How does a refractometer work?

So I'd like one of these to elimite my glass hydrometer and the accompanying shard hazard. I was under the impression that it was all you need for measuring specific gravity. When I went to ebay to have a look at them, there was one that had this caveat:


Beer & Wine: Use the Brix readings to get your wort sugars and help to determine your finished alcohol readings (use in conjunction with a hydrometer). All RHB models use ambient light, no battery or power source is required. Models with the “ATC” suffix are equipped with “Automatic Temperature Compensation” for accurate measurements without recalibration after shifts in ambient working temperature (field use).



So am I looking at the right thing here? Do I still need to keep my hydrometer? Seems counter-productive...

They are great for checking sparge and OG, but useless for FG. They can't deal with a three component solution: water, alcohol and sugar.

You cannot take the reading straight off the refractometer to get FG, you have to adjust for the alcohol thinning the sample. MoreBeer has put together a great little video and accompanying spreadsheet that makes the correction for you:
http://morebeer.com/learn_vids/vids_refract

Here's a nice calculator that takes into account the alcohol in the liquid for final gravities:

http://www.onebeer.net/refractometer.shtml

Well, what I really want it for is to take OG readings. I hung a wine theif full of today's batch from the ceiling hoping that it would cool enough to give me a general idea of pre-boil gravity and I couldn't help thinking how ridiculous that was. Of course, being a cheap bastard like most of my fellow homebrewers, I have to budget my purchases. Not that I can't afford the toys, but it's kind of fun having to 'save up' from my expense accounts to buy my brew stuff. You really have to think about what you buy then.

I know promash has a calculator for using a refractometer to take readings once fermentation has started. I have no idea how accurate it is (I don't have a refractometer) but I would think it's pretty good if they bothered to include it. I would imagine the other programs have a similar tool.

From an accuracy standpoint wouldnt it be better to take your OG and FG with the same instrument to eliminate variances in calibration? Or is the refractometer a just a quick way to test for OG requiring the use of a hydromete still?

Virtuous said:

...is the refractometer a just a quick way to test for OG requiring the use of a hydrometer still?

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For OG, the refractometer alone is fine. For FG, the refractometer has trouble. Refractometers are good for taking gravity readings during the mash and boil, but you're better off with a hydrometer once fermentation has begun.

Virtuous said:

From an accuracy standpoint wouldnt it be better to take your OG and FG with the same instrument to eliminate variances in calibration? Or is the refractometer a just a quick way to test for OG requiring the use of a hydromete still?

Click to expand...

I suppose it's a necessary evil....what's the difference in a few tenths of points of gravity or fractions of a percent of ABV?

For what it's worth, I used my refractometer for both OG and FG readings, using the calculator built into beersmith to make the corrections due to alcohol. However, after a few batches I decided to test that method side by side with a hydrometer for post-fermentation gravities and I can't get them to line up very well at all. I see differences of 5 or more gravity points. I need to do a full-fledged experiment where I more meticulously check gravities before and during fermentation with both methods before coming to any real conclusion, but I have my doubts about it.

I'm hoping it's just that I haven't been careful enough in taking my readings, and not that the sugar+alcohol+water+unfermentables combo renders the refractometer unusable after fermentation begins, even with calculators.

In reality, a refractometer reading will always just be an estimate, even for OG. There is an assumption when using a refractometer: only pure water with dissolved sugar (sucrose, specifically) is present. We all know that there are FAR more compounds in beer than just sugar and water. Also, the color of the liquid will tend to change the reading slightly as well (since you are measuring the way light is affected when it passes through the liquid). The darker the beer, the less accurate the reading.

Because sugars make up the largest portion of the dissolved compounds in unfermented wort, a refractometer reading is relatively reliable in obtaining OG. However, the presence of alcohol(s) and other fermentation by-products makes an accurate SG reading somewhat difficult, even with calculators...which make even more assumptions.

Now, for homebrewing, is it really that important to be so accurate? Perhaps not. I consider a reasonably accurate OG to be important in determining how well I've done with the mash, and I think a refractometer is accurate enough for that. I also consider an unchanged SG to be important when it comes to conditioning/kegging/bottling. An unchanged refractometer reading will work the same as an unchanged hydrometer reading in that case. The available calculators will estimate ABV well within 1%, and that's close enough for me, as ABV is the least important figure in the whole process, IMHO.

Yuri_Rage said:

Now, for homebrewing, is it really that important to be so accurate? Perhaps not. I consider a reasonably accurate OG to be important in determining how well I've done with the mash, and I think a refractometer is accurate enough for that. I also consider and unchanged SG to be important when it comes to conditioning/kegging/bottling. An unchanged refractometer reading will work the same as an unchanged hydrometer reading in that case. The available calculators will estimate ABV well within 1%, and that's close enough for me, as ABV is the least important figure in the whole process, IMHO.

Click to expand...

I think what I will probably be doing in the future is using the refractometer for OG and comparing its day-to-day values during fermentation to see when fermentation stops (because, as you say, although its absolute value may not be that useful, its relative value should be), but then pulling a bigger sample while racking to secondary/keg to get a more accurate absolute reading using a hydrometer.

They are grate for OG because you don't need to cool a volume of wort but I don't see the problem with sticking to the hydrometer for the FG, it's already cool.