Using a refractometer for end of fermentation...

[Alex.mc]

Can anyone help me? I use a refractometer for determining OG. I know it's resonably giving the same readings at my Hydrometer, having tried both together when I got it.

I understand that alcohol affects the refractometer readings post fermentation, but I cannot get my head around the adjustment formula. OG was 1.070, and now the refractometer is reading 1.030 for 4 days straight now.
I'll assume that fermentation is finished, given it's been stable for a while now.

Wort correction factors seem very close on the whole, with people saying 0.96-1.04
If i plug some numbers into brewers friend(which seems to need plato?) then I get an ABV of 7-8%, which is in the ball park of what I would expect.

Am I thinking that out correctly?

Edit:
I just pulled out a sample and dropped the hydrometer in it. FG is 1.014. So....... 1.070 to 1.014 is 7.35% ABV.
Can I extract a wort correction figure from that?

 

[Bigcol49]

Can I extract a wort correction figure from that?

Hi!
The best way to get a wort correction factor is to measure FV with hydrometer and refractometer over 15 to 20 brews.
Your way will give you a close enough approximation.

 

[Alex.mc]

So how do I get a factor from a hydrometer reading of 1.014 and a refractometer reading of 1.030?
I've tried plugging those figures into BF but it's not making any sense at all.

 

[MmmBeer]

When I first bought a refractometer, I went searching for a formula to correct the FG reading for the alcohol content, but didn't find anything satisfactory. Instead I found a formula for calculating ABV value, using just the FG readings from both a refractometer and a hydrometer, has come in very useful when I have forgotten to measure or write down OG values.

It's a bit long-winded, so best suited to excel or similar.

ABV = (277.8851 - 277.4 x H +0.9956 x R + 0.00523 x R^2 + 0.000013 x R^3) x H/0.79

Where H = Hydrometer Reading (e.g. 1.012), R - Refractometer reading in brix (e.g. 6)

 

[geigercntr]

If i plug some numbers into brewers friend(which seems to need plato?) ...

Can't help with anything else, but to all intents and purposes, Plato ≈ Brix (down to about 4 decimal places)

Edit: assuming that your refractometer gives one or the other!

 

[MrRock]

Can I extract a wort correction figure from that?

The wort correction factor is taken from comparing your refractometer & hydrometer over a number of brews, but importantly it's done with the wort before fermentation has started
https://www.brewersfriend.com/how-to-determine-your-refractometers-wort-correction-factor/

After fermentation has started I tend to use my refractometer just to get a rough idea what's going on and if it stays steady over a few days and is roughly around the expected ABV I know it's done, I'll then use the hydrometer to get my "actual" reading.