A Hyrdrometer vs. Refractometer for Beer Measurement
The hydrometer is based on Archimedes principle that a solid suspended in a liquid will be buoyed up by a force equal to the weight of the liquid that it displaces. Therefore, the lower the specific gravity of the substance, the lower the hydrometer will sink.
Hydrometers are usually constructed of glass or plastic. They often have a cylindrical stem with a measurement scale sealed inside and a carefully weighted bulb to make it float upright. In use, the hydrometer is gently lowered into the liquid until it floats freely. The reading is taken at the point where the surface of the liquid (the meniscus) crosses the scale on the stem of the hydrometer.
Although the hydrometer is affected by the density of the fluid, it may have a scale inside with any particular unit of measure that is related to density. One important point to note is that whatever the unit of measure, the hydrometer scale is only valid at one particular temperature. Hence, they must be used in conjunction with a thermometer, and any variation from the reference temperature must be noted and compensated for.
Also, due to the nature of the hydrometer, a relatively large sample size must be used so that the hydrometer can float freely. Since the hydrometer is made of glass, great care must be taken to keep it from breaking. Obviously, the hydrometer is not designed to be used as a field instrument.
A refractometer is an optical instrument designed to measure the concentration or mixture ratio of water soluble fluids. It measures refractive index, the speed at which light passes through a liquid. The denser the liquid the slower the light will travel through it, and the higher its reading will be on the refractometer. Like hydrometers, many different scales are available that convert refractive index into a unit of measure that is more meaningful, i.e., Brix, specific gravity, °Plato, etc.
Refractometers designed for field use are usually rugged and portable. An advantage over the hydrometer is the relatively small sample size that is required for testing. This also saves time required for cleaning and sanitizing the hydrometer equipment.
Although refractometers are also dependent on temperature, certain refractometers are made with automatic temperature compensation; in fact, you shouldn’t rely on results from a refractometer without temperature compensation.
Traditional Analog Refractometers
The traditional analog refractometer is comprised of lenses and prisms that focus a shadowline on a tiny glass reticle with a scale etched on it. You point one end at a light source and look through the other end. The scale inside is likely to be Brix (a measure of sucrose), wort specific gravity, or both. These units, likely made in China, can be found for as little as $35 to $60.00 and are aimed at the home hobbyist.
The Traditional Analog Refractometer can be further split into two categories, those with and without automatic temperature compensation. However, since refractive index (the underlying method of measurement) is very temperature dependent, refractometers without temperature compensation should not be considered. However, analog refractometers with temperature compensation are compensated based on sucrose, which is problematic since there is so little sucrose in wort.
Digital Handheld Refractometers
The second type of refractometer is the digital handheld refractometer. Digital instruments read the refractive index of a fluid and use an internal scale to convert refractive index into a useful unit of measure, such as Brix, Plato, or specific gravity, and display that value on an LCD display. Like analog refractometers, there are many different brands of digital refractometers whose quality and accuracy can vary widely.