Specific gravity is measured by a hydrometer, refractometer, pycnometer or oscillating U-tube electronic meter. In beekeeping, a brix refractometer is used to measure the amount of water in honey. A traditional handheld refractometer is an analog instrument for measuring a liquid's refractive index. A refractometer is the instrument used to measure refractive index. A water sample is also taken back to test its salinity using a refractometer. A refractometer is the instrument used to measure refractive index ("RI"). There is also the Rayleigh Refractometer used (typically) for measuring the refractive indices of gases. In the automobile industry, a refractometer is used to measure the coolant concentration. The refractometer allows for in situ measurements of oil concentration within industrial plants. In drug diagnostics, a refractometer is used to measure the specific gravity of human urine. Water content can easily be measured with a refractometer. Most operate on the same general "critical angle" principle as a traditional handheld refractometer. For this purpose, it is usually measured by using a refractometer. When using a refractometer, one should report the result as "refractometric dried substance" (RDS).