The Stress-Resistance Test

Measuring filter performance under "real-world" conditions

Hydraulic System Stresses

Most hydraulic filters, due to the nature of their function, are subjected to variable stresses that include:
  • Elevated heat during operation
  • Cold start-up
  • Vibration
  • Cyclic (variable) flow
  • Increased pressure drop due to filter loading

Many hydraulic filters on the market are designed and tested in the laboratory without ever being subjected to these stresses during testing.


How Stresses Affect Hydraulic Filters

Elevated heat causes accelerated media degradation and loss of strength and performance. This is evaluated per ISO 2943.

Cold start-up causes compression of filter materials and pleat "bunching". Effects of cold start-up can be evaluated per ISO 2943.

Vibration can cause mechanical damage or loss of efficiency and desorption (release of previously captured particles). There is no current test to evaluate this effect.

Cyclic (variable) flow causes fatigue of filter structure (pleats). Fatigue resistance to cyclic flow is evaluated per ISO 3724. Cyclic flow can also potentially cause reduced efficiency and particle desorption (release).

Increased pressure drop due to filter loading can cause loss of efficiency and particle desorption (the more particles captured and held in the filter, the more that can be released).


What Makes a Filter Stress Resistant?

Stress-resistant filters have the following design attributes:
  • Uniform pore size (in efficiency control layer)
  • No movement allowed during operation
    • Strong resin bonding of fibers
    • Strong support and bonding of pleat structure
    • Compatible materials with no "softening"
  • Protection of media from any other damaging or moving layers

In order to properly evaluate stress-resistance, especially to cyclic flow and filter loading, Pall Corporation developed the Stress-Resistance Test


Primary Limitations of the Multi-Pass Test

  • Uses only steady flow
  • High dust injection rate
    • 1,000 to 10,000 times actual field experience
  • No stresses such as heat, cold start, vibration
  • Normal reporting of Beta ratios is based on averages and not the worst point in a filter's life


The Stress-Resistance Test

In order to address the deficiencies in the Multi-pass test, Pall Corporation has developed the Stress-Resistance Test, which provides a more realistic measurement of filter performance. This laboratory test examines a number of areas of operation: steady state performance, cyclic flow performance, and the effects of contaminant loading on the retention and unloading characteristics of the filter. During the clean up portion of the test procedure, stabilized particle count levels are measured for cyclic flow conditions at different stages of the filter's life. The SRT test is not meant to measure fatigue resistance or fatigue induced failure but "normal" performance under cyclic conditions.

Typical Filter Duty Cycles in Hydraulic Equipment Most hydraulic systems experience cyclic flow conditions. This figure provides examples of typical Filter Duty Cycles in hydraulic equipment.

Flow Cycle Profile Used in the Stress-Resistance Test This figure represents the flow cycle profile used in the Stress-Resistance test. You will notice how this is similar to many of the typical flow cycles in the figure above.

Clean-Up Curve (Upstream Particle Counts) for a New Filter with an Initial Injection of Contaminant and Steady Flow This figure represents the clean-up curve (upstream particle counts) for a new filter with an initial injection of contaminant and steady flow. No additional contaminant is added during clean-up. Notice how the contaminant is quickly removed from the fluid alowing stabilization to occur with almost no remaining contaminant.

Clean-Up Curve (Upstream Particle Counts) for a New Filter under Cyclic Flow Conditions If we now examine the clean-up curve (upstream particle counts) for a new filter under cyclic flow conditions, stabilization again occurs quickly with a cleanliness level nearly as good as under steady conditions.

Stabilized Cleanliness with Cyclic Flow (Upstream Particle Counts) on a Filter at 2.5% of Terminal Pressure Drop When we examine the stabilized cleanliness with cyclic flow (upstream particle counts) on a filter at 2.5% of terminal pressure drop, a drop-off in clean-up is clearly visible. A 2.5% pressure drop increase typically represents the filter being 30-50% plugged with contaminant.

Stabilized Cleanliness with Cyclic Flow (Upstream Particle Counts) Shows a Marked Deterioration At 80% terminal pressure drop, the stabilized cleanliness with cyclic flow (upstream particle counts) shows a marked deterioration. This is the point near the end of a filter's service life when its performance is most severely challenged by stresses.

Similarly Beta Rated Filters Do Not All Perform the Same under "Real-World" Stress Conditions Comparing downstream particle counts different filters with similar steady-flow Beta reveals that [similarly Beta rated] filters do not all perform the same under "real-world" stress conditions.

Filters can be given an ISO code rating based on the cleanliness level that can be achieved under stress conditions. Rating a filter at 80% pressure drop provides the user with the best understanding of what level of performance the filter will demonstrate at the worst operating condition.

Filter Stabilized Particle Count per mL ISO Code
>4 µm(c) >6 µm(c) >10 µm(c)
A 4200 540 20 19/16/<11
B 7200 970 47 20/17/<13
C 3400 420 18 19/16/<11
D 1100 70 0.8 17/13/<07
SRT 380 31 1.4 16/12/<08
Ratings established at 80% of terminal pressure drop representing worst case operating condition.

Stress-Resistance test results for four pairs of filter elements (E1 and E2, F1 and F2, G1 and G2, H1 and H2) show that the test exhibits excellent repeatability.

Filter >4 µm(c) >6 µm(c) >14 µm(c) ISO 4406 Class
E1 78 2.4 0.09 14/09/05
E2 69 2.7 0.07 14/10/04
F1 166 6.5 0.07 16/11/04
F2 149 4.7 0.10 15/10/05
G1 395 34 0.15 17/13/05
G2 447 42 0.12 17/14/05
H1 932 363 0.27 18/17/06
H2 1,061 402 0.39 18/17/07



The Stress-Resistance Test offers an improved filter performance reporting method over traditional Beta ratings, providing a more realistic measure of how a filter performs in actual service. The Stress-Resistance test also gives the user (via ISO Codes) a more understandable representation of the level of contamination control that can be maintained by the filter during its service life.