on line SG calculations

[otuatail]

Hi This is odd

Hi I am getting very confused over SG readings. I wanted to find a calculator on the internet that gave a corrected SG for temperature. What I found is a lot of confusing calculations. I have an OG of 46. On the hydrometer this relates to 7.5% alcohol. If the fermentation is taken down to near 1.000. Pretty strong beer? On line calculators have values of around 5.3 to 5.8% I know alcohol weighs less than water but these figures are very different.

I added one kilo of sugar to a Brupacks. This did not require sugar. It claims to be 4.0 - 4.3%. but I have a reading of 46 after adding the sugar. What would the alcohol content have been without the sugar? According to the online calculator it will be about 5.5% with the kilo of sugar. I have taken the temperature into consideration. No adjustments needed.

Can't get my head around this sorry.

 

[TheMumbler]

Roughly speaking:
Gravity readings tell you the density of the wort. Some of that density is due to sugar being in solution. Also density varies with temperature hence the need for corrections if you take the measurement at a different temperature to the one your hydrometer is calibrated for (usually 15 or 20 degrees celsius).

In order to get an estimate of how strong your beer using a hydrometer you need two (temperature corrected if necessary) readings:

Original Gravity (OG) - the density before the yeast is added
Final Gravity (FG) - the density after the yeast has used up all of the fermentable sugars.

A calculation for abv can then be made on the assumption that the change in density is due to sugars being converted into alcohol. I don't know how the formula is derived or even what it is off the top of my head, but there are tons of online tools including one in the Calculators section here.

I would expect your FG to be around around 1010 if you are using an ale yeast but it does vary a bit depending on the strain and temperature etc. That would give you beer of around 4.8% abv accoring to the calculator on this forum.

edit see this for a how to on hydrometers

Hope that helps

 

[otuatail]

Hi Yes I realize that temperature and taking both readings are required, but if my start is 1046 and my finish is 1005 then according to the hydrometer it should be about 7.5%. One website says take (in this case) the (46 - 5) / 8 = 5.125. This is a long way away from 7.5%

 

[TheMumbler]

Hi Yes I realize that temperature and taking both readings are required,(snip)

OK, you need both readings and clearly your hydrometer can't know what the FG will be... So in the end it is a waste of time worrying about what it estimates.

I think the discrepancy is probably because your hydrometer is giving you an estimate assuming that you are making wine and not beer. I don't really make wine but I think that they typically see FGs of 1000 and lower. Beers don't. So it is estimating a FG that is too low for making beer. A FG of 990 would put you in the ball park of 7.5% according to the calculators here.

If you want a pretty rough rule of thumb for beer, the abv tends to be in the region of the OG (usually a bit higher). In your case 1046 would predict around 4.6% which is wrong, but not far off for a fairly normal FG of 1010 (4.8%) and a good bit closer than 7.5%.