refractometer or hydrometer for FG

[Zephyr259]

Joining this debate against my better judgement but I've had a beers already so I have an excuse.

An sg reading from a hydrometer is always correct for that fluid sample, assuming correctly calibrated and tested at calibration temperature.

What we do is use sg as a measure of sugar content which is fine for OG which is sugar in water. When measuring FG we get a measurement which is a combination of sugar dissolved in an ethanol and water mixture. That mixture of ethanol and water doesn't have sg of 1 since the ethanol portion decreases it so assuming the between 1 and fg is due to sugar isn't accurate. But it should be close enough or we'd have a more complicated formula to account for abv. It's also why we call it apparent attenuation, real attenuation is 81% of the apparent but you'd then need a different formula for abv.

I use both tools, refractometer is very handy for checking during brew day and to do see if fg is stable but I'll still take a hydrometer reading to confirm fg, but it tends to be with 2 points which is about the confidence level of me reading a hydrometer anyway or due to temp difference.

 

[bobukbrewer]

peebee - we agree ! let us say it falls to 0.970 so alcohol depresses the FG reading
you started with a wort with SG (hydrometer) of 1.050
you finished with FG (hydrometer) 1.008
you know that the 1.008 reflects the fact that you have approx 4.5% abv
if you could miraculously take out that alcohol then the FG would be 1.010
so back to my original question, why cannot I use SG brix 12.8 FG brix 6.7
the calculation of abv using only BRIX has been worked out by people far cleverer than me...

 

[Zephyr259]

You can use brix for both provided you have a formula to account for the alcohol which makes the refractometer read high. Brewers friend has a calculator and I have my own spreadsheet which uses similar maths, the site won't allow me to attach it for some reason, disallowed file extension? I've uploaded it here a few times recently so a search should find it.

 

[peebee]

peebee - we agree ! ...

Well yes. We had never disagreed that 40% or so alcohol has a lower SG than water, or beer. But it's late. Fortunately @Zephyr259 came to the rescue:

… What we do is use sg as a measure of sugar content which is fine for OG which is sugar in water. When measuring FG we get a measurement which is a combination of sugar dissolved in an ethanol and water mixture. That mixture of ethanol and water doesn't have sg of 1 since the ethanol portion decreases it so assuming the between 1 and fg is due to sugar isn't accurate. But it should be close enough or we'd have a more complicated formula to account for abv. It's also why we call it apparent attenuation, real attenuation is 81% of the apparent but you'd then need a different formula for abv. ...

Nothing is ever new. Go read up on "apparent" and "real" attenuation. I'm pretty sure this is where you are heading. I've never been bothered by it all, like the majority of brewers, and studying it will probably uncover why we hang on to hydrometer readings, warts and all, and do not get weighed down by all the intricacies that has the reward of … well not very much at all.


(EDIT: I've done the donkey work; this from the Beersmith site:
http://beersmith.com/blog/2010/09/07/apparent-and-real-attenuation-for-beer-brewers-part-1/
http://beersmith.com/blog/2010/09/14/apparent-and-real-attenuation-for-beer-brewers-part-2/
)

 

[-Bezza-]

Cor! Fight? Lets stick my oar in:

The OP isn't right! @xozzx is. Gravity measured as "specific gravity" (SG) is an absolute measure of gravity in reference to pure water (1.000). Doesn't matter what the liquid is.

What you say is strictly true - the SG is the SG regardless of whether it's water, alcohol, wort, meths or unicorn spit. However I had understood the point of the question to be around the accuracy in measuring the SG at two different points of fermentation, i.e. the way a homebrewer would most commonly use a hydrometer. That being the case, I stand by my assertion that a hydrometer does inherently result in inaccuracy. However, it's likely to be completely imperceptible in the context that it's not worth worrying about.

 

[foxbat]

There is a more complex hydrometer reading formula that takes into account alcohol content in the finished beer.

It makes a significant difference in the strongest beers. I believe Beersmith uses it by default and the others may offer you the choice.

Reference

 

[-Bezza-]

There is a more complex hydrometer reading formula that takes into account alcohol content in the finished beer.

It makes a significant difference in the strongest beers. I believe Beersmith uses it by default and the others may offer you the choice.

Reference

That's helpful and good to hear that Beersmith uses this (planning on grabbing this in the new year).

The simple formula gives us an ABV of 11.81%. The more complex formula gives us a mark of 12.05% ABV for this massive beer.

Interesting to actually see somewhere how much of a difference it makes, although not sure I would count that as significant - it's not the final 0.24% that would floor me!

 

[Zephyr259]

There is a more complex hydrometer reading formula that takes into account alcohol content in the finished beer.

It makes a significant difference in the strongest beers. I believe Beersmith uses it by default and the others may offer you the choice.

Reference

That reference formula is the one I have in my spreadsheet but the article doesn't tell you have to calculate real extract which is kinda complicated. I compare the abv and they tend to be similar enough that I don't overly care.

 

[The Baron]

do you know after all this we'll all still use the hydrometer for reference absolutely correct or not lol

 

[peebee]

... However I had understood the point of the question to be around the accuracy in measuring the SG at two different points of fermentation, ...

Heck, I wish you'd mentioned that earlier! Here, you can take this bit of paper off me (it's only got a black spot on it). If @bobukbrewer comes back can you tell him I've gone away for Christmas (I haven't decided what year yet).

 

[bobukbrewer]

peebee - so I stay with refractometer, quicker, less waste, less chance of contamination, as accurate as an hydrometer, merry Xmas to all our readers - Santa is right out of hydrometers.............

 

[Chippy_Tea]

Time to bring this one to a close, happy Christmas to one and all.