[dmtaylor]

Here's what I know based on my experience.  It will seem lengthy, but if you can digest all of it, it should make good sense I believe.

1) First and foremost, you need to check calibration of your refractometer with every single use.  That's right -- EVERY use.  The readings on a refractometer really seem to bounce around a lot each time I use mine.  First measure plain distilled water and ensure it reads exactly zero.  If it doesn't, you can either adjust the screw in the thing (I haven't even done this) or else figure out about how far off it is from zero, then add or subtract that number of points when reading wort.  For example, mine often seems to read about negative-0.2 Brix with plain water.  So then I dry the thing off, then add a few drops of wort and add +0.2 Brix for an accurate reading.  If you don't check calibration every time and adjust from zero, then the readings are pretty much useless and you're wasting your time.  The next time you use the thing, even just 5 minutes later, check it with water again, because it probably moved!  Readings are not stable, that's been my experience, so I calibrate every single time I'm going to use it.  Even check it with water again immediately after measuring wort, and if the readings aren't the same in water each time, take the average of how far off it is from zero and apply that.  So, for example again, if I check in plain water again and now it reads exactly 0.0 Brix, but I got negative-0.2 Brix before, figure I should add the average +0.1 Brix to the reading in wort instead of +0.2.  This will provide the utmost accuracy.  The more measurements and experience you gain, the more accurate your readings will be.

2) You must use both your refractometer (recording your readings IN BRIX) and hydrometer together many times with OG readings (zero ABV) along with ANY refract-to-hydro calculator (it does NOT matter which one) such as Sean Terrill's (http://seanterrill.com/2012/01/06/refractometer-calculator/) or Petr Novotny's (see info in the OP above) so that you can learn the "correction factor" for your specific refractometer.  Every refractometer is a little different in this respect.  Sean Terrill suggests an average correction factor of 1.04 for many refractometers his cohorts have tested.  However, do NOT assume your refractometer's correction factor is 1.04 without testing it out.  Mine definitely has a correction factor of exactly 0.99.  I have learned this after measuring the OG of at least 11 batches, and now that I know this correction factor, the results are always extremely consistent and predictable.

3) Okay.... so now let's say you know your correction factor.  You've measured several OGs of several different worts, and it's matching up perfectly with your hydrometer reading every single time.  Okay.  Now you're ready to use it for FG readings!  Until this point, your FG readings will be a crapshoot!  But now that you know how to use the thing and apply the proper correction factor, you can use it for FG.  Yay.  So to do that........ I personally recommend either Petr Novotny's formulae available from the OP, or use the "Old Cubic" formula which very ironically is available in Excel spreadsheet format from Mr. Sean Terrill at: http://seanterrill.com/wp-content/uploads/2011/04/fg_calculator_v3.0.xls 
Check calibration of your refractometer in water as before, then measure your final wort in BRIX, then pump that into Terrill's spreadsheet but use the result from the column that says "Old Cubic".  That will give you the results you see in the chart above as red squares -- almost dead-nuts accuracy.  The one goofy data point above, by the way, was where my OG was like 1.098.  None of the refractometer calculators are very accurate with high gravity worts of greater than about 1.080 or 1.085.  They'll still work okay for high gravity, but in my experience you'll probably need to subtract 0.003 or so to match a hydrometer reading.

Note: Hydrometer readings will ALWAYS have the greatest accuracy.  Refractometers with good calculators can get you very very close indeed to a hydro reading, within 0.001-0.002, but only that accurate if you follow appropriate standard scientific practice as I've attempted to provide in all the guidance above.

I certainly hope this helps somebody more than it causes confusion.  It's working really awesome for me, with my small batches (average 2 gallons).  I have a lot of confidence now when using refractometers for FG and ABV that I am getting very accurate results consistent with the more reliable hydrometer readings.  So now I don't need to waste a big hydro sample.  Just a few drops and I'm good to go.

Cheers.

[denny]

After reading the post above, it's confirmed my decision to stick with a hydrometer.

[dmtaylor]

After reading the post above, it's confirmed my decision to stick with a hydrometer.

It's really not complicated.  Calibration and knowing your correction factor are the keys.  Once you've got those two things down, you're golden.  Makes more sense to do for small batch brewers too.  Love my refractometer.  I actually find it a little faster and easier to use, too.

[denny]

After reading the post above, it's confirmed my decision to stick with a hydrometer.

It's really not complicated.  Calibration and knowing your correction factor are the keys.  Once you've got those two things down, you're golden.  Makes more sense to do for small batch brewers too.

And I have to do none of that with a hydrometer.  I can get a hydrometer reading about as quickly as a refractometer reading, especially including all the calibration.  I don't see any reason to not use a hydrometer.

[dmtaylor]

After reading the post above, it's confirmed my decision to stick with a hydrometer.

It's really not complicated.  Calibration and knowing your correction factor are the keys.  Once you've got those two things down, you're golden.  Makes more sense to do for small batch brewers too.

And I have to do none of that with a hydrometer.  I can get a hydrometer reading about as quickly as a refractometer reading, especially including all the calibration.  I don't see any reason to not use a hydrometer.

It's just another thing to nerd out about.  Like IBUs and SRMs and LODO and pH and water and........ we are all friggin nerds in this hobby to some extent -- ain't we!?!?

[Robert]

After reading the post above, it's confirmed my decision to stick with a hydrometer.

Me too, not even absolutely sure where my refractometer is!  But this thread leads me to a related question.  I've noticed some people use a refractometer to determine when their mash has gone to completion. I rely on an iodine test.  What am I missing?

