Refractive index

OELCHECK checks not only our gas engine oils, but also coolants from these engines. The lab reports for coolants now also include the refractive index at 20 °C. How is this value determined? What does it mean if this has changed? And is a refractive index relevant for oils as well?


Table of contents

  1. OELCHECK answers: 
  2. Refraction of light
  3. Determining refractive indices with a hand refractometer
  4. Exact values can only be determined in the lab
  5. Refractive indices of coolants and oils

Refraction of light

The refractive index has its origins in optics. When light passes from one medium into another, the rays of light are refracted at the junction, changing their direction and their speed. Figuratively speaking, the ray of light bends. The strength of the effect depends on the refractive index of the medium.

Determining refractive indices with a hand refractometer

The easiest way to determine refractive indices is by using a hand refractometer. This device measures the behaviour of light at the junction between a prism and the material being tested. A typical application is measuring the sugar content of grape must, which is a criterion in determining the quality of wine. Particularly in the case of foodstuffs, the refractive index is still traditionally given in °Brix, or in the case of wine in °Oechsle. In metalworking, hand refractometers are an essential tool for monitoring the concentration of oil-in-water emulsions. Refractometers can also be very helpful when a mixture of incompatible oils needs to be checked.

Exact values can only be determined in the lab

Due to the different wavelengths of light, using a hand refractometer is not the most accurate way of determining refractive indices. The value varies slightly at these different wavelengths, and is also affected by the light source and temperature. Even the human eye perceives colours slightly differently depending on differences in wavelength. In order to exclude deviations and to make values truly comparable, a specific wavelength that can only be produced under laboratory conditions has been defined for the determination of refractive indices. Sodium D line light is produced when table salt is burned in a gas flame, It consists of the wavelengths 588.9951 nm and 589.5924 nm. In our lab reports for coolant fluids, we state the refractive index determined at this wavelength at a media temperature of 20 °C. The higher the refractive index of a medium, the more strongly the light refracts. By way of illustration, a vacuum has a refractive index of 1.0, the refractive index of air (1013 mbar) is 1.000272, and drinking water has a refractive index of 1.333. Ethylene glycol, which is used to prepare coolants, has an index of 1.43. When a refractive index is needed to indicate a relatively exact quantitative share of glycol in water, it is therefore better to have this determined in a laboratory.

Refractive indices of coolants and oils

OELCHECK measures refractive indices using the RX40 Refractive Index Cell Module from Mettler Toledo. This produces the wavelengths of light required with high accuracy, and determines refractive indices within a value range of 1.3200 to 1.7000. The refractive index can then be used to determine the mixing ratio of two components.

In the case of coolants, the concentration and quantity of ethylene glycol or propylene glycol mixed into the water are determined. This is important information, because the glycol content affects the thermal characteristics of the coolant. Concentrations of inhibitors, which give the water additional protection against corrosion, can also be determined in this way. As it is difficult to identify exact proportions of ethylene and propylene glycol using the refractive index alone, OELCHECK also determines the density of the coolant mixture. Comparison of these values then makes it possible to make a clear assessment.

In the case of base oils of varying origins and compositions, refractive indices enable us to get to the bottom of what mixtures contain. In order to do this, we generally measure not only refractive index, but also density, viscosity index, and IR spectrum. The values for hydrocarbons (mineral or synthetic), esters, and glycols vary less than they do in the case of water-glycol mixtures. For silicone or perfluorinated polyether (PFPR) based oils, on the other hand, there are clearly distinctive values. Particularly with mixtures involving silicones and PFPEs, which are not compatible with other lubricants, the consequences can be very severe. Silicone oils, such as those used in some non-water miscible metalworking fluids or as insulating oils in transformers, must remain entirely free from other types of oil. PFPE-based lubricants, which have an exceptional performance capacity at high temperatures and are resistant to chemicals, must likewise not be contaminated with other lubricants. Determining their refractive indices in the laboratory allows mixtures that are hazardous with regard to these products to be easily identified.


OELCHECKER Winter 2018, page 8