Contamination and Filter Selection 381

"Contamination and Filter Selection" describes common contaminants that may affect a fluid system. Contaminants include solid particles, liquids, and energy. Hydraulic and pneumatic systems are both subject to contamination, but hydraulic systems are more susceptible due to their higher operating pressures and recirculation of fluid. Filters are used in a fluid system to clean fluid and control contamination. Filters are selected based on the target cleanliness for a system or component.

A knowledge of the filter selection process helps fluid system operators to determine the most efficient and appropriate filter to use. Being aware of common contaminants that may damage a fluid system helps machine operators prevent malfunctions and reduces downtime.

Class Details

Class Name:
Contamination and Filter Selection 381
Number of Lessons:

Class Outline

  • Fluid System Failures
  • The Role of Fluid in a Fluid Power System
  • Sources of Contamination
  • Types of Contaminants
  • Fluid System Contamination Review
  • Pneumatic System Filters
  • Hydraulic Filter Types
  • Filter Locations in a Hydraulic System
  • Hydraulic and Pneumatic Filter Review
  • ISO Cleanliness Code
  • Target Cleanliness
  • Multipass Test for Hydraulic Filters
  • Fluid Cleanliness and Filter Rating Review
  • Filter Condition Indicator
  • General Fluid and Filter Maintenance
  • System Flushing
  • Final Review


  • Describe fluid system failures associated with contamination.
  • Describe the role of fluid in a fluid power system.
  • Describe sources of fluid system contamination.
  • Describe the types of fluid system contaminants.
  • Describe pneumatic filters.
  • Distinguish between surface filters and depth filters.
  • Distinguish between filter locations in a hydraulic system.
  • Describe how an ISO 4406:1999 rating indicates a fluid's cleanliness.
  • Describe target cleanliness.
  • Distinguish between hydraulic filter ratings.
  • Describe how a filter condition indicator monitors filter performance.
  • Describe preventive maintenance for fluid and filters.
  • Describe system flushing methods.

