# Help with gravity calc?

Hi all -I’m fermenting a chocolate cherry stout now. I produced 4.5 gallons of wort with OG of 1.073. After a couple days in primary, I added the cherry juice concentrate. Specifically, I added 64 ounces of liquid with 704 grams of sugar within.

I am struggling to come up with OG including the cherry juice. My math seems to give me an OG of 1.090 including the cherry juice, but intuitively that seems low to me.

Any help would be appreciated! Cheers!!!

Hmm…this is trickier than it looks. I think your effective OG may be lower than your calculation. Here’s how I did it:

You started with 4.5G at 1.073, so that’s 329 gravity points (4.573). You added 1/2 gallon of liquid with 704G (which is 1.55#) of “sugar”. If we assume that sugar has the same ppg as corn sugar (1.046), that’s 87 more gravity points (1.5546).

Add those two and divide by the total volume (415/5) and you get about 1.083.

Thanks Rusty.

I got to the bottom of it, kind of.

I purchased a refractometer. I used it to measure the OG of 1.073, which was right on top of my target. After 10 days of fermentation, took another reading with the refractometer. Reading was 1.050, so backed in to an ABV of around 5%,which had me scratching my head. I figured adding the cherry juice must have increased the gravity by a LOT but left a lot of unfermentables.

Fast forward to tonight. Took another gravity reading using my buddy’s hydrometer (broke mine, which was the whole reason I bought the refractometer) and it was 1.020, so an ABV around 8.25%, which was pretty much my initial expectation.

I know you have to adjust a refractometer for FG readings because of the alcohol present, but that difference seems crazy to me. I think a normal adjustment is a factor of like 4% (not 60%).

Going to have to read up on my new gadget, but I calibrated it with water, and worried I have a defective unit.

In any event,thanks again for your response!

Cheers,
Drew

I use a refractometer as well and I really like it, but the adjustments during fermentation are not trivial at all.

Here’s a very good calculator that is based on equations that are curve-fit to measured results:

http://seanterrill.com/2012/01/06/refra ... alculator/

You can find a lot of different calculators out there, but none of them that I’ve used come close to the accuracy of Sean’s. You can check it for yourself by taking a refractometer reading during fermentation, plugging it into the calculator, then do a hydro measurement and see if you get the same result.

I’ve done this a couple times and it’s been dead-on for me.