Published: Jan 1957
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In order to evaluate direct-reading spectrographic analysis for used railroad oils, a fuller understanding of the two types of spectrochemical analysis is helpful. The one method utilizes a photographic emulsion as the light integrating and measuring device. The light from the ignited oil sample is directed to a photographic plate which, upon development, and evaluated in terms of density, is related to the concentrations of the various elements. This device has now reached a high degree of perfection for quantitative analysis. Although its application will continue to expand, the sphere of activity is rather well defined and limited by the inherent properties of photographic emulsion. Because of these limitations, we found it necessary to consider other means of intensity measurement to realize the full capabilities of spectrochemical methods employing emission spectra. The latest direct-reading instruments, such as the Production Control Quantometer, employs multiplier phototubes instead of the photographic emulsion. Obviously, this high degree of precision readily lends itself to special techniques where extremely small amounts of elements are burned in the arc, as with the Rotrode method on used lubricating oil samples. In the multiplier phototube method, the diffracted light is picked up by the phototubes which change the light into current and amplify it up to a million times. This means that measurement of extremely small concentrations of an element is possible in a given lubricating oil sample. On the Production Control Quantometer, the current from the multiplier phototubes is fed to condensers, the charge of which is amplified by an electrometer amplifier, then fed to the Leeds & Northrup recorder where the apparent concentrations of the different elements in the sample are consecutively recorded in sequence. The apparent values on the recording chart must then be corrected to actual concentrations by means of a calculated chart.
Barth, V. C.
Chief Metallurgist and Engineer of Tests, Chicago and North Western Railway System, Chicago, Ill.