Job Roles


  • MSSC Fast Track Maintenance


Vocabulary Term Definition
absorbent filter A filter consisting of porous materials and resin for increased strength. Absorbent filters are used in all hydraulic systems and some sensitive pneumatic systems.
additives A substance added to another substance or material to improve its chemical and physical properties. Additives are included in hydraulic fluid to improve a variety of properties, such as lubricity and viscosity.
air breather A device that filters ambient air let into a hydraulic reservoir. Air breathers may be a source of ingressed contamination.
air filter A device that removes solid particulates from the air. Air filters are used in a filter-regulator-lubricator to condition the air in a pneumatic system.
air receiver A component that stores compressed gas for a pneumatic system. An air receiver balances the air compressor capacity and airflow demand.
ambient air Air coming from the surrounding environment. Ambient air let into a hydraulic reservoir may contaminate the oil in the reservoir.
bacteria Biological microorganisms that can contaminate fluid. Bacteria growth in pneumatic fluid is a sign of contamination.
beta ratio A filter rating that indicates a filter's ability to trap particles of a specific size at a constant flow rate. Beta ratio is calculated during a multipass test by counting particles upstream and downstream of the filter.
built-in contamination A source of fluid system contamination that occurs when foreign particles are present in new fluid system equipment. Built-in contamination can form during manufacturing and assembling or result from improperly stored fluid.
cake filtration A filtration method formed from the layer of particle buildup on a surface filter. Cake filtration causes increased flow resistance and the layer of particles eventually has to be removed.
catastrophic failure An effect of contamination characterized by the sudden and complete loss of performance in a system or major component. Catastrophic failure can cause a pump to seize or an actuator to jam.
cavitation The wearing away of a surface as a result of low fluid levels that draw air into the system. Cavitation in hydraulic fluid results in an inadequate amount of fluid being delivered to the pump.
check valve bypass A flow control valve that routes fluid around a specific element. A check valve bypass is included with many filters to provide an alternate flow path if the filter becomes too clogged.
chemical flushing A flushing technique that involves forcing a low-viscosity fluid through a hydraulic system. Chemical flushing pushes fluid through the system at high speeds and at a turbulent flow pattern.
clearances The small space between components that allows for movement and lubrication. Clearances are necessary for the successful operation of many fluid system components.
coalescing filter A filter that removes liquid and solid contaminants from compressed air. A coalescing filter combines the functions of a water separator and an air filter.
code numbers A number between 6 and 24 that represents a range of a specified size of particles in a fluid sample. A code number is assigned for each defined particle size in a fluid's ISO 4406:1999 rating.
compounds A substance composed of two or more separate elements. Compounds are broken down by chemical processes.
compressed air A gas that has been squeezed to a greater pressure than the air surrounding it. Compressed air is a fluid used to transmit power in a pneumatic system.
compressible Capable of being made smaller in volume in response to pressure. Compressible gases are used in pneumatic systems.
contaminant Any foreign substance in a hydraulic or pneumatic fluid that causes a loss of efficiency or a breakdown in a fluid system. Contaminants include solids, liquids, gases, and forms of energy.
contamination The presence of any foreign material in a hydraulic or pneumatic fluid that has a negative effect on the fluid system. Contamination may be material or a form of energy.
control valve A mechanical device that regulates fluid flow by opening, closing, or obstructing one or more passageways. Control valves contain movable parts that can malfunction if subjected to contaminants.
corrosion The gradual chemical attack on a material caused by moisture or atmospheric agents. Corrosion in a fluid system can be caused by contamination or a small leak.
cylinder actuator An actuator that directs force in a straight line. Cylinder actuators include double- and single-acting cylinders.
degradation failure An effect of contamination characterized by the gradual deterioration in component performance. Degradation failures are caused by wear, corrosion, and cavitation.
depth-type filters An absorbent hydraulic filter that traps fine particulate matter throughout the filtration media. Depth-type filters are thicker than surface-type filters and provide an indirect fluid flow path.
dirt tolerance An indication of the internal clearance of a fluid system component. A high dirt tolerance indicates a large amount of internal clearance.
dirt-holding capacity DHC. A filter rating that indicates the total weight of particles a filter is able to trap before reaching its terminal pressure drop. Dirt-holding capacity is weighed during a multipass test.
double oil and filter change A basic flushing technique that requires an operator to change the fluid and filter, circulate new fluid, and then change the fluid and filter once more. A double oil and filter change is often done in conjunction with more intense flushing methods.
energy contaminants A type of contamination that can negatively affect the performance of a hydraulic or pneumatic fluid, as well as the environment. Energy contaminants include heat, magnetic fields, and radiation.
entrained Trapped and suspended in hydraulic oil. Entrained air can form a layer of foam on the surface of hydraulic oil.
expandability The tendency to increase in volume. The expandability of compressed gas allows it to expand to fill a container.
fiberglass A lightweight and strong material composed of fine glass fibers. Fiberglass fibers are woven together and used as filtration media.
filter condition indicator An instrument attached to a filter housing that monitors filter performance during operation. A filter condition indicator may sense check valve bypass actuation or increased pressure drops.
filter-regulator-lubricator FRL. A device that conditions compressed air for use in a pneumatic system. A filter-regulator-lubricator removes moisture and solid contaminants from compressed air, regulates pressure, and provides lubrication for downstream components.
filtration efficiency A filter rating that indicates how many particles of a given size a filter is able to trap. Filtration efficiency is calculated using the beta ratio.
filtration media A layer of absorbent material used in a filter to trap particulate matter. Filtration media materials include a paper-like material, fiberglass, and composites.
flow rate The total volume of fluid that moves through a fluid system or individual component in a given period of time. Flow rate is measured in gallons per minute (gpm) or liters per minute (lpm).
