A question on Gravity

[Jimbo75]

Hi,

I am just finishing the primary fermentation stage of my first kit brew and am a little confused on hydro readings and potential ABV. I took an OG reading of 1040 just after pitching the yeast and today took a reading of 1016 (8 days later). According to the ABV calculator I found that should give me an ABV of only 3.2% however, the kit I am using (Woodforde's Nelson's Revenge) should produce 5.0%. The instructions on the kit say that when the gravity reaches below 1014 it is time to keg the wort. Admittedly, I did add one litre too much water to the wort initially so I have 21.5 litres in stead of 20.5. I assume this will make a small difference but nothing significant. According to my calculations the wort will have to reach an FG of 1002 to attain the advertised 5% ABV for this ale! Surely this can't be right?

So I have a couple of questions.:

- Will the brew increase in alcoholic strength at all after kegging and in the conditioning phase?
- If not, is there any point in pitching a little more yeast in an effort to kick off some more fermentation to get a bit closer to the intended ABV for this ale?

Cheers
Jimbo

 

[Paulie Walnuts]

Hi,

I did add one litre too much water to the wort initially so I have 21.5 litres in stead of 20.5. I assume this will make a small difference but nothing significant.

Someone will, I'm sure, be able to do the maths on this but by adding the extra water you have, in effect, diluted your brew.

Gravity is pretty much the same as density when it comes to liquids. The 'thicker' it is, the more resistance is created to stop your hydrometer sinking.

 

[EskiBrew]

In order to get 5% you need an Original Gravity reading somewhere about 1052. Assuming you had achieved that with 20.5 litres then your additional litre would have reduced the gravity to about 1049 (leading to 4.75%).

The SG will continue to drop slowly as it matures although I would be very surprised if you got down to anywhere near 1002.
It therefore seems likely that you did not achieve the expected starting gravity for some reason or the reading you took is incorrect:

1) what temperature was the wort when you took the reading?
2) have you checked your hydrometer in water (at the right temperature)?

The best thing to do is ignore the alcohol content and just enjoy your first brew - if it tastes nice then that's all that matters :thumb:

 

[evanvine]

Gravity is pretty much the same as density when it comes to liquids. The 'thicker' it is, the more resistance is created to stop your hydrometer sinking.

Specific Gravity and Relative Density are one and the same.
It is the number of times the measured fluid/liquid is heavier/lighter than an equal volume of pure water.
The action of the Hydrometer is based on Archimedes “Principle of Flotation”.
That is to say that as soon as the Hydrometer has displaced it’s own weight a state of equilibrium exists and the Hydrometer cannot “sink” any more.

 

[Jimbo75]

Thanks for the responses guys, they're really usefull. That's a very good point eskimobob and I was thinking along the same lines. If it tastes good (and it does, because I tasted it after the last hydro reading) then I'm just going to enjoy the ale and not worry about the ABV.

In answer to your questionthough , the temp of the initial reading was around 30 celcius and the temp of the latest reading was 18-19 celcius. Therefore according to a calculator I found the initial reading was more likely to be around 1044 or 1045 and the latest one would be pretty much accurate at 1016 so the final ABV will more likely be around the 4% mark which I am pretty happy with. Very nice to know a bit more about the science behind it now!