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Old 03-11-2006, 05:15 AM   #1
ROAD MUTANT
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O.K finally got tired of filling the beeker. Decided to get a refractometer.
Cool toy but general knowledge says you can't use it to measure final gravity, only original gravity.

Then I read this obscure article that says you CAN use a refractometer to measure F.G. One catch, the conversion math is far to complicated for mere mortal man. I'll need to buy this CD (that aslo does alot if crap I'll never need) to do it for me.

I've decided to throw myself on the mercy of the court, appeal to the god(s)
of refractometery / specific gravity and seek this elusive knowledge (without paying through the nose for it). Does anyone possess this gravity grail and can ya send me a copy of it?

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Old 03-11-2006, 06:18 AM   #2
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Quote:
Originally Posted by ROAD MUTANT
O.K finally got tired of filling the beeker. Decided to get a refractometer.
Cool toy but general knowledge says you can't use it to measure final gravity, only original gravity.

Then I read this obscure article that says you CAN use a refractometer to measure F.G. One catch, the conversion math is far to complicated for mere mortal man. I'll need to buy this CD (that aslo does alot if crap I'll never need) to do it for me.

I've decided to throw myself on the mercy of the court, appeal to the god(s)
of refractometery / specific gravity and seek this elusive knowledge (without paying through the nose for it). Does anyone possess this gravity grail and can ya send me a copy of it?
damn i was thining of buying one but...now...i do remeber using one in my observational astronomy class and i remeber just looking through it, and the star would be broken into different wavelenthgs.i dont recall doing the math though...
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Old 03-11-2006, 03:19 PM   #3
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Quote:
Originally Posted by ROAD MUTANT
O.K finally got tired of filling the beeker. Decided to get a refractometer.
Cool toy but general knowledge says you can't use it to measure final gravity, only original gravity.
That doesn't make any since to me...SG is SG. If it will measure SG before fermenting, then it will measure it after fermentation too. Can you post some of this "general knowledge" so we can debunk it?
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Old 03-11-2006, 03:30 PM   #4
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Yeah, I'm pretty sure they just measure SG as long as your SG is within range of your refractometer. What they won't tell you is abv (no more than a hydrometer)...you still have to do a relatively simple calculation.

I've been thinking of getting one of the el cheapo China jobbie refractometers of ebay for $15-20...anybody have any experience with those? Worth it? Apparently they have temperature compensation...

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Old 03-11-2006, 03:56 PM   #5
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I was looking into getting one as well, but after further consideration, just worth filling the beeker.....They don't measure FG from what I can tell, but if you have ProMash software, that does convert it for you with pretty much ease. But is it worth the $60 or more for them, still not sure it is worth it. Still thinking it through more......

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Old 03-11-2006, 06:53 PM   #6
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You can't measure FG with it, because they are calibrated to measure the amount of sugar dissolved in "pure" water. The alcohol throws the values off. You can adjust the numbers if you know the OG, but it's messy.

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Old 03-11-2006, 07:41 PM   #7
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Quote:
Originally Posted by El Pistolero
That doesn't make any since to me...SG is SG. If it will measure SG before fermenting, then it will measure it after fermentation too. Can you post some of this "general knowledge" so we can debunk it?
A refractometer doesn't measure S.G. It measures the refractive index of a solution. (i.e. the amount a beam of light is bent when passing through the solution.) In the case of an unfermented wort, the dissolved sugars are about the only thing that affects the refractive index and the S.G, so it's easy to estimate the S.G.
As David_42 said, after fermentation there are still some dissolved sugars, but also alcohol and CO2 that affect the refractive index.. You can do it, but you need to eliminate the CO2 by putting a sample in a blender, or pouring back and forward between two containers, and then apply some fairly complicated formulae.

This as an oversimplification of what acrually happens.

See http://byo.com/feature/1132.html for more detailed information

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Old 03-11-2006, 07:54 PM   #8
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This is very cool...from the BYO article ajf linked to:

Quote:
To calculate your correction value, measure the specific gravity with your refractometer. Then chill a sample of your wort and measure the gravity with a hydrometer.
So you buy a refractometer so you don't have to use your hydrometer, but you still have to have a hydrometer to calibrate your refractometer...for each batch?
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Old 03-11-2006, 11:22 PM   #9
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Quote:
Originally Posted by El Pistole
This is very cool...from the BYO article AF linked to:

So you buy a refractometer so you don't have to use your hydrometer, but you still have to have a hydrometer to calibrate your refractometer...for each batch?
The calibration is only necessary to make very small corrections. I (and I suspect most home brewers) don't bother.

The advantages are:
One drop sampling size.
No temperature compensation. (The thermal mass of one drop is so small it can be ignored, and most instruments have built in temperature compensation.) - You can take a drop from the boil and check within a few points in a couple of seconds.
More accurate than a cheap hydrometer if you have poor eyesight.
They don't roll off the counter and smash into thousands of pieces when they hit the floor.
Great for doing what they are designed to do (measuring the sucrose content of fruit).

The disadvantages are:
Not as accurate as a good hydrometer (With good eyesight.)
Good for OG, but not very good once fermenting has started
More expensive.
The one drop sample size is useless for tasting.

-a
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Old 03-12-2006, 03:16 AM   #10
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Quote:
Originally Posted by ajf
The disadvantages are:
Not as accurate as a good hydrometer (With good eyesight.)
Good for OG, but not very good once fermenting has started
More expensive.
The one drop sample size is useless for tasting.
That's what I read too. So it pretty much go to the bottom of the need-to-have list. I'd be more interested in a PH meter now. But they are even more expensive and require some maintenance. And the probes wear out too.

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