A refractometer with ATC (Auto Thermal Compensation) is unaffected by temperature, fermentation gases and only takes one to two drops to measure the sugar content of your must. However, it is effected by the different refractive index of ethanol. As the amount of alcohol increases the refractive index changes algorithmically. Unlike a Hydrometer the reading in a refractometer will never go to zero.
In theory the "ethanol effect" is predictable and a formula should be able to compensate for it's refractive change.
The formula tracked like rocket science. Over the 10 day fermentation we averaged less than 1% difference and that falls into the realm of user error.
Ok now that the secret is out -- That you can not pick up a Refractometer and expect it to work like you use a Hydrometer because different laws of Physics are at play here. If you don't know, its understandable that you put it away in the back of the drawer and rubbish it publicly because you didn't understand how the tool works. But now you do, so its time to learn how to use your tool and see if it was tool error or operator error all along.
Making a Calibration Test Kit
(If your refractometer passes the calibration with flying colours but your wort measurements are still all over the place, then its a pointer to your technique. Else, it will show you have a dodgy refractometer that needs replacement.)
One of many ways. To be more accurate you can build differing test samples based on accurate weights of both water and sugar alone.
1. Obtain the following materials (Fig. 6).
a. 4 bottles of drinking water containing 500ml (16.9 fl oz) each.
b. A 100-count box of sugar packets with each packet containing 3 grams of sugar. The amount of sugar is specified on the box where nutrition information is listed.
If you cannot get 3 gram packets, make sure you have an accurate scale to measure out 3 grams of sugar to use.
c. Felt tip marker.
d. TC or ATC refractometer.
e. A clean, soft, lint-free cloth.
2. Carefully remove 5 capfuls of water from each bottle to allow room for the addition of sugar.
3. Clearly label each bottle with one of the following designations: 0, 5, 10, 20. Marking the bottle and cap using a permanent marker will maintain the identity of the bottle contents. Count out the correct number of sugar packets and place them in front of the bottle with the corresponding number.
4. Hold the sugar packets up to the light to make sure contents are uniform. Although the box may indicate that each sugar packet contains 3 grams of sugar, it was found that there was a lot of variability in the actual weight of the sugar packets (use an accurate scale to be sure!). Carefully pour the contents of the specified number of sugar packets into each bottle. The bottle marked with 5 will receive 5 sugar packets, etc.
5. Secure the cap and vigorously shake each bottle until the sugar is completely dissolved. At the end of this step, four bottles containing 0, 5, 10, and 20 packets of sugar make your calibration test kit.
--- done ---
CALIBRATING
1. Inspect the refractometer prism for scratches, chips, separations or other aberrations that may interfere with proper readings. If dusty, rinse (do not submerge) with water and wipe with a clean, soft, lint-free cloth.
2. Note the temperature at the time of readings. Most ATC refractometers operate properly in the 68-86F (20-30C) range. A laboratory or office at room temperature should be used if outside temperatures exceed the manufacturer's recommended temperature range.
3. Place a few drops of pure water on the prism surface. If it is an analogue refractometer, close the cover. If bubbles form, gently pressing the cover will remove the bubbles and help disperse the water over the entire surface. For digital refractometers, make sure that bubbles in the well are eliminated prior to making a reading.
4. Hold the refractometer up to natural light or an incandescent bulb to obtain the reading. Looking into the eyepiece, one should see a distinct separation between a blue and white section, often called a contrast line. If the contrast line is not directly at zero, then adjust by turning the screw on the top of the refractometer until it reads zero. Replace the plastic cap after adjusting the calibration screw to prevent water from entering the refractometer. You can adjust the focus by twisting the eyepiece until the scale can be seen clearly. Once the refractometer is calibrated to zero with pure water, dry the surfaces with a clean cloth. Digital refractometers should be calibrated by pressing the zero button with water in the well.
5. Place some of the solution from the 5-packet bottle on the prism and close the cover, making sure that the entire surface is filled and void of bubbles. Note the value and write it in Table 1 under Your Value. For digital refractometers, place the solution in the well and press the start button. Record the value in Table 1. The value will remain on the LED display until the next sample is read.
6. Between samples, clean the refractometer prism surface with pure water and wipe dry.
7. Repeat steps 5 and 6 with bottles marked 10 and 20. Record the values in Table 1.
8. Compare your values with those in Table 1. If your values are outside the expected range, follow the calibration steps a second time. If they are still outside the range, read the following Trouble Shooting section for help.
--- done ---
TROUBLE SHOOTING
1. Check to make sure that the bottled water used was unopened/new.
2. Check to make sure that 500ml water bottles and 3g sugar packets were used.
3. Check to make sure that 5 capfuls of water were removed before adding sugar.
4. Check to make sure that the packets had close to the same amount of sugar in them and that the appropriate number of packets were added to each of the bottles.
5. Check to see if the refractometer is temperature compensating. If it is not, a corrections table is needed to make adjustments. The corrections table is only valid if the temperature was recorded for each reading.
6. Check to make sure that an ATC refractometer was used within its range of 68-86F (20-30C).
7. Check to make sure that the solution is within the temperature range of the ATC refractometer.
8. Replace the batteries in the digital refractometer at the beginning of each season.
Hopefully this is the end on my piece on Refractometers
Cheers,
Brewer Pete
