[a10t2]

Aahhhh, so "wort correction factor" is derived empirically.  I was looking for a "how to calculate".

Right, although like narvin said one data point is generally sufficient to establish a reasonable WCF for your brewhouse. Really, though, I've only seen WCFs range from about 1.00-1.06, so guessing isn't going to cost much in terms of accuracy.

So Toby is right.  Refractometers can show trends in highly attenuated beer but their accuracy sucks!

I guess it just depends on what kind of accuracy you want/need in your brewery. I honestly stopped comparing against a hydrometer after a while, but for those 30 batches the mean deviation is 0.0002 SG, standard deviation is 0.0014 SG, and maximum deviation is 0.0039 SG (that was for an 11% ABV barley wine). About the same as the $10 triple-scale hydrometers we all started out with.

[a10t2]

Yes, the two (almost) agreed on the OG.  Hydr. read 1.056, Refrct said 14.1 (calcs to 1.057)

FWIW, that's a WCF of ~1.02.

[majorvices]

I trust my bench top refractometer 100% for OG readings. For FG readings I only trust my lab grade hydrometer.

[Philbrew]

For FG readings I only trust my lab grade hydrometer.

I definitely need to get one of those!

[blair.streit]

Other than the BeerBug, has anyone seen a hydrometer type device that you can leave in fermenting beer?

[Philbrew]

Other than the BeerBug, has anyone seen a hydrometer type device that you can leave in fermenting beer?

Well, waaaay back in the day, that's how you brewed.  When the kreusen fell, you put your hydrometer in the open 12 gallon crock of beer.  The hydrometer had a red line on it.  When it got down to the red line you bottled.  As a broke, thirsty college student, I made 1500 gallons of beer that way.  Don't ask how long ago that was.  Yes, I did graduate.