Accidentally recorded OG before adding water to batch...any equation to figure out OG with water?

[Conner James]

I made a hopslam clone about 2 weeks ago, and I took a reading of it with my hydrometer and it came out to 1.130 which was insanely high but for some reason I just went with it. It didn't occur to me until a week later that I didn't record it after I had taken the 3.5 gallons I brewed and added 2 gallons of water to it. It's getting closer to bottling day and I'm just wondering if there is some way to figure out or estimate what the OG should have been with the added water?

 

[Qhrumphf]

Do you precisely know volumes prior to and after top-off? If so, then yes.

G1*V1=G2*V2.

So if you had 3.5 gallons of 1.130, and added two gallons of water...

130*3.5=G2*5.5

455/5.5=G2

=82.7

Your gravity topped up with 2 gallons of water to 5.5 total would be 1.083.

Now, changes in volume will obviously throw off that math. But this works for any instance where water is removed (boiloff) or added (top off). And as long as thermal expansion is accounted for (volumes adjusted to or read at the same temperature).

 

[BrewnWKopperKat]

Convert OG (130) in concentrated wort volume (3.5 gal) to gravity points: 130 * 3.5 = 455.

Add water. Gravity points remain unchanged.

Convert gravity points (455) to wort volume (5.5 gal): 455 / 5.5 = ~ 83

 

[Conner James]

Perfect! Thanks everyone

 

[Qhrumphf]

I will note, if you're topping off, reading prior to top off and calculating will get you a more reliable OG than reading after topping off- assuming your volume measurements are correct. Otherwise I'd be hesitant to trust a gravity sample to be representative no matter how well I mixed it.