How to measure alcohol content after cider restarted fermentation?

[bensira]

I racked a cider from primary into secondary and topped up the secondary carboy with some apple juice to get rid of head space. The sugars in the apple juice have restarted a gentle fermentation which has been going on for five days now. Will I be able to measure the alcohol content with gravity readings now?

Day 0
Juice has gravity reading of 1.040. Nottingham yeast added

Day 15
Primary fermentation finished. Gravity reading of 0.994, giving an estimated ABV of 6.04% One litre of apple juice added to top up carboy. SG reading remains the same after juice was added - though this was quite likely a faulty reading, as the apple juice didn't mix with the cider for a while, and I may have just been measuring the fermented cider without including the juice! I didn't want to shake them for fear of oxidation.

Day 16
Fermentation restarts

Day 19
Fermentation continues

When fermentation finishes and I take another gravity reading, how can I estimate the ABV? Will it be unknown because of the addition of an unquantified amount of sugar after racking?

 

[Maylar]

You need the "Day 15" number, so you know how much sugar you added.

 

[MindenMan]

There may be an easier way, just calculate the total amount of fermentable sugar added. It doesn't matter if you added it in "stages" or not. You don't say how big your batch is, so w/o knowing that we can't help you calculate your numbers.