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Gravity/Alcohol question

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Skiddy au

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Hi,
Just finished fermenting my first attempt at making my own recipe, as a partial boil, but I have noticed something strange with my gravity readings.

Beersmith gave me a predicted OG of 1.051, but I had it in the fermentor at 1.059. The finishing gravity predicted at 1.013, and actual at 1.012, so far so good, with a predicted alcohol of 6.2%. Too high for the style, but so what?

The problem is this: As a sanity check, I tested the sample with a hydrometer and a refractometer (1.012 and 1.025 respectively). Putting these numbers in online calculators reveals ABV of about 4.2%, which is too low. This implies my starting gravity was about ~1.045.

These leads to two questions:
- Which alcohol measurement is correct; and
- If it's the second one, what did I do wrong in my initial measurements?

Any guidance would be appreciated in finding the flaws that lead to this situation.
 

MHB

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The standard way to calculate Alcohol By Volume (ABV) is change in gravity / 7.5 (people use points rather than SG, so) 59-12= 47 Points change, ABV=47/7.5=6.299999 Ok 6.3%ABV.
Note that SG is non-unitary, its just a comparison to the mass/volume of water, in this case 1L of you wort would weigh 1.059kg rather than the 1kg pure water would.

A refractometer measures the amount light is bent, water bends light, dissolve sugar in it and it bends it more, turns out we can tell how much sugar is dissolved by the amount of bending. Problem is Alcohol bends light to, just a different amount to what water and sugar do. At the end of fermentation you have a blend of Water, Alcohol and unfermented Sugars (mostly, with other bits and pieces).
It's impossible to tell just from the final reading which of the three main constituents is doing how much bending of the light. You can find equations or calculators that given the OG will tell you your alcohol content from the final refractometer reading.
For most brewers the refractometer is really handy before the ferment but a bit of a PITA once you add yeast.
Best bet is to have a decent hydrometer you can trust. Use it for your OG and FG readings, use the refractometer for a quick reference, I use a digital one, often several times during the mash and boil just to make sure everything is on track. The numbers we send to the tax mam have to be taken with a certified hydrometer (or refractometer...) break one of those and you will notice a hole in your weeks income.
Mark
 

Skiddy au

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Just to be clear, I am not relying on the refractometer for the final gravity, the 1.012 from the hydrometer is the correct number. Rather I am relying on calculators such as this one which take as input both the final hydrometer reading and final refractometer reading.
 

MHB

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I'm not getting much sense from that calculator either.
Hard to tell exactly what they want in terms of inputs, I would try a different calculator. Or just go with your Hydrometer readings.
Not a bad notion to make a sugar solution and calibrate both your Hydrometer and Refractometer, preferably at two points 0/ 1.000 (pure water) and a constructed standard 10%/1.040 are pretty common.
Depending on the accuracy of the scales you can access, you need to be aware of the errors you get when weighing your sugar/water to make the second standard.
Mark
 

Skiddy au

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I put the numbers into this formula from this byo.com article:

ABV = [277.8851 – 277.4(SG) + 0.9956(Brix) + 0.00523(Brix2) + 0.000013(Brix3)] x (SG/0.79)

This gave me a result of 4.5%. Also revisited the recipe, and put in the brewers friend recipe builder, with a predicted SG 1.049 FG 1.011. If I was a point wrong each side (total change 36 points), this would result in ABV of 4.8%, which is near enough.

Finally relying on a taste test, this is not a ~7% beer. You know when you drink something that strong.

I think I must have got my OG reading wrong somehow, but not sure how.
 

rizrah

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as this was a patial boil, are you sure you dissolved all your brewing sugars perfectly? when i was extract brewing, i often got incorrect hydro results as i didnt dissolve 100%, and took readings from the tap of the fermenter, down the bottom where all the heavy sugar was.
 

Skiddy au

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I was wondering about that, but I thought it unlikely, because all the malt went into the boil, so I thought it would be evenly distributed once I poured into the fermentor. Something I'll watch more carefully next time.
 

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