This puts the 4% error (assuming the default shrinkage of 4%) in the pre-boil gravity estimation done by the software.  This is where the error in the calculation resides. 

To do a proper gravity point comparison, the volumes must be measured or converted to the same standard temperature.  In BeerSmith, it is assumed that room temperature (around 20C/68F) is that standard since this is also representative of the specific gravity measurement temperature (most hydrometers are calibrated to 60F/15.6C or 68F/20C).  To properly conduct the gravity point balance then, it is more productive to convert the hot temperature of the wort pre-boil to room temperature.  The problem here is that the thermal expansion in BeerSmith is a flat rate and, at least where I measure the volume from the mash, is at around 148F/65C to 160F/71C at which the thermal expansion is around 2.2%.  I really don't want to wait until the wort is just about to boil before measuring to find I need to top off a little.  This right here creates a nearly 2% difference from the program estimation of volume and consequently in the prediction of pre-boil gravity, since it uses the estimation of thermally expanded wort at 4% and the sugar value to calculate the pre-boil gravity.

Now for most of us homebrewers working on a small volume scale, the measurement of volume has a reasonable inaccuracy of +/- a half liter (half a quart) so that accounts for around 2% error in measurement right there, assuming a 5 gal finished batch, 7 gal pre-boil volume. 

If you really wanted to be precise, because I know there are some who will be pedantic about it, the actual density of water at 20C/68F is 0.998 and not 1.000, which reflects a 0.2% thermal expansion of the water over the accepted density of 1.000 which occurs at 4C/39F.  Given the other inaccuracies in the calculations, I think we can kind of gloss over this one.
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Offline gizzygizmo

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Re: Inconsitency in Pre-Boil Gravity/Volume and Post-Boil Gravity/Volume
« Reply #5 on: September 22, 2019, 12:21:39 PM »
I think this is more about the pre-boil gravity not accounting the for the volume correctly.  We measure SG at room temp (68degF) and the software isn't clear if the SG given is room temp or mash temp.  If i multiply the SG times volume and divide by the end volume i'm not getting the same final gravity number.  So if BS things the SG should be 1.047 and the pre-boil volume is 12.5; i'd expect the formula for final gravity to be (pre-boil gravity * pre-boil volume at 68degf) / (final volume at 68degF) for the actual OG.    Setting shrinkage to 0% makes the numbers line up so I believe the pre-boil graivity is actually the Gravity at mash temp without cooling the sample.

Also, trub loss appears to be off... on a 10g recipe where I have 1 gallon of trub loss I noticed that if i set the tub loss to "10 gallons" it does give me a nice 23.38 pre-boil volume to account for the volume loss  but the gravity actually goes "up" for post mash gravity and pre-boil gravity.  I would expect these numbers to go down as the wort is diluted with an extra 9-10 gallons of water.   This is very reproducable, just change equipment profile trub loss to 100gallon and watch your pre mash gravity go UP, your IBUs go DOWN, and your OG stay the same even though boil-off rate doesn't remove the extra losses and you did not add any more malt.

It may help everyone if the software is more clear about temps, volumes, and pH (lots of people still argue if 5.2-5.4 is room temp or mash temp reading.

For instance, pre-boil volume 14g with gravity of 1.040 ... does this mean (14*.97=13.58g at 68degF)?  13.58 * 40 = 543.2 total points.  Divide 543.2 / post boil volume 12g = 1.045 OG?  no.. BS says 1.047 OG.  So i think it means 14g preboil * 40 points = 560 total points, divide by post boil volume 12g to get 1.0467.    So it really means 1.040 at whatever temp pre-boil volume is, so i should really have 1.041 (560 / 14*.97).   This isn't as bad at 5g and 10g batches for 1.050 and below; but doing a RIS or larger 1.070+ and you end up dbl checking your efficiency numbers.

This can all be fixed by making shrinkage 0% and making your target batch volume exactly what you need with shrinkage.  But then i'm not taking advantage of the software.  I'd love to be able to choose which formula is used for displaying pre/post volumes or gravities.  So even if we do not agree on cooling a sample to 68degF for an SG reading that I can display the SG at 68degF instead of at temp. 

