In mobile equipment applications, in the absence of kidney-loop systems (off-line or sidestream filtration), oil changes are often the preferred method for dealing with dirt that has entered the system, and/or any metallic wear debris that has been generated since the previous oil drain. This practice prevents the offending dirt or wear particles from causing additional surface damage. In cases where components are suspected to be failing or experiencing an accelerated wear mode, monitoring and trending the condition of the component between drains and oil top-up may be tricky and can generate false conclusions.
Normalizing the wear data to take into account the “period of oil in use” has been adopted by a majority of laboratories, and viewing these wear rates in some graphical form is a common practice.
The potential pitfall arises when the person interpreting the trend fails to take into account the residual oil left behind in the component after the drain and refill. In engines, this varies anywhere between 5 percent and 20 percent of the full charge. In hydraulic systems, the percentage of residual oil could be much higher depending on the number and size of the circuits and cylinders.
Values as high as 40 percent are common. Failing to factor this residual oil into the wear rate distorts the calculated wear rate, and the distortion is often further aggravated with low oil hours.
For example, an engine oil sample with an iron level of 80 ppm taken after 250 hours and before the drain would translate into a wear rate of 0.32 ppm/hour or 32 ppm/100 hours - borderline by most guidelines. If the residual percentage of oil is assumed to be 15 percent, then the initial starting iron value after the new oil is added is 12 ppm (80 ppm x 15%). If the engine is sampled after 10 hours, the wear rate (assuming no additional wear was generated and added to the oil) would be 1.2 ppm/hour or 120 ppm/100 hours - enough to ring alarm bells through any organization. The wear rate would appear to have increased sharply, and the false conclusion may lead to pulling the engine.
It is important to have reliable information about the percent of residual oil that is left behind during a normal drain and refill. If this information has not been determined through experience or from the original equipment manufacturer, then it is necessary to take an initial sample approximately 10 hours after the unit has circulated the oil, before taking a second sample at a reasonable interval afterward. This is to offset the residual levels and determine the new wear rate.
In the case of kidney systems, it is even more important to establish the initial wear level because the filtration system may take out the majority of larger particles and leave the finer particles to circulate in the system. These finer particles are the particles more likely to be measured by spectrometric elemental analysis, and may lead to even greater error.
A conclusion from wear rates calculated from a single sample taken soon after a drain, major top-up or off-line clean-up is a recipe for a wasteful but sometimes interesting wild goose chase.