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Original Gravity Discrepancy

Hi All,

I’ve just started brewing extract kits again, after about 2 years, and noticed something that I didn’t pay attention to before.

First brewed a Cream Ale, but forgot to measure OG when transferring to fermentation. Therefore at the end of fermentation I just used the OG on the box (1.048) and the FG I measured (1.01) and got ~5% ABV.

Second brew was an Amber Ale, this time I did measure OG when transferring to fermentation and noticed OG was 1.036 (instead of 1.047 on box). FG was measured at 1.01, giving me only ~3.4% ABV. However if I use the box OG, it calculates to ~4.9% ABV.

My question is, what can be causing the discepancy between the measured OG and the stated (box) OG?

If you’re doing partial boils, your most likely culprit is not getting the top-off water mixed well with the wort. I always take the reading, but for the most part, I use the OG on the box. As long as you add all the extract, and finish with 5 gallons in the fermenter, the OG’s will be the same.

Plus or minus a point or two, but yes, it sounds like you probably measured some not very well mixed in wort. There’s no reason you’d have a huge gravity swing from what the OG should be per the instructions, it’s not like you’re mashing.

What temp are you taking your OG readings at and at what stage?

I am doing full boils, I start wit around 6.5 gallons, and after the boil, it ends up at 5 gallons. Is it better to boil 5g and add the extra water after boil
Also I’m measuring the gravity at 70F just before I pitch the yeast, and just before I keg.

Are the kits you are brewing from Northern Brewer? I have brewed probably 20 and have never been under on my OG. I do partial boils but that should not matter. You should be on the button if you end up with 5.25 gallons in your bucket, no matter how you get there. Unless you spilled, forgot to add something and/or did not empty the extract containers fully.

Have you calibrated the hydrometer?

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