No OG reading, can I figure it out?

[RichSib]

I couldn't take a post boil gravity reading today.
I took a pre boil reading, 1.042 for 7 gallons of wort.
Post boil volume was 5.5 gallons.
Using pretty straightforward arithmetic, I figure that my post boil gravity should be 1.051.
Does this compute with anyone else?
Thanks, Rich.

 

[Polboy]

for me its coming up as 1.053

 

[weirdboy]

Yeah 1.053 is what I get too.


42 * 7 / 5.5 = 53.45

 

[RichSib]

I originally got 1.053 too but that gives me 88% efficiency, which seems high, so I changed my methodology a bit and got 1.051 which sounds more realistic. I guess math is math though, 1.053 it is.
Thanks, Rich.

 

[gypsyhead]

I thought efficiency was based on pre-boil gravity... right? Otherwise you could just boil longer to artificially increase your extract. Using .042 would give 70% efficiency, which seems about right.

 

[RichSib]

Yes and no, It was a redundant statement on my part, I wasn't having a very bright day yesterday.
Efficiency can be based on either pre or post boil gravity readings, the extracted sugars are the same in both, the concentration is different due to the reduced post boil volume. Regardless of whether you measure pre or post boil gravity the efficiency should be about the same, in this case 87-88%. You can not however measure efficiency without knowing the original grist bill.

 

[Polboy]

There is actually a difference in efficience pre and post boil as pre boil you are not accounting for BK lost (trub and dead space mostly), in my case 6gal in BK after boil will end up as 5.5 or less in the fermenter, this is almost 10% drop in my efficiency