Accuracy of Proportion Measurement Instruments
Two manufacturers that specialize in proportion measuring devices are Sarin Technologies and Ogi Corporation.
The base models of both have a limited accuracy. More accurate models have different sized ‘windows’ that make it possible to use a larger portion of the field of view to measure different size stones. The most advanced models have a multiple lens capacity within the same unit, so the most appropriate lens can be used for an individual diamond.Measuring a 3mm stone with a lens designed to measure 20mm diamonds will reduce the accuracy by a factor of three compared to using a lens designed for a 7mm maximum stone diameter.
Another problem is the fact that Sarin proportions often do not add up to an accurate total depth as calculated by dividing depth by diameter. There are several reasons for this, however the solution applied by Ogi is to adjust the other parameters to equal the known weight. This is akin to a surveyor adjusting rise and fall measurements to ‘close the loop’. This approach adds an error to at least one or more of the measured proportions, rather than accepting the best assessments of the instrument. While it is difficult to explain this error, it is one of the likely reasons why all the major laboratories choose Sarin over Ogi.
Used properly a well-calibrated Sarin measures to an accuracy of 20 microns and gives results to within 0.2 degree. This level of inaccuracy will provide significantly different results using the Cut Adviser at Pricescope, but this is a constraint faced by all cut grading systems. The most positive comment that can be made is that the most accurate facet measurement is the pavilion facet angle. This is because it is the longest measurement made and the methods employed measure inclination more accurately. The same method is used for short facets, the inclination is measured and interceptions are plotted, so the shorter the distance between two points on the facet edge being analyzed, the greater the margin for error. The pavilion angle has been is the single most critical proportion on a diamond. It is four or five times more critical than crown angle or any other proportion in its effect on diamond beauty.
