Abstract
The U.S. Military specifies the use of Mil-PRF-2104 engine oil in the hydraulic system of certain nontactical military vehicles. Skid-steer loaders and other heavy equipment also use engine oils in their hydraulic systems. These vehicles are required to meet roll-off cleanliness specifications in order to improve hydraulic equipment reliability. Automatic particle counters are used to verify the cleanliness of these systems. Occasionally, particle counters detect phantom particles that cannot be removed by filtration. This paper examines the possible role of base oil and additive selection in the appearance of phantom counts. Filtered Group I and Group III base oils were doped with the components of an engine oil formulation. Particle levels were monitored before and after filtration using an on-line automatic particle counter. The results show that base oil selection has minimal bearing upon appearance of phantom counts while additive selection is a significant factor. Results from three different particle counters are compared. Two laser particle counters that operate by the light-blockage principle were found to produce phantom counts from polydimethylsiloxane antifoam additives. A direct-imaging laser particle counter classified antifoam particles as water droplets and was less susceptible to phantom particle interferences from silicone antifoam additives.
