Machine condition monitoring through oil analysis is a proven and cost-effective maintenance management technique. Periodically, however, the ability and need of the analytical techniques to detect large wear particles are questioned. Today there is general consensus as to the maximum wear particle size capability of atomic emission spectrometers (AES) and atomic absorption spectrometers (AAS), but few agree on the ways to improve them. This paper discusses three techniques that can be used to improve the ability of optical emission spectrometers to detect and quantify large wear particles in lubricating oil.
The single spark technique is based on an atomic emission spectrometer readout system that stores the intensity of individual excitation sparks for each element and presents the data as a particle size distribution. The system can operate in real time and provide information on each analysis, or the output can be suppressed and the user alerted only if wear particles exceed a preset size limit. Test data has shown an ability to detect and identify wear particles up to 40 μm in size.
The ashing rotrode technique is based on a sample introduction and analysis system in which the oil sample is deposited on a graphite electrode, then resistively heated to desolvate the oil matrix. The desolvated matrix is subsequently excited by a pulsed D-C arc, and the spectra is analyzed by a polychromator. This technique minimizes matrix effects, enhances spectral sensitivity, and greatly improves detection of large wear particles.
The acid digestion differential technique is based on two analyses of the same oil sample. The sample is analyzed with and without acid to dissolve wear particles. The difference in the two readings indicates the presence of large wear particles.
All three techniques discussed in this paper improve the ability of spectrometric techniques to detect, quantify, and identify wear particles. Capabilities and limitations of each are discussed. The technique of choice varies with the requirements and philosophy of the analytical laboratory.