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The Lovibond® Petroleum Oils Comparator is a 3-field instrument for visually determining the ASTM Colour of samples by direct comparison with coloured glass standards. It incorporates the 16 glass standards which make up the scale in a pair of discs:

Disc Colour Standards

1 0.5,1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5

2 1.0,2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0

With a 3-section field of view, the sample and two adjacent standards on the ASTM Color Scale are viewed simultaneously, making it easier to achieve the optimum colour match. For rapid colour grading within predetermined colour limits, the glass standards can be set to the two limiting colours, making it easy to see if the sample is within tolerance. The tungsten halogen light source is colour corrected to CIE standard illuminant C, which guarantees constant lighting conditions for colour grading, day or night and irrespective of ambient lighting.

Three cylindrical glass sample jars with an internal diameter of 33mm are housed in the 3-section sample container; the central jar is half filled with the sample and the outer ones are filled with distilled water. The sample is viewed through a prism which brings the sample and the standards into adjoining fields of view. The two discs containing the colour standards are rotated by turing the control knobs on the front of the comparator until the colour of the oil sample falls between two standards which are 0.5 apart, or until it exactly matches one of the standards. The reading given directly as ASTM Color is then taken from the scale on the control knobs. Samples which are not clear, such as petroleum waxes, can be heated to above the cloud point; samples that are darker than 8.0 colour can be diluted with solvent kerosine.