Offline Oginme

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Re: Inconsitency in Pre-Boil Gravity/Volume and Post-Boil Gravity/Volume
« Reply #6 on: September 23, 2019, 07:10:13 AM »
We measure SG at room temperature because that is standard conditions.  You will note that the software has a hydrometer adjust tool.  If you have looked at this, you will have noted that the adjustment is ALWAYS to the calibration temperature of the hydrometer.  The reason for this is that this is where density is normally reported and where the program estimates the gravity.  In scientific literature, when someone cites a density which is not at standard conditions, it is always denoted with the temperature at which this reading has or is to be measured. 

Next, the volumes are very clear when you follow them on the 'vols' tab in the recipe.  Starting from the bottom, the user sets the following:  batch size (fermenter volume), loss to trub and chiller, thermal shrinkage (thermal expansion coefficient), top off water.  The program starts with the fermenter volume, subtract any top off water, and add in the loss to trub and chiller to get a post boil volume when chilled down to room temperature.  It now applies the shrinkage factor to give you a hot post boil volume.  The program adds in the loss for boil off (also at hot temperature), to give you a pre-boil volume -- at hot temperature.  Further, the program will then add in non-recoverable losses in the mash tun and subtract any top off water to give you a total water required for the mash AT HOT TEMPERATURE.   The program does not remove the thermal expansion to give you a cold water volume needed. 

Since it is unreasonable for the users to chill down their wort to room temperature just to get a volume reading, it is not an unreasonable assumption on the part of the software to assume the pre-boil volume is at an elevated temperature. 

Now, on to your changing of the trub loss.  Go back to my previous post on how the program calculates the OG of the recipe.  It takes this value and calculates out the gravity point losses for the loss to trub and chiller (at cold volume).  It further then back calculates the amount of sugar needed from the grist in order to satisfy this requirement set by the user.

Now, if you change this value, and solely this value, the program will calculate a larger amount of sugar lost at this step and back calculate to extract more sugar from the grist.  Because it is just a model and is totally driven by USER INPUT, it will and does calculate additional sugar requirement from the grist, even going to the extent of calculating a mash efficiency above 100%.  If you go back to your exercise, you will see the mash efficiency change in response to your sudden demand for more sugars to be drawn from the mash. 

I have already acknowledged the error caused in the program model by not accounting for the temperature of the wort on the gravity point balance.  From your arguments, it appears that you are trying more to justify your ending number instead of adjusting the BHE to account for this value.  When you work your calculations, please start with the ending volume (batch size) and gravity and work back from there.  This is how the program model is set up.  While some of us would prefer to have the program use mash efficiency (since I find this more consistent and predictable) as the determiner of sugar extraction and work downstream from there, it is not the way Brad has set the model.

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Offline gizzygizmo

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Re: Inconsitency in Pre-Boil Gravity/Volume and Post-Boil Gravity/Volume
« Reply #7 on: September 24, 2019, 11:57:43 AM »
Okay... this is good to know on SG.  It should be always displayed as a value at room temp... i think the issue is that preboil SG is not actually room temp, but boiling temp.  It's doesn't appear to be corrected at all... and the preboil volume is used based on 4% expansion. Then this is compounded by trub losses.

The statement that "the program will calculate a larger amount of sugar lost at this step and back calculate to extract more sugar from the grist" i believe is the main issue here.  I'm sure there's a way to fix it... Adjusting the trub loss does not logically make me think that my mash eff is going up.  i didn't not change crush, or anything related to mash eff.  I'd much rather see BH eff and OG go down... because total volume is going up with the same ingredients.  This I can deal with though; now that you said this I checked and noticed the mash eff % is going up when trub loss is added.  I can go to "design", edit the Bh eff number, go to "session" check mash eff number, toggle back and forth until mash eff goes back to normal.  Then I can edit the equip profile and fix the Bh eff number (because none of these are on the same page).  It would be nice to have a more graceful way to edit this, but i'm only editing this when i mess with my equipment every few months.

Back to preboil SG... i still can't get this to line up.  I could def relate to innacuracies with volumes when measuring ... 1 quart isn't much to be off.  However; the SG is a bigger deal... i'm using this value to see if i'm going to hit OG and deciding what todo about it.  Things you can do during the boil (boil harder, add DME), and then there are things you can do after (liquor up).  But you can't just throw some more DME in after the boil, and you can't boil harder after you've added additions.   The error is larger than 4%, there appears to be more going on or trub losses are "helping".  I think BS is using the full 4% expansion volume to figure out SG.   If so, it seems like we could just reduce it by 4% to give the proper preboil SG to the user.

