Which gravity reading is good...

[bzwyatt]

I brewed a 5-gallon batch yesterday, measured the gravity with refractometer of the wort after sparge was done and all wort collected, right when I turned the heat up to start the boil, OWG was 1.047. Volume went from ~6.9 gal to 5 gal in the hour-long full boil. I forgot to take a gravity reading before pitching and covering carboy. When I realized it during cleanup, I went back and took a sample with some tubing. This gravity reading was 1.057.

I anticipated a gravity reading of 1.060, and if I increase gravity by the ratio the volume decreased, I get ~1.060.

5/6.9=0.725

2-0.725=1.275 (does that work to determine the increase in gravity?)

1.275(47)=59.94 (can I multiply the increase by the OWG to find calculated OG?)

I should probably get Daniels' book out, I am pretty sure I could find it there... but can you let me know if I'm right?

I think it is possible that some of the dissolved sugar settled, because my tubing just dipped into the wort, so my sample came from the top inch, and I think that gravity reading may be off.

 

[LLBeanJ]

Try this...

47 x 6.9 / 5.0 = 64.86 ----> 1.065

 

[bzwyatt]

So if my original reading was right, my OWG (I call it that - original wort gravity, not sure if that is a 'real' term I read somewhere, or if it is just what I call it because it makes sense in my brewing journal) or 1.047, and the volumes are accurate, then my OG has to be 1.065.

Right?

 

[LLBeanJ]

Correct. And what you're referring to as "OWG" would be "pre-boil gravity."

 

[user 141939]

There is some error that could be introduced by gravity readings and volume measurements. My final OG is close but never exactly what I would predict from my pre-boil OG. I think it got closer since I more accurately marked my "measuring" mash paddle.

And perhaps the yeast took it down a couple points already? They eat sugar when they are multiplying too.