We aren't using the post boil volume in the equation- it’s there to optionally enter and save if desired.
Hey
@Yooper ,
Still seeing the bug on my end where Postboil gravity != batch OG, but I probably didn't give a thorough enough explanation.
Post boil gravity should match recipe OG 99% of the time, with the exceptions of a water top off or a sugar addition being added between flameout and transfer to fermenter. Most recipes will currently show a lower post boil gravity than OG where there are volume losses at postboil.This is unrealistic.
Right now, it
appears that the post boil gravity is being calculated via
postBoilGravityPoints = batchSize*(OGPoints)/postBoilSize
where gravity points = (SG -1)*1000 -> 1.030 = 30 grav points
1.066 <= 66.36 = (5 * 73)/5.5.
Or equivalently,
PostBoil gravity = grainWeight * grainPotential * brewhouseEfficiency / postBoilSize
65.93 = 14 * 37 *.70 / 5.5 => 1.066
Where in reality the post boil gravity will almost always be the same as the recipe OG, unless water is added or additional fermentables are added. Working backwards, preboil gravity is a result of concentration of sugars in the wort so the above formula works, but will have a different value than current as it should be concentrating the to be updated postboil gravity value. But postboil is not a matter of concentration but separation.
If you take a gallon of wort out of the kettle and put it in a carboy, then put the rest of it in another carboy, the two volumes are still the same gravity. But if you added a gallon of water to the larger volume, then it will be a lower gravity, or if you add a lb of DME to one of the two carboys then it will be a higher gravity. this is the same as postboil calculation.
The general formula should be (or similar equivalents in plato/brix/extract). Specific gravity is slightly less accurate but easier to follow.
PostBoilGravity = ( recipeGravPoints - addedGravPoints ) * (batchSize) / ( postBoilSize - postBoilLosses).
in the use case of no additional sugars, and no dilution they should match.
1.073 = 73 = ( ( 73 - 0 ) * ( 5 ) / ( 5.5 - .5 )
Another way of thinking of it is via the total gravity points method. Total batch should equal postboil total - kettle loss total.
totalBatch = batchSize * batchOG = ( postBoilSize - kettleloss ) * postboilGrav = 73 * 5 = 365 = 73 * (5.5-.5). Note that since all the gravities are the same, it's equivalent to volume consistency 5 = 5.5 - 0.5.
Recipe total points = 5 * 73 = 365.
Kettle loss = 0.5 * 73 = 36.5.
Postboil = 5.5 * 73 = 401.5. This all checks out.
Preboil should be 401.5 / PreboilSize. ie 401.5 / 6.5 = 61.7 => 1.0617 (currently pre 1.056, post = 1.066).
In the use case of additional sugars, postboil should be less than OG. Lets add 1lb of sugar to the recipe after the transfer. OG is now 1.082, where 9.2 SG is contributed by the 1lb of sugar according to the "OG" stat in the fermentables section.
1.073 = 72.8 = (82 - 9.2 ) * ( 5) / ( 5.5 - .5 ). As you can see the postboil gravity did not change, but the OG is higher. This makes sense.
In the use case of a water addition, it gets a little tricky depending on the order of operations. But if you're transferring from the kettle to your fermenter, then adding water then it should behave like so. So postboilSize = 5.5 still @ what should be the original recipe OG of ~1.073. Loss of 0.5 gallons @ 1.073, then diluting the recipe by 1 gallon.
1.073 ~= 72.516 = (60.43 - 0 ) * ( 6) / ( 5.5 - .5 )
Recipe total points = 6 * 60.43 = 362.58.
Kettle loss = 0.5 * 72.516 = 36.258.
Postboil = 5.5 * 72.516 = 398.838. This all checks out.
In the use case of the opposite order, diluting by 1 gallon in the kettle and then losing half a gallon to kettle losses we get an identical scenario similar to the first use case as if the postboil size is 6.5 gallons and the recipe size is 6 gallons. Recipe OG is 1.060
1.060 ~= 60= (60 - 0 ) * ( 6) / ( 6.5 - .5 ). Once again as this is a matter of separation, and not concentration/dilution, the recipe OG and the postboil SG match.
Recipe total points = 6 * 60 = 360.
Kettle loss = 0.5 * 60 = 30.
Postboil = 6.5 * 60 = 390. This all checks out.