To reproduce this error create a simple recipe with 10lbs malt and 5gallon batch size.   Equipment was a "new profile" with nothing changed.  Don't change anything but the trub loss... change from 3 gallons to 0 gallons.  When it's at 3 gallons we add 3 gallons of water pre-boil to account for the ending 3 gallon loss.  I did the math both ways, based on "cooled" preboil volume vs actual pre-boil volume to see if one was correc vs the other.  Then i took the OG and multiplied by total volume to get points... divided by "cooled" preboil volume to see what the real target preboil SG would be.   


0 gallons trub loss
  1.053 OG
  preboil 5.71g
  preboil SG 1.046
  est mash eff shows 72%
  post boil 5.21g
  shrinkage .21
  ending kettle volume 5g (not shown)
  batch volume 5g
  5.5g pre-boil cooled volume * 46 = 253 points, divide by 5 ending volume = 1.0506 (OG is off..., 6% off)
  5.71g "hot" pre-boil volume * 46 = 262.66 points, divide by 5 ending volume = 1.05253 (expected OG, as expected)
  Actual pre-boil OG should be (53 * 5g / preboil volume cooled) = 1.048
3 gallons trub loss
  1.053 OG
  preboil 8.83g
  preboil SG 1.048
  est mash eff shows 115%
  post boil 8.33g
  shrinkage .33
  ending kettle volume 8g (not shown)
  batch volume 5g
  8.5g pre boil cooled volume * 48 points = 408, divide by ending kettle 8g = 1.051 (OG is low... expected 1.053, 4% error)
  8.83g "hot" pre-oil volume * 48 points = 423.84, divide by ending kettle 8g = 1.05298 (this is the expected OG)
  Actual pre-boil OG should be (53 * 8g / preboil volume cooled) = 1.049


Suggestions to make this more user friendly:

  1) for thermal expansion on preboil SG/volumes calculate based on the last mash temp.  If their mash temp is 154 use that, if they have a "mash out" temp at 168 then use that.  This will avoid issues with larger recipes (1.050+ gravities) where my OG and preb-oil SG are off.  This would help with teh 1-2% error in preboil volume due to 4% being used rather than 2%, adn it would fix the SG/OG issue.  If this is too difficult, we could just deduct the 4% thermal expansion added by BS when calculating SG rather than displaying the SG at "212degF".

  2) When for adjusting trub losses allow the user the choice to have BH eff go down /w OG/IBU or mash eff up to make up for losses.  This will help alleviate the user nightmare of toggling between "design" and "session" after adding trub losses... fixing the BH eff until mash eff ends up matching reality.

  3) Allow users to edit mash eff and have this change BH for them.  This would go a long way to help users.  I have yet to meet any users who know what their BH eff is... and can enter that when creating profiles.  Instead, we all know what our Mash eff and losses are... we enter those values.  Since we can't enter mash eff, we end up editing BH eff until it shows the correct mash... and the values are not on the same screens.

Offline Oginme

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Re: Inconsitency in Pre-Boil Gravity/Volume and Post-Boil Gravity/Volume
« Reply #8 on: September 24, 2019, 12:35:34 PM »
Okay... this is good to know on SG.  It should be always displayed as a value at room temp... i think the issue is that preboil SG is not actually room temp, but boiling temp.  It's doesn't appear to be corrected at all... and the preboil volume is used based on 4% expansion. Then this is compounded by trub losses.

Go back and read what I wrote on my first response to you and I clearly state how the calculation for the pre-boil gravity is made. 


The statement that "the program will calculate a larger amount of sugar lost at this step and back calculate to extract more sugar from the grist" i believe is the main issue here.  I'm sure there's a way to fix it... Adjusting the trub loss does not logically make me think that my mash eff is going up.  i didn't not change crush, or anything related to mash eff.  I'd much rather see BH eff and OG go down... because total volume is going up with the same ingredients.  This I can deal with though; now that you said this I checked and noticed the mash eff % is going up when trub loss is added.  I can go to "design", edit the Bh eff number, go to "session" check mash eff number, toggle back and forth until mash eff goes back to normal.  Then I can edit the equip profile and fix the Bh eff number (because none of these are on the same page).  It would be nice to have a more graceful way to edit this, but i'm only editing this when i mess with my equipment every few months.

