Understanding Phantom Particle Counts 

Phantom particle counting is a documented problem, encountered when using laser-based LBM particle counters to verify the cleanliness of mineral based fluids containing certain insoluble additives. Silicone-based antifoam agents such as polydimethylsiloxane are common additives causing problems. Fluids used in mobile fluid power systems are typically formulated to contain high antifoam concentrations. Research has suggested that through differential surface tension, the silicone-based antifoam additive agents adhere to the inner wall of microscopic, entrained air bubbles, resulting in micelle-like encapsulations of the additive agents that then take on the apparent shape characteristics of a water droplet. The micelle-like antifoam agent encapsulations typically range in 4-10µm in size. Once the encapsulations rise to the fluid surface, the agents quickly pierce the air bubble and return back into the fluid. In a related study on the effects of a variety of nonsolid contaminants and additives on LBM particle-counting accuracy, it was found that a base stock mixture containing 0.02% (by weight) of a silicone antifoam agent increased the particle count by a minimum factor of three. The erroneous particle accounts were particularly evident in the 4µm and 6µm channels of the ISO 4406 cleanliness standard.