Digital Refractometer for Seawater Analysis - HI96822

Refractive Index

The HI96822 takes measurements based on a sample’s refractive index. Refractive index is a measurement of how light behaves as it passes through the sample. Depending on the sample’s composition, the light will refract and reflect differently. By measuring this activity with a linear image sensor, the sample’s refractive index can be assessed and used to determine its physical properties such as concentration and density. In addition to the linear image sensor, the HI96822 uses an LED light, prism and lens to make measurement possible.

Variations in temperature will affect the accuracy of refractometry readings, so the use of temperature compensation is highly recommended for reliable results. The HI96822 contains a built-in temperature sensor and is programmed specialized algorithms then apply temperature compensation to the measurement and convert the refractive index to PSU (Practical Salinity Units), ppt (part per thousand) or S.G. (Specific Gravity) (20/20). PSU is defined as the conductivity ratio of seawater to a standard KCl solution. It is based upon the work of the UNESCO, ICES, SCOR, and IAPSO. This information is published in The Joint Panel of Oceanographic Tables and Standards. An older salinity scale is ppt (10-3), where salinity is defined by “the salt content is the weight of the inorganic salts contained in 1 kg of seawater if all bromide and iodide are replaced by an equivalent amount of oxides” (Knudsen, 1901). Specific Gravity (20/20) is based upon the published relationship between density at 20 °C and the mass of dissolved salts in the seawater sample (CRC Handbook of Chemistry and Physics, 87th Edition).