foam A thick filtration medium used in coalescing filters. Foam filtration materials are flexible and reusable.
friction A force that resists motion between two moving objects that are in contact. Friction between the moving components in a fluid system can lead to excess wear.
gaseous contaminants An air particle that can contaminate hydraulic fluid. Gaseous contaminants can become trapped in hydraulic fluid.
gear pumps A fixed-displacement hydraulic pump that has two meshing gears that rotate to move fluid. Gear pumps are relatively inexpensive and compact.
hydraulic fluid A liquid, typically oil, that flows through a hydraulic system to transmit power. Hydraulic fluid is an incompressible liquid.
incompressible Relatively incapable of losing volume in response to pressure. Liquids are incompressible, while gases are compressible.
inert gas Chemically inactive or having little ability to form chemical reactions. Inert gases are fluids used in pneumatic systems.
ingressed contamination A source of fluid system contamination that occurs when foreign particles enter from the surrounding environment. Ingressed contamination occurs during system operation or maintenance.
inlet strainers A coarse surface-type filter installed on a hydraulic pump inlet to filter fluid entering the pump. An inlet strainer has a micron rating of 75 or higher.
intermittent failure An effect of contamination characterized by unreliable and unpredictable component performance. Intermittent failure is often the result of an untreated degradation failure.
International Organization for Standardization ISO. An international organization that establishes documented standards, rules, and guidelines to ensure that products, processes, and services are fit for their purpose. The International Organization for Standardization rates the cleanliness of hydraulic fluid using the ISO 4406:1999 standard.
ISO The International Organization for Standardization. An international organization that establishes documented standards, rules, and guidelines to ensure that products, processes, and services are fit for their purpose. ISO rates the cleanliness of hydraulic fluid using the ISO 4406:1999 standard.
ISO 4406:1999 A standardized method for measuring the average size and quantity of particles present in a sample of working hydraulic fluid. ISO 4406:1999 measures particles measuring 4, 6, and 14 microns.
ISO medium test dust A synthetically produced contaminant with a specified particle size distribution and composition. ISO medium test dust is used in a multipass test.
lubricating Using a substance to reduce friction between two surfaces in relative motion. Hydraulic fluid lubricates components in a hydraulic system.
material contaminants A type of contamination that hinders the normal performance of a hydraulic or pneumatic fluid. Material contaminants include solids, liquids, or gases.
mechanical flushing A flushing technique that requires an operator to use high-pressure air to push a sponge through a conductor in a fluid system. Mechanical flushing may be performed in conjunction with a double oil and filter change.
microns ยต. A metric unit of measurement equal to one thousandth of a millimeter. Microns are often used to measure solid contaminants.
multipass test An ISO test that measures filter performance under controlled operating conditions. A multipass test measures dirt-holding capacity, beta ratio, and filtration efficiency.
nominal peak pressure A maximum pressure value assigned to a fluid system or component. Nominal peak pressure is the maximum pressure a system should encounter during operation.
pore size An opening in a filter that determines the largest size particle that can pass through the filter. Pore sizes are usually expressed in microns.
power variables Any characteristic of a fluid system or component that can be measured and quantified. Power variables include pressure, flow rate, and temperature.
preventive maintenance Regularly scheduled service and upkeep performed while a component is still in working order. Preventive maintenance can prolong equipment life and increase production.
reservoir The main storage container for fluid in a hydraulic system. The reservoir supplies fluid to the hydraulic pump.
sampling valves A type of valve connected to a port in a fluid system that allows operators to safely extract a representative sample of working fluid for testing. Sampling valves allow fluid to be taken during operation of a fluid system.
sealing Providing a barrier for internal components to prevent leakage and block contaminants from entering a fluid system. Sealing may be provided by hydraulic fluid.
self-generated contamination A source of fluid system contamination that forms internally due to component wear and fluid deterioration. Self-generated contamination will move through the system until it is removed.
servo valves An electrically operated control valve that continuously regulates output flow as a function of electrical input. A servo valve is installed on a pressure line downstream from a depth filter.
silt A small solid contaminant. Silt is generally small enough to fit through most dirt tolerances but can also flow through many filters.
sludge A form of liquid contamination that can form in the reservoir when oil and water mix. Sludge is gummy liquid matter that interferes with the oil-cooling process.
solid contaminants A particle that can impair fluid flow and block clearances between components in a fluid system. Solid contaminants include small pieces of metal, dirt, and sand.
standard An established policy regarding a particular practice or method. The standard for quantifying particulate matter in a hydraulic fluid is ISO 4406:1999.
surface-type filters An absorbent hydraulic filter that traps large particulate matter on the surface of the filtration media. Surface-type filters have uniform pore sizes and a direct fluid flow path.
target cleanliness The minimum cleanliness level for a fluid system. Target cleanliness levels influence filter selection.
terminal pressure drop The highest allowable difference between upstream and downstream pressure across a filter. Terminal pressure drop for a filter is when a filter is too clogged to allow adequate flow.
thermal energy Energy in the form of heat. Excess thermal energy can negatively affect the performance of hydraulic or pneumatic fluid.
turbulent When fluid flows roughly and erratically. Turbulent flow is necessary during chemical flushing in order to effectively clean the system.
vane pumps A hydraulic pump that uses a series of flat protrusions that extend within a rotor to trap and move fluid. Vane pumps tend to have small dirt tolerances.
volume The amount of three-dimensional space an object occupies. Volume is measured in cubic inches or cubic centimeters.
water separator A device used to remove water droplets from compressed air. A water separator is used in combination with an air filter in a pneumatic system.
wear The gradual removal of material on a surface caused by contact and friction. Wear is reduced by using lubricants.
wear The gradual removal of material on a surface caused by contact and friction. Worn fluid system components can generate contaminants during system operation.