Two different efficiencies for the same numbers.

[BEERngineer]

For some reason there seems to be a ~5% efficiency deduction between mash efficiency and preboil. Why does this happen?
In this example, I completed my mash with 28L of wort at 1.040. I get 83.6% conversion efficiency.
My preboil efficiency with the same 28L and 1.040 only has 78%. Where did I lose 5%

 

[UgliestLemming]

Looking at this It appears that conversion efficiency isn't looking at your complete mash and lauter but just first runnings? Based on this the pre-boil efficiency is your whole boil volume. This is just a guess though..

 

[BEERngineer]

Thanks for the reply. I BIAB, so the two figures should be the same for me.
http://www.brewersfriend.com/brewhouse-efficiency/ describes your picture with words, yet I still do not see where my 5% is going.

 

[cassa]

http://www.brewersfriend.com/brewhouse-efficiency/ describes your picture with words, yet I still do not see where my 5% is going.

At that link, it says the volume used for conversion efficiency is "how much mash water was used (do not count grain absorption or mash tun dead space)." I.E. not the amount of wort that made it to the kettle. With BIAB, you don't have dead space, but you still have grain absorption, so I think the Mash Complete log should have a volume larger than 28L.

 

[valtert]

Yeah but if he had ZERO grain absorption (if it was possible) then he wouldn't have lost efficiency due to grain absorption, and should have the same efficiency calculated for conversion and pre-boil efficiency. That would be reasonable if you look to the brewing efficiency diagram from the FAQ.

I'm getting different numbers as well... but for pre-boil and ending kettle efficiency. I know it's wrong because if I use the "Dilution and Boil Off Gravity Calculator" and enter the pre-boil volume and gravity plus the ending kettle volume, it gives me the ending kettle gravity exactly as measure, so PPG should be the same (thus the same efficiency) for pre-boil and ending kettle. But its not. And It happens only for this particular recipe. I've used custom fermentables, if that matters.

Maybe there could be a bug here, but as mentioned in another thread, this forum seems to be unmonitored, so no staff will ever know about it, which makes of me a frustrated subscriber.

 

[J A]

I've assumed that the two numbers are just using different data and I'm entering the only data I have for both. The one that matches the OG of the wort is the one that makes sense to me. It all works out once I get a good post-boil gravity. From there I may top up slightly ( up to a couple of quarts) if my boil-off was more than expected and I really want to keep the numbers a little lower.
Once it's in the fermenter and I can accurately measure the volume and OG, I know what I can expect from the beer. Before that, not so much. :?