How to calculate abv with a second/fed fermentation

[Longhorn]

I recently mixed a Tom Caxton kit of real ale with 2lb Golden Syrup and achieved an OG of 1.040. The sugars are now pretty much all done and I'd like to transfer to a secondary fermentation bin and add some more syrup to feed the yeast and get a bit more abv. So a couple of questions:

1. Is this a good idea or a stupid one?
2. If it's a good idea, how do I calculate the abv at the final stage? I guess there will be a gravity now, then another gravity when I've add more syrup, and then a final gravity at the end.

OG and FG is simple, but how much does this secondary stage complicate the issue?

Thanks in advance.

 

[RobWalker]

Err, there should be points on the side of your hydrometer. A 450g tin of golden syrup will add 7 points to the gravity in a 5 gallon batch, so just count your OG as 1.047 instead - used beersmith for that.

It will be thin with that much fermentable sugar though, you might want to consider malt extract instead!

 

[Pinck]

What kind of info do you have on the syrup you're looking to add? What is your batch size?

Gravity is nice because it's nice and linear. If you know how many gravity points your syrup would add to your entire batch, then you can simply add that to your beer that you're looking to boost.

Example:
If you don't know your brewing specs for your syrup (it's generic from the grocery store, no brewing specs), then you can do a mini mixture with water and check the gravity. For the sake of simple math, lets say your batch size is 20L. If you mix up 200ml of water (1/100 of 20L) with 10 grams of syrup and the result is the OG of the water raising from 1.000 to 1.015, then you can assume that if you were to mix 1kg of the same syrup into your 20L of wort, you should see the same boost of 1.015. In your case, it would make your 1.040 act more like a 1.055.

Be very careful about how you're doing this measurement though, since the amount of sugar that sticks in your spoon when you do your mini mixture won't necessarily scale 100x when you add it to your wort :)