Understanding phantom particle counts

18 May, 2021

Imagine this: You have been filtering 150 litres of hydraulic fluid off-line with staged 5 and 1 micron filter elements at a rate of 25 litres per minute (1,500 litres per hour). You have been using a laser light-blockage method (LBM), automatic particle counter to verify the fluid cleanliness. The starting cleanliness per ISO 4406:2017 was 25/22/15. After eight hours, the cleanliness has only dropped to 22/20/13. You’ve checked that the filter element is not clogged and not in bypass. You even went through the trouble of making sure there was an element actually in the filter. How is this possible? What you are likely experiencing is a classic case of ‘phantom’ particle counting.

Demystifying 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 micellelike 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.

Limitations of Laser LBM particle counts

LBM particle counters are useful in many applications and the principle of the method is fundamentally simple. Fluid flows through a measurement cell containing a light source on one side and a receiver (photodetector) on the opposite side. The light source becomes partially blocked as particles pass through the measurement cell, creating a light radiation differential (a shadow) – corresponding to the circular cross-sectional size of the passing particles. Because of this fundamental operating principle, LBM particle counters suffer from a key drawback: the inability to discern solid contaminates from other contaminates like water, air bubbles, phantom particles, and so on. So, how can the phantom particles be overcome using automatic particle-counting technology? There is an alternative solution.

Digital imaging particle counting

Digital imaging particle counting uses size and shape-recognition technology, based on advanced algorithms to distinguish contaminants. For example, a direct imaging particle counter can distinguish contaminants into fatigue, cutting and sliding wear, as well as fibre and air bubble categories based on shape characteristics. It can also recognise water droplets (though water droplets are typically not recorded). The distinction of contaminants allows for more accurate particle counting, by negating nonsolid particles like entrained air bubbles and water droplets.