Using Your Refractometer
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Another instrument that is popular with many homebrewers is the refractometer. It instantly reads gravity, in Brix, of unfermented wort or fruit juice by measuring the degree that light passing through the sample is bent. Unlike a hydrometer, only a few drops are required for a sample. To use, apply 2-3 drops to the prism face, close cover, and look through the eyepiece while aiming the instrument at a light source. Most refractometers measure from 0-32° Brix, with an accuracy of +/- 0.2° Brix. Some also include automatic temperature compensation for temperatures between 50-86° F, which eliminates the need to consult temperature correction charts.
You must consult a chart to convert the reading in °Brix to Specific Gravity.
Click Here For SG to Brix Conversion Table
Most refractometers are scaled in terms of °Brix because they are most often used in industries that use the Brix scale. Use caution with your refractometer because light refraction is temperature dependent just like specific gravity. Your sample temperature should ideally be around 68° F.
The great thing about a refractometer is that the sample size is small (about 1 mL) and very easy to cool to room temperature before measuring. You just have to wait ten to fifteen seconds prior to taking the reading for the sample's temperature to equalize with the environment. A couple of drawbacks to the refractometer are that they are difficult to read unless they are really high-end, and they are not reliable for checking samples from the fermenter because alcohol interferes with the measurement differently than it does with a hydrometer.
Many brewers will use the hydrometer to express specific gravity from the fermenter. If you do want to use he refractometer, use it only on wort and you will be alright. °Brix can be approximately converted to specific gravity (SG) by a simple equation:
SG = 1 +(0.004 x °Brix)
Most people just remember the multiply by four rule. Take the Brix reading, multiply by 4 and this will give you specific gravity in “gravity points.” For example, if your reading is 11 °Brix, multiplying that by 4 yields 44, which corresponds to a specific gravity of 1.044.
Most brewing software has a refractometer °Brix to specific gravity conversion algorithm you can use real time. If you are using a model with ATC, you can simply use the reading you have. If you do not have ATC, then you must use a chart (usually included with the refractometer) to get the compensated reading. You simply use the ambient temperature and the reading to obtain a value that must be added or subtracted from the reading to make the compensation.
Do not use any temperature corrections when calibrating your refractometer. Instead, ensure that the calibration is made when the meter is at the correct temperature, 68 °F (20 °C) for most meters.
Measurements of the specific gravity of wort using a refractometer will not agree with the measurements of gravity using a hydrometer. Brix refractometers are meant to measure the percentage of sugar in a pure sucrose solution. Since wort is not simply sugar and water, you need to make a small correction because of the non-sugar components of the wort.
There are ways to calculate this factor, but most brewing software have a refractometer tool that will calculate and then apply this correction factor. You must obtain a clean sample of unfermented wort and take both a refractometer reading and an accurate hydrometer SG reading. You then enter these values into the software's refractometer tool and the result is applied to future refractometer wort readings.
So how does the refractometer work?
The speed of light is always the same in a vacuum. But when light travels through a liquid it changes direction and travels more slowly because it is being absorbed and reemitted by the atoms in solution.
Using the constancy of the speed of light in a vacuum, the ratio to the speed of light in any other liquid is called the liquid's Index of Refraction or refractive index or n. It actually gets a little more complicated when the light's angle is taken into account but that's for the engineers out there to work out.
The refractive index for most compounds is between 1.3000 and 1.7000 (out to 5 digits) and has no units. But to be totally accurate, since the index of refraction depends on the temperature of the sample and the wavelength of light used, these units are usually indicated when reporting the refractive index of a substance.
A liquid's refractive index is commonly used to identify certain characteristics of a liquid, much like the melting point of a solid is used to identify certain characteristics in solid compounds. A liquid's refractive index may also be used to identify a sample by comparing its index to known values, assessing the purity of a sample by comparing the refractive index to that of a pure sample, and to determine the concentration of a solute (like sugar or sucrose) in solution by comparing the liquid's refractive index to a standard curve.
For most of us homebrewers, the refractometers we will use utilize a a °Brix scale that is calibrated to give us the percentage (weight/weight) of sucrose dissolved in water at a reference temperature. To adjust for temperatures other than that reference temperature, utilize the following formula:
nD20 = nDT + 0.00045(T - 20°C).
n=the refractive index
This formula uses the yellow light of a sodium source which has the wavelength of 589.3 nanometers as the base-line source and is indicated by the subscript "D". Most refractive indexes you see will be measured with this light source.
"T" is the observed temperature of the sample in °C.
Most refractive indices are reported at 20°C (68°F) or 25°C (77°F).
As an example, If a substance has an observed refractive index of 1.2821 measured at 18°C (64.4°F), the corrected and reported value for this substance should be: nD18=1.2821+(0.00045(18-20))=1.2821-0.0009=1.2812. As you can see, the Refractive Index (RI) of a substance is different at different temperatures but the change is usually very small. In fact, water changes 0.01% per degree Celcius.
Apart from the refractive index scale, the °Brix scale is the most widely used scale on a refractometer. °Brix, or %sugar, is an internationally recognized standard in the food and beverage industry. It relates the concentration of sucrose in water at 20°C to the Refractive Index of the solution. There are many advantages of using a refractometer while brewing. It allows you to take instant gravity readings at any point during the boil or during the run-off of the sparge. Taking these readings during the brewing process allows you to add water or boil longer if needed to hit your target gravity. The amount of sample is minimal reducing waste and the readings are instant, no need to cool down a sample before measuring.
Information for this article was adapted in part from Bulletin No: Bellingham + Stanley Ltd/Technical Bulletin No: R001entitled Principles of Refractometry.
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