Hydrometers and Refractometers Use, Care, and Calibration Verification William Menzl January 2009
Web References Ever Ready Thermometer Company - http://www.ertco.com/how_to_read_a_hydrometer.html Understanding Specific Gravity and Extract http://www.brewingtechniques.com/library/backissues/iss ue1.3/manning.html Alternative Hydrometer Callibration Verification - http://www.honeycreek.us/hydrocal.php Refractometer - http://www.grapestompers.com/articles/refractometer_us e.asp
Hydrometer Types Plain Form Hydrometers Hydrometers which do not incorporate their own thermometers, an auxiliary thermometer should be used to measure the temperature of the sample and correct for any temperature variations. Combined Form Hydrometers (Thermohydrometers) Hydrometers that incorporate their own thermometer inside the body or stem to measure the temperature of the sample.
Useful Hydrometer Scales Brix - The Brix scale is used almost exclusively by the sugar industry. Degrees on the scale are equal to the percentage of sucrose by weight at a standard temperature. Specific Gravity - Specific gravity hydrometers measure the ratio of the density of a sample compared to the density of pure water at the same temperature. The specified temperature for these types of hydrometers is 60/60 F (15.6/15.6 C). Plato (Saccharometer) Scale is defined as percentage of Sucrose by weight (12 Plato means 12 grams of Sucrose per 100 grams of solution)
How to read a Hydrometer
How to read a Hydrometer Degas the solution if necessary Greatest accuracy is when the test solution is at the hydrometer calibration temperature (no correction necessary) Spin the hydrometer in the solution to ensure no gas bubbles are on the hydrometer Read at the bottom of the meniscus Verify the hydrometer is floating/not touching the sides Rinse your test jar and hydrometer first with the solution to be tested.
Temperature Correction (Specific Gravity)
Hydrometer Correction Table (Specific Gravity - From the Brewing Calculator)
Conversion Calculator (From the Brewing Calculator)
Calibration Verification Once the hydrometer is made, it can no longer be calibrated, it can only be verified (with possible correction factor applied) Prepare two sugar solutions of known Plato (see later slides) and measure with the hydrometer at the temperature of the hydrometer calibration. Distilled water can be one of the solutions If a difference exists between the standard and the measured value, determine the correction factor. If the correction factor is invalid when measuring the second standard, the scale is non-linear and the hydrometer should be discarded or a correction equation determined.
Calibration Verification Standard Preparation Prepare 10 Plato (1.0400 Sp. Gr. @ 68 F) solution and a 20 Plato (1.0830 Sp. Gr. @ 68 F) solution as follows: Tare a clean and dry 0.5 liter plastic pop bottle on a balance that discriminates at least 0.1 grams and up to at least 500 grams. Add exactly 40.0 ± 0.1 grams of pure sugar into the bottle. Add distilled water to the bottle to 400 ± 0.2 grams total. Cap and shake the bottle until all the sugar is dissolved. Label the bottle 10 Plato. Repeat the procedure using 80.0 ± 0.1 grams of pure sugar up to 400 ± 0.2 grams total. Label the bottle 20 Plato.
Example Verification Data A 10 P solution (1.040 Sp. Gr) solution was made by diluting 31.82 grams of table sugar to 318.2 grams with DI water. RI values at 25 C were 1.3473 and 1.3473 which compares well to a literature value of 1.34783 (but this value is at 20 C) Density meter (measuring Specific Gravity) results at 25 C and calibrated at 25 C were 1.0397 and 1.0399. Hydrometer reading at 77 F (25 C) where the hydrometer was calibrated at 60 F/60 F was a Specific Gravity of 1.046.
Relative usefulness of the Measurement A hydrometer measurement system (which includes everything you do) has a large amount of error making the hydrometer useful, but only to a general sense To statistically determine how good your measurements are, measure a solution repeatedly (at least 5 times but 10 is better) ensuring to dump the solution, clean the test jar and hydrometer, and pour new solution between tests.
Relative usefulness of the Measurement (continued) Determine the differences (ignoring the sign) between each result in chronological order. These are called ranges. Average the ranges and multiply that result by 3.268. Replicate 1 2 3 4 5 Result 1.038 1.041 1.04 1.039 1.041 Range 0.003 0.001 0.001 0.002 Average Range 0.0017 UCLr 0.0057
Relative usefulness of the Measurement (continued) The value calculated (called UCLr in the last spreadsheet) is a good estimate of test-retest expectations. If you test a solution, the next time you test the same solution, your result will be within the UCLr of the first result. This will give you an idea if there really is a statistical difference between 1.040 and 1.043 (THERE ISN T!) and if a hydrometer reading to the nearest 0.0001 is worth the money (Again, it isn t!).
Refractometers Refractometers measure the refraction of light as it passes through a transparent substance (your sample)
Portable Refractometer Types Analog or Digital Automatic Temperature Compensation (ATC) or specific temperature All refractometers have error if not at the temperature of the calibration. ATC refractometers will read correctly within a small range of temperatures (68 F to 86 F is common)
Automatic Temperature Compensation ATC refractometers are temperature compensated by means of a bi-metal strip that expands or contracts depending on the temperature of the instrument. This bimetal strip moves the optics inside the instrument to adjust for temperature variations.
Examples of Refractometers (Digital and Analog)
Refractometer
How to use the Analog Refractometer Verify the instrument is clean, free of scratches, the prism has no occlusions and is not detached. Place a few drops of sample on the prism avoiding making bubbles. Close the daylight plate so the Sample spreads across the entire surface of the prism without any air bubbles or dry spots.
How to use the Analog Refractometer Allow the test sample to sit on the prism for approximately 30 seconds before taking a reading. This allows the sample to adjust to the ambient temperature of the refractometer. 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. Read the scale where the light and dark fields intersect.
Analog Refractometer views No Sample on the Prism Distilled Water on the Prism
Refractometer Calibration Follow the Instrument Manual as calibration method is instrument specific! An easy verification is to measure distilled water, it should be 0% (zero) Brix and then measure the sugar standard prepared for your hydrometer verification earlier. If both solutions read correctly, no calibration is needed and further checks can be done with just DI water.
Refractometer use during fermentation Once fermentation begins, the alcohol (Sp Gr < 1) and the unfermented sugar (Sp Gr > 1) cause the near linear correlation between Brix and Specific Gravity to no longer be valid.
Refractometer use during fermentation To calculate Specific Gravity with a refractometer once fermentation begins, you need to know the original gravity and use the following equation SG = 1.001843-0.002318474(OG) - 0.000007775(OG 2 ) - 0.000000034(OG 3 ) + 0.00574(AG) + 0.00003344(AG 2 ) + 0.000000086(AG 3 ) OG is original gravity in Brix AG is apparent (current) gravity in Brix
Refractometer use during fermentation Or use the Brewing Calculator!