The "easy" way to do this is to take your post boil volume with and without the additional trub loss and recalculate your BHE based upon that ratio.  So if you have 10 liters of wort and 1 liter of trub with a BHE of 80% and you increase the trub loss to 3 liters, then the new BHE to maintain about the same mash efficiency would be 80% * (10 + 1) / (10 + 3) = 67.7%  It is a pretty straight forward calculation which will get you close enough for starters.


Back to preboil SG... i still can't get this to line up.  I could def relate to innacuracies with volumes when measuring ... 1 quart isn't much to be off.  However; the SG is a bigger deal... i'm using this value to see if i'm going to hit OG and deciding what todo about it.  Things you can do during the boil (boil harder, add DME), and then there are things you can do after (liquor up).  But you can't just throw some more DME in after the boil, and you can't boil harder after you've added additions.   The error is larger than 4%, there appears to be more going on or trub losses are "helping".  I think BS is using the full 4% expansion volume to figure out SG.   If so, it seems like we could just reduce it by 4% to give the proper preboil SG to the user.

With my setting the thermal expansion to 2.2%, I end up right on with volume and within a point on gravity.  Once you get a feeling for your process, you pretty much know if you are on target or not.


To reproduce this error create a simple recipe with 10lbs malt and 5gallon batch size.   Equipment was a "new profile" with nothing changed.  Don't change anything but the trub loss... change from 3 gallons to 0 gallons.  When it's at 3 gallons we add 3 gallons of water pre-boil to account for the ending 3 gallon loss.  I did the math both ways, based on "cooled" preboil volume vs actual pre-boil volume to see if one was correc vs the other.  Then i took the OG and multiplied by total volume to get points... divided by "cooled" preboil volume to see what the real target preboil SG would be.   


0 gallons trub loss
  1.053 OG
  preboil 5.71g
  preboil SG 1.046
  est mash eff shows 72%
  post boil 5.21g
  shrinkage .21
  ending kettle volume 5g (not shown)
  batch volume 5g
  5.5g pre-boil cooled volume * 46 = 253 points, divide by 5 ending volume = 1.0506 (OG is off..., 6% off)
  5.71g "hot" pre-boil volume * 46 = 262.66 points, divide by 5 ending volume = 1.05253 (expected OG, as expected)
  Actual pre-boil OG should be (53 * 5g / preboil volume cooled) = 1.048
3 gallons trub loss
  1.053 OG
  preboil 8.83g
  preboil SG 1.048
  est mash eff shows 115%
  post boil 8.33g
  shrinkage .33
  ending kettle volume 8g (not shown)
  batch volume 5g
  8.5g pre boil cooled volume * 48 points = 408, divide by ending kettle 8g = 1.051 (OG is low... expected 1.053, 4% error)
  8.83g "hot" pre-oil volume * 48 points = 423.84, divide by ending kettle 8g = 1.05298 (this is the expected OG)
  Actual pre-boil OG should be (53 * 8g / preboil volume cooled) = 1.049


Suggestions to make this more user friendly:

  1) for thermal expansion on preboil SG/volumes calculate based on the last mash temp.  If their mash temp is 154 use that, if they have a "mash out" temp at 168 then use that.  This will avoid issues with larger recipes (1.050+ gravities) where my OG and preb-oil SG are off.  This would help with teh 1-2% error in preboil volume due to 4% being used rather than 2%, adn it would fix the SG/OG issue.  If this is too difficult, we could just deduct the 4% thermal expansion added by BS when calculating SG rather than displaying the SG at "212degF".

You do this by setting the thermal expansion coefficient to the value which corresponds to the temperature you usually take the volume measurement. It is completely in the control of the user.  The program does not scale based upon the temperature and does not know at what temperature the user is taking the reading.  Nor can it, it is a program, not a mind reader.

  2) When for adjusting trub losses allow the user the choice to have BH eff go down /w OG/IBU or mash eff up to make up for losses.  This will help alleviate the user nightmare of toggling between "design" and "session" after adding trub losses... fixing the BH eff until mash eff ends up matching reality.

See my calculation above.  Again, since the program is designed to calculate sugar extraction based upon the BHE it is up to the user to make the changes.  While I would prefer to have it work off of mash efficiency, the designer and others are fine the way it is.  I accept that and make the adjustments needed as I change parameters in the equipment profile.


