The level of acceptability depends on three features
•   the contamination sensitivity of the components
•   the operational conditions of the system
•   the required reliability and life expectancy

New ISO Standard Test Dust and its effect on ISO Contamination Control Standards

When General Motors gave advance warning to the International Standards Organisation (ISO) that it was intending to stop the production of AC Fine Test Dust (ACFTD), work commenced immediately on finding an improved replacement dust. ACFTD was used extensively within the fluid power and automotive industries for calibrating Automatic Particle Counters (APCs) and for the testing of components. APCs are used for testing oil filters, and also for contaminant sensitivity testing of hydraulic components.

For 25 years, APCs have been the main stay in the measurement of solid particles in hydraulic fluids. The growth in demand for measuring fluid cleanliness in a variety of industrial processes, including fluid power, has resulted in APCs moving from the laboratory environment out into the factory. In fact, they are now a critical part of many production processes. It is therefore essential that the data they provide is both accurate and consistent.

Calibration
ACFTD has been used as an artificial contaminant since the 1960s and its original particles size distribution was determined using an optical microscope. This particle size distribution subsequently formed the basis of ISO 4402, the method for calibrating APCs. Due to the limitations of that method of measurement, the particle size distribution was questioned below about 5 microns. It was also not traceable to any national standard of measurement - a critical requirement for today’s quality management systems.

There was also an absence of formal controls over the distribution of the test dust, and batch-to-batch variability was much greater than is acceptable nowadays.
ISO therefore defined the requirements for the replacement for ACFTD and asked the National Institute of Standards and Technology (NIST) in the USA to produce a standard, traceable reference material. The new dust’s particle size distribution has been accurately determined with the aid of modern scanning electron microscope and image analysis techniques.

New Test Dust Benefits
The new ISO Medium Test Dust (ISOMTD) consists of similar materials to the old ACFTD, but to minimise particle counting errors, it is of a slightly coarser grade because ACFTD included too many particles smaller than 5 microns which gave problems during testing.
ISOMTD is produced to a standard distribution and stringent quality control procedures, thereby ensuring excellent batch-to-batch repeatability. These procedures, combined with a revised ISO APC calibration method give:

•   A traceable and controlled reference test dust with greatly reduced variation in particle size distribution.
    This gives the traceability required by ISO 9000, QS9000 and similar quality management systems.
•   A procedure for determining the performance of APCs so that minimum acceptable levels can be set by
    the user.
•   Improved calibration techniques and procedures.
•   More accurate calibration.
•   Improved levels of particle count reproducibility with different equipment.
•   More accurate and consistent filter test results.

Effect on Industry
The introduction of ISOMTD has necessitated changes to certain ISO standards.
The standards affected include:-

ISO 4402 : 1991 Hydraulic fluid power
                     Calibration of liquid automatic particle counters.

ISO 4406 : 1987 Hydraulic fluid power
                     Code for defining the level of contamination by solid particles.

ISO 4572 : 1981 Hydraulic fluid power                    
                     Filters
                     Multi-pass method for evaluating filtration performance of a filter element.

The LPA2 is calibrated with ISO Medium Test Dust (to ISO 11171).
The correlation between particle sizes and the ACFTD (old standard) to the ISOMTD (new standard) is as follows :

Although ISO 4406 standard is being used extensively within the hydraulics industry other standards are occasionally required and a comparison may be requested. The following table gives a very general comparison but often no direct comparison is possible due to the different classes and sizes involved.

Modern hydraulic systems featuring highly effective filters have fluid cleanliness levels that approach that of the sample bottle themselves.

The use of un-cleaned bottles can greatly increase the particle counts. (Please note sterilisation kills bacteria but does not remove particles). Perhaps of even greater concern is the variability in their levels of cleanliness.

A sudden increase in contamination could be caused by the sample bottle. This apparent increase could instigate unnecessary corrective action.

Hydraulic Component Manufacturer’s* Recommendations

Most component manufacturers know the proportionate effect that increased dirt level has on the performance of their components and issue maximum permissible contamination levels. They state that operating components on fluids which are cleaner than those stated will increase life.

However, the diversity of hydraulic systems in terms of pressure, duty cycles, environments, lubrication required, contaminant types, etc, makes it almost impossible to predict the components service life over and above that which can be reasonably expected.

Furthermore, without the benefits of significant research material and the existence of standard contaminant sensitivity tests, manufacturers who publish recommendations that are cleaner than competitors may be viewed as having a more sensitive product.

Hence there may be a possible source of conflicting information when comparing cleanliness levels recommended from different sources.

The table below gives a selection of maximum contamination levels that are typically issued by component manufacturers. These relate to the use of the correct viscosity mineral fluid. An even cleaner level may be needed if the operation is severe, such as high frequency fluctuations in loading, high temperature or high failure risk.