Misconceptions and misinformation abound in the equipment maintenance business as to the correct use of appropriate standards to measure particle contamination in fluids. Komatsu Australia's John Hardy and Satchoo Padayachee explain.
There is considerable misunderstanding among maintenance personnel in the mining and construction industries about appropriate standards and methods to measure fluid contamination.
Much of this is due to what we can only attribute to a program of misinformation on the part of some suppliers of maintenance services and consumables.
The end result is unnecessary confusion on the part of equipment owners and users, who in turn are requesting OEMs and condition monitoring providers to supply results that are often meaningless and/or unrealistic and in many cases do not properly comply with ISO particle counting standards.
The problem we are facing is that non compliant systems have been introduced to the industry and test results from these systems are being reported to the ISO Standard.
Feedback and discussions between clients and OEMs such as Komatsu in recent years is showing growing confusion through improper use of ISO systems in certain industries.
The end result is misinterpretation of data when applied to machine maintenance. Potential flow-on effects include inadequate maintenance, leading to component failure and unscheduled downtime or even fruitless, time-consuming and expensive efforts to fix "problems" which don't actually exist.
Data integrity and interpretation are critical areas of any successful maintenance strategy; misapplication of inappropriate standards, and the resulting misinterpretation directly affects maintenance costs, inventory, equipment availability, resource management and safety.
It is essential that the ISO code system is properly understood by maintenance professionals and then applied appropriately to enhance contamination control as it applies to machine condition monitoring.
Certainly, the ISO code system complements oil analysis data. However, it should never be confused with the primary indicators for used oil which have been proven to be effective over many years in preventive maintenance of equipment.
The ISO Code System
The International Organisation for Standardisation, otherwise known as ISO, is a global international standards organisation that qualifies test criteria and standards for several industries including oil manufacturing.
The federation of regulatory bodies that endorse the ISO standards have introduced a system that provides scaling for contamination levels in new hydraulic fluids using particle counting methods.
Essentially, the ISO code system was developed to qualify levels of environmental contamination in new, unused hydraulic fluids and hydraulic fluids that transmit power to a main drive, such as hydrostatic transmission.
The most common instruments used for particle counting are Automatic Particle Counters (APC) and Optical Microscopes. APC instruments use a laser to penetrate the oil sample and detect particles; particle counting by optical microscopes involves physically counting particles distributed on a mesh grid.
Over the years, a number of standards for particle counts in fluids have been developed. The two that are most relevant to the mining and construction industries are:
ISO 4406:1999 Hydraulic fluid power Fluids Method for coding level of contamination by solid particles (automatic particle counter method)
This standard uses a three-size system for automatic particle counters in the size ranges of ?4 m ( m = microns), ?6 m and ?14 m. It is important to note that the ?5 m and ?15 m still apply to the particle count determined by the optical microscope method (ISO 4407:2002).
ISO 4407:2002 Microscope Method
This is a manual counting process using a microscope and micron scale attached to an objective (lens) to determine quantities of particles of selected sizes. Generally particles sizes ?5 m and ?15 m are counted by this method.
The microscope method, when run to the standard can be laborious, allows plenty of room for error and is time consuming. The time and labour resources required to apply this method in these industries appears to have led to substitute in-house methods that deviate significantly from the Standard further compromising its useability and reliability.
Applying ISO Codes to non-hydraulic fluids
As its title implies (note that it includes the words "hydraulic fluid power fluids) the ISO 4406 Code standards are not intended to qualify contamination levels in non-hydraulic fluids.
Unfortunately however, in recent years there have been many instances in the mining, construction, civil, marine, transport and food industries where these ISO Standards are used for analysing non-hydraulic fluids, including fuel.
While both the automatic particle counting instrument and the manual microscope method can provide a quantitive evaluation of particle contamination by particle counting in any fluid or semi-fluid, any test result should never be quoted to the ISO 4406:1999 standard if it is a non-hydraulic fluid and not carried out to the Standard.
Test methods
Now let's look at some of the test methods being used to determine particle contamination in fluids, using ISO standards (however inappropriately!).
Automatic Particle Counters: Laser Method
Particle counting by laser instrument is a reliable test method with good repeatability provided that the samples are prepared using consistent and reliable sample preparation methods, and the laser instrument is calibrated to the relevant standard (ISO 11171).
The laser method uses a laser beam to pass light through a specimen. The beam undergoes diffraction. When the interference of the laser beam occurs, the instrument assumes that a particle caused the interference and reports the presence of a particle.
The quantity and size range of particles passing through the path of the laser beam are determined by the software, and reported as the number of particles per ml of sample for each micron size range detected.
It is important to note that non-wear generating particles are also counted using this method, even though they do not contribute to component wear out i.e. not all particles counted by the ISO system generate wear.
In other words, the ISO count achieved using this method is not directly related to component wear.
Comparator Method
An alternative to the laser method (which has high capital and material costs) that has been popularised in recent years is known as the 'Comparator' method. This involves a series of images with fixed (or theoretically "known") ISO 4406:1999 values "compared" against a photographic image.
The images are that of the filter residue obtained after passing a sample under vacuum through a membrane filter, often referred to as a "patch" (hence the term "patch test"). This comparator reference image is then assigned an ISO code allocation from a sample result processed on an automatic particle counter.
These photo images of particles are subject to many variables, depending on sample volumes, membrane diameter, pore size, particle distribution and even operator bias used to produce the patch. Bias occurs where the analyst can focus on the cleaner part of the membrane filter to report a lower ISO code or, alternatively, a dirtier area to report a higher code.
For these reasons, image diagnosis will not be conclusive or qualified against an APC test result. In addition, the test result should NEVER be reported to an ISO standard if the method or means of measurement does not comply with the standard.
The automatic particle counter reports the number of particles per millilitre of sample and therefore can be converted directly to ISO codes since ISO 4406:1999 codes are related to the number of particles per millilitre.
Some patch test methods used for images on the other hand will specify 100ml of sample which can potentially give a particle concentration in the order of 100x when compared to a reference sample tested on automatic particle counters
Any resulting inaccurate or unreliable test results can lead to costly (and potentially completely unnecessary) maintenance on a piece of operating machinery or on the other hand, give a false sense of security that all is well, when that is not the case.
Similarly, unreliable results can cause data inaccuracy if trending models are used.
The resulting maintenance actions or inactions will have a cumulative effect on productivity.
While patch tests and comparator methods have their place in a maintenance program, they cannot provide conclusive results and should not be used as primary indicators in wear monitoring in oil analysis or condition monitoring programs.
But whatever tests are used, analysts should always consider the accuracy of the technology against the compliance to the current International Standard to gauge the conclusiveness of the test result and the associated reporting level.
The fashion for "fluid cleanliness"
"Fluid cleanliness" is a relatively new term which appears to have been introduced by the filter industry. The term 'cleanliness' only appears in the latest ISO 11500:2008 standard possibly due to it's being popularised in more recent times.
The term "fluid cleanliness" also seems to have been correlated to the term "ISO cleanliness" although in reality this is a misnomer as ISO codes deal only with solid particulate contamination, whereas fluid cleanliness includes liquid contamination.
Fluid cleanliness suggests that a fluid's contamination level to ISO 4406:1999 is either acceptable or non-acceptable.
If this is the way forward, users then need to run several test samples in their own environment to the documented best practices and from these "ideal' test results determine acceptable levels to suit a (site specific) operation.
It is extremely challenging, in uncontrolled dusty environments, trying to achieve published ISO codes that have been derived from ideal laboratory and manufacturing processes.
Notwithstanding, these published guidelines often appear in warranty documentation and filter manufacturers' brochures.