Hey guys!
I thought iBrewMaster should officially chime in here! Yes! It is because of the Kettle Trub Loss. It seems as though some other software has never taken this into consideration and many users tend to immediately believe that iBrewMaster is wrong because of their years of experience with those other apps! :-D But Kehaar hit the nail on the head! Your gravity isn't diluted just by the wort that you remove from the kettle of course, but is diluted by the entire kettle volume including what you left behind! We receive a lot of Q's in regard to this, and have an entry in our FAQ's about it, but here's a detailed description of the issue that explains the calculations and the math behind them...no secrets there!! I hope it helps!
FAQ: Why did the OG calculations change in my recipes? They don't appear to be correct compared to other applications out there!
Answer: We corrected a minor bug with the OG calculations as we were originally calculating OG by not including the Kettle Trub Loss. Obviously, whatever is left in the Kettle has still diluted the fermentable sugars. So, if you had a recipe size of 5.00 gallons, but a Kettle Trub Loss of .25 gallons, we were only using the 5 gallons instead of 5.25. You can adjust what your Kettle Trub Loss is by changing it in the Setup → Mash / Eq Profiles, then use that profile for your recipe. So, if you dump the entire kettle into your primary, you would want it set to 0. (This should bring your calculations back to the original values you used to see.)
For the curious
heres the formula:
currentGrainGravity = (grainAmount * ((grainSG 1) * 1000) * (Efficiency / 100)) / (recipeSize + defaultKettleLoss)
For each grain, you take the amount (in lbs) and multiply it by the number of gravity points for that grain, or SG. (The * 1000 is because a gravity of 1.037 would yield .037 after subtracting the 1, and we need to convert that to 37.) Then you multiply that by the efficiency. You need to divide that by the recipeSize including the Kettle Loss. So heres an example:
2 lbs of a 1.037 gravity grain, efficiency of 70% and with a recipeSize of 5.00 gallons and a Kettle Loss of .25 gallons
currentGrainGravity = (2 * ((1.037 1) * 1000) * (70 / 100)) / (5.00 + .25)
currentGrainGravity = (2 * (.037 * 1000) * .7) / 5.25
currentGrainGravity = (2 * 37 * .7) / 5.25
currentGrainGravity = 51.8 / 5.25
currentGrainGravity = 9.866
Now, lets look at it from a practical standpoint. If you get 37 gravity points per lb of grain per gallon of water, then 2 lbs would give us 74 points. However, we only have an efficiency of 70%, so you get 51.8 gravity points. But thats per gallon
.if you have 5.25 gallons of wort in your kettle before you transfer 5 gallons to your primary, you need to divide the 51.8 gravity points by the entire 5.25 gallons, not 5 gallons as we ourselves were also originally doing. Therefore, you end up with 51.8 / 5.25 or 9.866 gravity point contribution from this one grain. Do this for every grain you have, and add em up.
The bottom line is that while iBrewMaster may have been showing you accurate OG calculations for your setup (assuming you dont have any Kettle Trub Loss), they were incorrect for those users who do have a Kettle Trub Loss. So all you have to do to correct it is to set your Kettle Trub Loss to 0 for the Mash / Eq Profile that you use for your recipe if you have none.
Cheers all!