Sometimes referred to as the "mineral oil color value", the ASTM Colour Scale is widely used for the grading of petroleum products such as lubricating oils, heating oils and diesel fuel oils.
Mineral oils are constantly checked for colour during processing in order to establish when they have been refined to the required grade. Colour is also used as a means of confirming that the correct oil or fuel is being used for its intended use and that no contamination or degradation of quality has occurred.
ASTM D1500 is a single number, one dimensional, colour scale ranging from a pale straw through to a deep red in sixteen steps (0.5 – 8.0 units in increments of 0.5 units). Visual Comparators can achieve a resolution of 0.5 units, automatic Lovibond PFX and PFXi instruments achieve a resolution of 0.1, ASTM D1500 superseded the 12-step D155 NPA (National Petroleum Association) scale in 1960.
Other petroleum products that do not fall within the scope of ASTM D1500 such as undyed gasoline, white spirit, petroleum wax and kerosene may be graded using the Saybolt test ASTM D156 or IP (Institute of Petroleum) 17.
ASTM D1500 Colour discs 4/81 and 4/82 are also available for checking the colour of F16 diesel fuel before on loading. The discs are particularly important where fuel is taken onboard in isolated locations where there no laboratory facilities. The discs are used with the Lovibond 2000+ Comparator and a W680/OG/33mm cell and cover the ranges 1-5 (4/81) & 4/82 (0.5-4.5).
ASTM D 6045 for colour testing of petroleum products by the automatic tristimulus method covers electronic methods of determining Saybolt Colour and ASTM Colour. ASTM D 156 for Saybolt Colour and ASTM D 1500 for ASTM Colour both refer only to visual colour grading instruments (the Saybolt Chromometer and the Lovibond Petroleum Oils Comparator respectively).
Thus ASTM D 6045 is the electronic equivalent of the ASTM D 1500 method and the ASTM D 156 method. Additional performance criteria quoted in the ASTM specification such as accuracy, linearity and reproducibility are all met by the Lovibond PFX and PFXi ranges of automatic colorimeters.
Why we need to know?
- A colorimeter is a device used in colorimetry. In scientific fields the word generally refers to the device that measures the absorbance of particular wavelengths of light by a specific solution. This device is most commonly used to determine the concentration of a known solute in a given solution by the application of the Beer-Lambert law, which states that the concentration of a solute is proportional to the absorbance.
- Determination of the color of petroleum products is used mainly for manufacturing control purposes and is an important quality characteristic, since color is readily observed by the user of the product.
- Darkening Color – for various reasons, as the oil oxidises, its color tends to darken.
- While dark oil is not always oxidized and degraded, it is one possible indication.
- The color may serve as an indication of the degree of refinement of the material.
- When the color range of a particular product is known, a variation outside the established range may indicate possible contamination with another product.
- However, color is not always a reliable guide to product quality and should not be used indiscriminately in product specifications