Refractometer accuracy measuring gravity - Strange readings

[Cockfighter]

Since the first brew I've done I've been using a refractometer to give me my gravity readings. It's easy to use and allows me to waste minimum liquid.

My boil gravity and OG readings have usually been spot on with my recipe calculations. Yet I'm never really sure about my FG readings...

I use the following calculator to properly convert my FG, but I often get inconsistent results.
My FG is usually a little higher than what is calculated for the recipe, even if I've made an effort to mash low.
Also, I have had gravity readings vary wildly... eg. 8 brix to 7.2 brix and then back to 8 brix. How can I get a higher reading if I had a lower reading the day before???
This is after about 2 weeks in primary but the way (ale yeast, fermentation no longer visible).

What could be causing this erratic behaviour? Are refractometers just too inaccurate for a proper final gravity meeting? Maybe the increasing alcohol content is causing the reading to go back up?

 

[jbb3]

Since the first brew I've done I've been using a refractometer to give me my gravity readings. It's easy to use and allows me to waste minimum liquid.

My boil gravity and OG readings have usually been spot on with my recipe calculations. Yet I'm never really sure about my FG readings...

I use the following calculator to properly convert my FG, but I often get inconsistent results.
My FG is usually a little higher than what is calculated for the recipe, even if I've made an effort to mash low.
Also, I have had gravity readings vary wildly... eg. 8 brix to 7.2 brix and then back to 8 brix. How can I get a higher reading if I had a lower reading the day before???
This is after about 2 weeks in primary but the way (ale yeast, fermentation no longer visible).

What could be causing this erratic behaviour? Are refractometers just too inaccurate for a proper final gravity meeting? Maybe the increasing alcohol content is causing the reading to go back up?

Refractometers work by measuring how the light bends when it passes through the liquid. The presence of alcohol changes how the light passes through thus throwing off the FG reading. There are calculators that are supposed to help adjust for this change but I wouldn't trust it for FG. Invest $7-$8 and get yourself a trusty hydrometer for FG readings.

 

[ajdelange]

The refractometers we buy are for measuring the sucrose content of liquids. Wort isn't a sucrose solution and fermented wort isn't either. Thus we can't expect refractometers to give us very good representations of the extract in wort and fermented beer. With unfermented wort the situation is generally better than it is with fermented and in fact a refractometer will give you a good answer most of the time. The problem, of course, is knowing when your situation isn't a most of the time situation. Wort OG readings can be off by 1 Bx or more.

 

[zam216]

Make sure your refractometer is calibrated. If you use programs like Beersmith they have a calibration tool you can use. You want to make sure it is calibrated to 0 with distilled water and calibrated to a hydrometer reading before and after alcohol is in solution. This is what I do and my readings are always where they need to be. I to am not a fan of losing that much liquid to measure gravity so the refractometer is my preferred method.

 

[lolcats]

For FG readings you can use
http://seanterrill.com/2012/01/06/refractometer-calculator/

But i found that it's never accurate. It usually gives me a higher FG by .002

Refractometer readings for FG are good to check that the brix readings are steady over 3 consecutive days and that's pretty much it