[Big Monk]

After reading the post above, it's confirmed my decision to stick with a hydrometer.

Me too, not even absolutely sure where my refractometer is!  But this thread leads me to a related question.  I've noticed some people use a refractometer to determine when their mash has gone to completion. I rely on an iodine test.  What am I missing?

You check gravity after mash rests to see how close you are getting to the target gravity. For instance, I use no-sparge and step mash, so I use Kai’s maximum first wort gravity prediction and refractometer readings to determine my conversion efficiency.

[Robert]

After reading the post above, it's confirmed my decision to stick with a hydrometer.

Me too, not even absolutely sure where my refractometer is!  But this thread leads me to a related question.  I've noticed some people use a refractometer to determine when their mash has gone to completion. I rely on an iodine test.  What am I missing?

You check gravity after mash rests to see how close you are getting to the target gravity. For instance, I use no-sparge and step mash, so I use Kai’s maximum first wort gravity prediction and refractometer readings to determine my conversion efficiency.

So since I sparge, no real advantage to my using a refractometer,  right?

[dmtaylor]

this thread leads me to a related question.  I've noticed some people use a refractometer to determine when their mash has gone to completion. I rely on an iodine test.  What am I missing?

Iodine test will tell you when much, but not all, of the starches are converted to sugars.  But results will vary and are not very reliable.  On the other hand, measuring gravity or Brix over time will tell you if conversion is still going on.  If more sugars are being produced, readings will continue to rise.  Only when readings stay stable over the course of ~10 minutes is conversion really "done".  And even then, it might only be telling you about conversion of starches into sugars, but not whether those sugars are very fermentable!  So even then, you might for certain styles want to mash even longer!

Ultimately, at a certain point, you need to make a decision and just stop mashing when you think it's been "good enough".  For me, I mash for only 45 minutes usually, but am starting to mash longer for some styles.  I don't even measure pre-boil.  I get good efficiency pretty consistently so I just keep on rocking.  The whole thing is all optional, and decisions can be based on personal experience where appropriate, and we might not need to measure every dang thing like many of us tend to do.  Even FG....... does it really matter?  If the final beer tastes good, who cares about the FG!!  Or the ABV!!  Does it really matter?  It's a decision we each need to make on our own.

[Big Monk]

After reading the post above, it's confirmed my decision to stick with a hydrometer.

Me too, not even absolutely sure where my refractometer is!  But this thread leads me to a related question.  I've noticed some people use a refractometer to determine when their mash has gone to completion. I rely on an iodine test.  What am I missing?

You check gravity after mash rests to see how close you are getting to the target gravity. For instance, I use no-sparge and step mash, so I use Kai’s maximum first wort gravity prediction and refractometer readings to determine my conversion efficiency.

So since I sparge, no real advantage to my using a refractometer,  right?

Wrong. You can still calculate the maximum first wort gravity using Kai’s formulas and determine the efficiency of conversion. It just works out good when you no-sparge because the first wort gravity IS your preboil gravity.

It has nothing to do with a refractometer. I just happen to use a refractometer instead of a hydrometer.

[Robert]

I mash 60, just because, iodine test to make sure everything's going as usual, mash off 10-20 and sparge.  I get good yield (>33ppg in homebrew terms) and good attenuation (>78% ADF).  I'll stick with that.

[Big Monk]

I mash 60, just because, iodine test to make sure everything's going as usual, mash off 10-20 and sparge.  I get good yield (>33ppg in homebrew terms) and good attenuation (>78% ADF).  I'll stick with that.

Whatever floats your boat. We are into knowing all the particulars of a batch so that we can improve and strengthen our spreadsheet so we get more in depth than most. Nothing wrong with doing what works.

[denny]

I mash 60, just because, iodine test to make sure everything's going as usual, mash off 10-20 and sparge.  I get good yield (>33ppg in homebrew terms) and good attenuation (>78% ADF).  I'll stick with that.

I've found the iodine test unreliable, so I use conversion efficiency to make sure the mash is done.

[klickitat jim]

1. Calibrate with H2O every use. Already do that, check

2. Compare hydrometer to refractometer several times with unfermented wort to confirm MY correction factor.

3. Use the calculator with MY correction factor, or be crazy and skip step 2 and just use 1.04 lol

Thanks! I will do step 2 for a while and learn my correction factor. How many batches do you think? And could I do this by creating several various samples of sugar water?

Meanwhile I will use the calculator with 1.04 but compare my results to hydrometer FG and calculated ABV.

PS, sorry I started a fight

[Big Monk]

1. Calibrate with H2O every use. Already do that, check

2. Compare hydrometer to refractometer several times with unfermented wort to confirm MY correction factor.

3. Use the calculator with MY correction factor, or be crazy and skip step 2 and just use 1.04 lol

Thanks! I will do step 2 for a while and learn my correction factor. How many batches do you think? And could I do this by creating several various samples of sugar water?

Meanwhile I will use the calculator with 1.04 but compare my results to hydrometer FG and calculated ABV.

PS, sorry I started a fight

I would calculate the correction factor for your equipment and run with it. Especially for just checking pre-fermentation wort.

You may want to be a little more rigorous for measuring fermenting wort but even then, I’ve never found it necessary to recalculate the correction factor unless you see something amiss in your numbers.

I firmly believe in Dave’s assertion before that the correction factor specific to you is key in getting reliable numbers out of your unit.