I did some research and found these articles on the subject:
http://valleyvintner.com/Refrac_Hydro/Refract_Hydro.htm
<H2>How To Use A Refractometer in Winemaking</H2>
It's important for winemakers to know how to use a refractometer, because it is used to measure the amount of sugar (actually, the percentage Brix) in the juice of grapes or other fresh fruit. Winemakers know there is a direct correlation between the amount of sugar present and the ability to make wine. This portable instrument (it'll fit in your pocket) allows the winemaker to assess the ripeness of fruit by measuring Brix in the field or vineyard so he or she can decide the proper harvest time depending upon the readings taken.
This page explains what a refractometer does, how it works, how to calibrate and use it, tips for buying the best model, and how to take care of it.
What Is A Refractometer?
A refractometer is a relatively inexpensive yet essential piece of test equipment used by vineyard managers and winemakers. The rugged exterior of metal, rubber and plastic protects the highly polished optical glass, mirrors and prisms that are contained within. Once the sample is in place underneath the daylight plate, the winemaker can see the percentage Brix reading by looking through the monocular / eyepiece and reading the scale that is seen when he or she holds the refractometer in natural light.
What Does A Refractometer Do, and How Does It Work?
As previously stated, a refractometer allows the winemaker to figure the percentage Brix (the relative "sugar weight" of a sample compared to distilled water) of the juice of grapes or other fresh fruit. Brix is sometimes referred to as Balling - don't worry, the terms are interchangeable. Depending upon the readings observed, a winemaker can monitor the progress of ripening and adjust his/her plans for harvest, if necessary.
In simplest terms, the refractometer works much like a prism. Remember how, as a child, you could use a prism to separate out the different wavelengths of light (red, orange, yellow, green, blue, indigo, violet) when a source of light was shone on the prism at the correct angle? Well, the modern refractometer works on the same principle - it reacts differently to light (by giving a reading on a scale) depending upon the amount of sugar that is available in the liquid sample held between the daylight plate and the main prism assembly.
How to Calibrate and Use Your Refractometer
Before you start taking readings, it's very important to calibrate the refractometer. Some refractometers require the use of a special calibration liquid to perform this task, while others (like the ones sold at grapestompers.com) are calibrated with distilled water.
Let's get to it!
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Begin the calibration of your refractometer by lifting up the daylight plate and placing 2-3 drops of distilled water on top of the prism assembly. Close the daylight plate so the water spreads across the entire surface of the prism without any air bubbles or dry spots.</TD></TR>
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Allow the test sample to sit on the prism for approximately 30 seconds before you attempt calibration in the next step. This allows the sample to adjust to the ambient temperature of the refractometer.</TD></TR>
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Hold the refractometer in the direction of a natural light source and look into the eyepiece. You will see a circular field with graduations down the center. You may have to focus the eyepiece to clearly see the graduations. Figure 1 (below) shows what you would see if you looked through the refractometer without any sample present.</TD>
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Turn the calibration screw (see photo at left) until the boundary between the upper blue field and the lower white field meet exactly at ZERO on the scale.
See example (Figure 2, shown below) of the interior view you'll see when you look through the eyepiece of the refractometer.</TD></TR>
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Once the refractometer has been properly calibrated, you are ready to take readings of grape juice or whatever else you want to sample. Put away the calibration screwdriver. Clean the instrument (both the daylight plate and the top of the main prism assembly) using a soft, damp cloth, then place 2-3 drops of the desired sample on top of the prism. Close the daylight plate and take your reading as before.
Figure 3 (see below) illustrates what you might see at this point.</TD></TR>
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FIGURE 1
The image to the left illustrates what the winemaker would see if he looked through the refractometer without any sample at all.
Notice how the entire scale is colored blue; no white at all.
When looking through the monocular, be sure you are using natural light to view the readings; you should not read a refractometer in the presence of fluorescent light.</TD></TR>
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FIGURE 2
This is what the winemaker sees once he has properly calibrated the refractometer.
Notice that the reading is taken where the blue and the white meet. Calibrate to ZERO using distilled water as the sample.
If your refractometer does not automatically compensate for the temperature of the sample, you must take this into account or your readings will be off.</TD></TR>
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FIGURE 3
Finally, we get to sample some real grapes! Don't fall into the trap of sampling only one or two grapes - select a group of grapes at random from across your vineyard and combine their juice to get a good cross section sample of your crop.
As you can see, this sample is reading 23% Brix. Looks like it's time to make wine!
Be sure to cleanse and dry the refractometer before putting it away in storage.</TD></TR></T></TABLE>
Warnings and Maintenance of Your Refractometer
Accurate measurement depends on careful calibration. Follow the instructions above closely.
A reminder: Differences between the ambient room temperature of the prism and the temperature of the sample will throw off the accuracy of your reading. Remember to allow the sample to rest on the prism assembly for 30 seconds before taking a reading.
Do not expose the refractometer to damp working conditions. Do not immerse the instrument in water. If the instrument becomes foggy, water has entered the body. Call a qualified service technician or contact your dealer to purchase a new refractometer.
Do not measure abrasive or corrosive chemicals with this instrument, because they can damage the prism's coating.
Clean the instrument between each measurement using a soft, damp cloth. Failure to clean the prism on a regular basis will lead to inaccurate results and damage to the prism's coating.
The refractometer is an optical instrument. It requires careful handling and storage. Failure to do so can result in damage to the optical components and its basic structure. With care, this instrument will provide years of reliable service.
Buying Tips
When you purchase a refractometer, you'll need to know:
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<LI>The range of readings (highest to lowest), to make sure it will suit your purpose. A standard range for home brewers is 0 to 32% Brix. For example, in order to achieve a 13% wine, you'll want to start your wine at a Brix of 23.
<LI>The ease with which the refractometer can be read and understood. Some less expensive refractometers are difficult to read, either due to a lack of a focus adjustment, inferior optics, or the eyepiece lacks a rubber seal and will not fit snugly over your eye.
<LI>The calibration temperature of the refractometer. The most common calibration temp is 20° C or 68° F. If your sample is not exactly 68° F, you will need to make mathematical corrections to compensate for the temperature difference. Luckily, many modern models of refractometers (like the ones stocked by grapestompers) are sold with ATC (automatic temperature compensation), so you never have to worry about the temperature of your sample.
<LI>How easy it is to calibrate. Must you purchase a calibration liquid, or can you calibrate with distilled water? Does it calibrate with a set screw or a dial or knob?
<LI>How easy it is to clean.
<LI>If it comes with a protective case (they're pretty fragile) and instruction manual. </LI>[/list]
Conclusion
There are many reasons why a winemaker might want to use a refractometer:
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<LI>To measure the percentage Brix of grapes or other fresh fruit
<LI>To determine progress of crop ripening
<LI>To measure progress of fermentation
<LI>To measure the amount of sugar present in grapes or other fruit
<LI>To allow the winemaker to determine when fruit is at its peak of ripeness and should be harvested </LI>[/list]
Here's some other refractometer pages we recommend:
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Refraction of Light - the first part of this page explains the principles of refractometry.
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How a refractometer works - the bottom half of this page has a great explanation of how </LI>[/list]
Edited by: masta 
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