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This SAE Aerospace Recommended Practice (ARP) describes the multi-pass method for evaluating the filtration
performance of fine lube filter elements, commonly utilized in aerospace power and propulsion lubrication systems: gas
turbine engines, auxiliary power units (APUs), helicopter transmissions, constant speed drives (CSDs), and integrated
drive generators (IDGs).
1.1 Introduction
Variation in filter element testing methods and requirements make comparison of results difficult. In order to minimize
these problems, this document describes standard filtration ratings and test procedures. Both manufacturer and
customer will have a common means to specify, control, and evaluate filter elements.
1.2 Filter Element Performance Ratings
1.2.1 Filter Element Efficiency
Filter element efficiency is the ability of a filter element to remove (and retain) contaminant particles from the fluid stream.
This procedure determines the particle removal efficiency of the filter element as a function of particle size. The particle
removal efficiencies for the various particle size ranges are expressed as filtration ratios, termed Beta Ratios. The
filtration ratio at a specified particle size 'x', designated βx
, is the ratio of the number of particles larger than the specified
size entering the filter element, Ux
, to the number of particles larger than the same size leaving the filter element, Dx:
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The techniques specified in this document allow measurement of filtration ratios up to 1000 (99.9% particle removal
efficiency) for the particle size range 4 μm(c) to 25 μm(c), as defined in ISO 11171.
1.2.2 Multi-Pass Filter Element Dirt Capacity
The multi-pass filter element dirt capacity is the mass of test contaminant introduced into the filter element test system
during the filtration efficiency test to produce the prescribed terminal filter element differential pressure. This value should
be used only for comparing filter elements having similar filtration efficiencies
It should be noted that the most commonly specified dirt capacity for lube filter elements utilized in aerospace lubrication
systems is the MIL-F-8815 dirt capacity, MIL-F-8815 (4.7.2.6). In this dirt capacity test, contaminant is added in discrete
increments, 'slugs', each increment consisting of a constant, predetermined mass of test contaminant, immediately
upstream of the test filter, via a 'slug' addition valve, at fixed intervals (usually every 4 minutes) during the test. The filter
element differential pressure is recorded 2 minutes after each contaminant 'slug' addition. The total mass of contaminant
added to achieve the prescribed terminal filter element differential pressure is reported as the dirt capacity.
Due to the extensive experience with the MIL-F-8815 dirt capacity test, and the extensive amount of dirt capacity test data
that has been generated with this test, the continued use of the test is recommended for aerospace lubricant filter
elements. However, recently, several filter element specifications for helicopter transmission lubrication systems, APU
lubrication systems, as well as other aerospace lubrication systems, have specified the multi-pass dirt capacity. In such
cases the multi-pass dirt capacity, defined above, should be determined as described in this document. Due to the
differences in the two dirt capacity tests, the dirt capacities determined from the two tests will be different.
1.3 Test Contaminant and Particle Counter Calibration
Historically, AC Fine Test Dust was the test contaminant specified for the multi-pass filter performance test, and the
calibration of automatic particle counters was in accordance with ISO 4402. Replacement test dusts for the AC Test
Dusts, no longer available, have been specified by ISO (ISO 12103-1). The corresponding ISO Test Dust for AC Fine
Test Dust is ISO Fine Test Dust (designated ISO 12103-A2).
In addition, ISO has also specified a calibration procedure ISO 11171 for automatic particle counters to replace the ISO
4402 (1991) calibration procedure which utilized AC Fine Test Dust. The ISO 11171 procedure uses ISO Medium Test
Dust, designated ISO 12103-A3, instead of AC Fine Test Dust. The definition of particle sizes per the ISO 11171
calibration procedure differs very significantly from the particle sizes defined in ISO 4402. In order to distinguish the
particle sizes defined in ISO 11171, they are designated as μm(c) or micrometer(c), the (c) indicating NIST certified sizes.
Both the change in test contaminant, from AC Fine Test Dust to ISO Fine Test Dust (ISO 12103-A2), and the change in
automatic particle counter calibration, from ISO 4402 to ISO 11171, have been incorporated in this document.
The change in test contaminant and the particle counter calibration procedure will lead to multi-pass filter performance
test results that are significantly different from test results obtained previously with AC Fine Test Dust and ISO 4402
calibration. It is necessary for users to take this into account when comparing historic filter element efficiency and dirt
capacity test data with data generated per the present procedure, and when comparing filter element efficiency and dirt
capacity results from the current procedure to historic specification requirements for filter element efficiency and dirt
capacity. AIR5455 discusses the impact of the change in test dusts and automatic particle counter calibration on
laboratory filter performance and filter ratings.
1.4 Filter Element Conditioning
Filter element performance ratings can be adversely effected by harsh operating environments. Filter elements should,
therefore, be subjected to procedures simulating these harsh operating conditions prior to performance testing.
Conditioning is the term covering these procedures. This document does not cover conditioning requirements. They
should be determined by the user and reported by the testing agency. AIR1666 discusses recommended filter element
conditioning methods for gas turbine engine lubrication filter elements. The methods discussed in AIR1666 can also be
applied to filter elements utilized in other aerospace lubrication systems.strRefField
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