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I was struggling to understand this same issue, although I discovered it in a different manner. I've always had my BF target set to fermentor and used to take my OG reading after transfer from kettle to fermentor, as I was brewing in smaller pot and needed to top off my fermentor. About a year ago I switched to a Mash & Boil unit and no longer need to top off my fermentor. I just started using the volume collected as my final volume and I noticed that OG changed based on fementor volume. I started looking at the options with my most recent brew (yesterday..LOL) and realized I should probably switch to kettle volume, since I now take my OG readiing in the kettle after chilling. When I switched it, I noticed the same isuue as RockyMtnGT.
I understand the difference between kettle efficiency and brewhouse efficiency. One is the volume of wort following the boil, the other is the volume of wort transferred to the fermentor (although, if it's true brewhouse efficiency I would think this would be the final volume of package beer). Like RockyMtnGT, I don't think OG reading should change based on fermentor vs. kettle volume, unless it is assumed that the brewer is topping off the fermentor under the former. The OG of the wort doesn't change unless it is dilluted.
For my purposes, I think I should be using kettle efficiency for the most accurate OG estimate in a recipe. Is this correct, based on what I have described? I'm trying to fine tune an equipment profile for the Mash & Boil that will give me accurate results.
Thanks.
Personally, I think brewhouse and fermenter makes the most sense and that's what I recommend everyone uses. There's a couple reasons, but the main ones are
1) The final product is what truly matters. Every time you ignore a part of the brewing process, you're ignoring part of the finished beer.
2) Communication. When people say OG, they mean the gravity at fermenter before pitching yeast, with almost no exceptions. If you're using kettle, and using the postboil SG to mean OG, then you have to needlessly translate or explain things instead of using the common terms.
I do see a couple scenarios where it may be better to use kettle instead, such as making a concentrated wort and diluting it, especially where there may be wort losses in the kettle due to undrainable volumes.
If you're using equivalent numbers, and converting the efficiency, then there's no difference in accuracy.
Kettle efficiency will be more consistent than brewhouse, if you're leaving different amounts of wort behind in different recipes, but I don't think the target: kettle is the best practice personally. Hopefully we can separate the two in the near future so you'll be able to use kettle efficiency, but keep the batch size as fermenter volume.