  3) Allow users to edit mash eff and have this change BH for them.  This would go a long way to help users.  I have yet to meet any users who know what their BH eff is... and can enter that when creating profiles.  Instead, we all know what our Mash eff and losses are... we enter those values.  Since we can't enter mash eff, we end up editing BH eff until it shows the correct mash... and the values are not on the same screens.

This would mean a major change in how the modeling in the program operates.  Brad has chosen the BHE to determine sugar extraction and until he decides that there is a better way and rewrites the model accordingly (I would not hold my breath on that happening) the software is the way it is.  When modeling a process, there are variables that you must lock down to make the open equations solvable.  In the end, the person doing the modeling chooses the key variable which fits their thinking and makes the model work the way their logic can follow.  As a user, you can either accept it and work with the model the way it is presented, fight it and continually battle the limitations of the model, or find a different program.

Just a side comment: I worked in process control and process engineering for many years.  Everyone tackles a problem or a mathematical model from their own angle of understanding and perspective.  They key to being able to fully utilize a model created by another person is to first understand the logic path being followed and then to determine the key aspects which will maximize the results you expect from the program.  Along with the key aspects are the key results which are intended.  For me, final volume and gravity are the object and the process output.  Pre-boil gravity and volumes are an indicator, but not the process output, therefore they are of secondary concern.
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Offline gizzygizmo

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Re: Inconsitency in Pre-Boil Gravity/Volume and Post-Boil Gravity/Volume
« Reply #9 on: September 26, 2019, 04:13:36 PM »
I read through these again.. i feel maybe a phone call may be better?  A simple webex to find out where i'm going wrong here...  mabey it's just user error on my part.  I feel like these numbers don't add up...

I get how to fix BHE when i adjust rub losses, seems like a bad way to go about it... bu it's workable.  If you adjust trub losses you need to write down your mash eff first, then adjust trub loss, then go back and adjust BHE until mash eff goes "back".  Sure, if BS woudl do this for me that'd be great... but that's a 'new features' request i suppose.

More importantly is the preboil SG error...  You indicate a couple times that SG is at room temp... but also conversly state that thermal expansion is adding a 4% error, which to me means it's not at room temp and is accounting for thermal expanison.

If BS is off by 4% because it does SG calcs with thermal expansion included means to me that the reading is not at room temp, but instead at "hot" temp.  I think this is an easy fix...  just remove shrinkage on the volume before dividing the points to obtain preboil SG.  We can do this math to dbl check and it seems to check out:


In my example... BS says preboil SG 1.046, OG 1.053.   Trub loss of 0.5g and boil off 0.21g.  If the preboil SG is at rooom temp then when i multiply it by the preboil volume (hot volume) and divide by ending kettle volume i should be "off".  Instead, i'm spot on...

 46 * 5.71g = 262.66 points
 262.66 / 5 = 1.0525, which rounds up to 1.053... that matches; but it was done using "hot" volumes not room temp.

Now, if i do the math and correct preboil volume to 5g which is what it would be at room temp (0.96 * 5.71) then i can see how far off I really am.

 46 * 5.5 = 253
 253 / 5 = 1.0506, this is off... if I actually measured 1.046 on my preboil i'd be 3 points off my OG.

If i set thermal exp to 0, then math works.  Now the preboil SG is 1.048. 

 48 * 5.5 = 264 points
 264 / 5 = 1.0528 which rounds up to 1.053.

A quick workaround seems to be to multiply the preboil SG by your shrinkage... this introduces rounding errors, the alternative would be to multiply the post boil volume * OG and divide by temp corrected preboil volume.  Just multiplying by shrinkage will exasterate any rounding errors.

Offline gizzygizmo

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Re: Inconsitency in Pre-Boil Gravity/Volume and Post-Boil Gravity/Volume
« Reply #10 on: April 22, 2021, 06:28:23 PM »
Just adding... i thought using java vars was a good fix but when editing "session" data and putting actual postboil vol and postboil SG the mash eff calc is using the jacked up numbers so you have to change shrinkage to 0% to learn your actual mash eff, and plug in adjusted volumes.

Seems like to use BS you have to adjust your numbers for the given scenario, which is something the software should be doing for us.