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Understanding Diesel Fuel Cleanliness Standards

Posted March 23, 2020 by Tyler Moore

With diesel fuel becoming susceptible to contamination after the initial fuel refinement, it should be a concern for fuel managers to understand the quality of the fuel they are receiving.

There are many different types of diesel fuel contamination, each with their own unique challenges in remediating.

Dirt, water, and microbial growth can not only reduce fuel quality, but also impact the operational reliability of the equipment relying on the contaminated fuel supply.

Fuel cleanliness can be measured through the ISO 4406 code that defines the quantity of solid particles in a fluid.

Through this standard, contamination in liquids can be quantified to a degree where necessary actions can be determined.

Fuel Cleanliness Standards

What do ISO numbers mean?

Fuel cleanliness levels using the ISO 4406 method was officially documented as a global standard only as recently as 1998 with the development of the Worldwide Fuels Charter (WWFC).

Since its inception, the charter has established a minimum cleanliness level for each of the diesel fuels under various available categories around the world.

The numbers in the ISO 4406 cleanliness code are used as the industry standard to measure contamination in various liquids, such as fuels and hydraulics.

Since contaminants come in a variety of shapes and sizes, the ISO cleanliness standard specifies measuring the size of particles in microns.

To make contamination levels easier to digest, a 100ml fluid sample is taken to analyze. ISO 4406 measures the number of particles larger than 4µm, 6µm, and 14µm within the 100ml fluid sample.

Using the table below, the ISO code number to the left is used to represent the number of particles per ml. For instance, a reported fluid sample with an ISO of 21 / 19 / 14 would mean that there are 10,000 to 20,000 ppm of particles larger than 4µm (ISO 21), 2,500 to 5,000 ppm of particles larger than 6µm (ISO 19), and 80 to 160 ppm of particles larger than 14µm (ISO 14).

How big is a Micron?

When talking about things in terms of “micron” measurements (µm), a micron is simply an abbreviation for a “micrometer”, or 1/1,000,000 of a meter. Being equal to roughly .00004 inches, a micron is not visible to the human eye (which is limited to about 40 microns).

Microns are a standard unit of measurement in many sciences, industrial manufacturing, and other industries reliant on fine precision measurements.

The diameter of a human hair, which can be clearly seen by the naked eye, is about 80 microns in diameter. A United States one-cent coin is 19,053 microns in diameter. White blood cells measure at about 25 microns, while bacteria range from a fraction of a micron to about 2 microns in diameter.

It is important to note the ISO testing standards of measuring for 4µm / 6µm / 14µm, as all of these micron sizes are invisible to the human eye without the use of a microscope.

These ISO numbers are extremely important to quantify considering mechanical equipment components, such as injectors with specific micron tolerances, or other small openings in pumps that are susceptible to mechanical wear driven by the presence of these microscopic particulates in the fuel.

Fuel Quality Testing

Testing for Fuel Contamination

There are preferred and recommended methods in testing for fuel contamination that are considered the standard in many industries. From bulk fuel storage sites to construction operations relying on petroleum energy, fuel quality is directly responsible for the operability and reliability of these industries and many others.

This is why fuel is to be regularly tested, adequately maintained, and properly remediated should it not meet the recommended cleanliness standards.

Fuel Contamination & Fuel Degradation

Creating a proactive fuel maintenance program requires an understanding of the processes that occur within the tank. In order to properly react to changes in your stored fuel supply, it is important to distinguish the differences between fuel contamination and fuel degradation.

Fuel contamination is typically some type of impurity that has entered the fuel supply, and often starts with water. Some forms of contamination can be removed through remediation or prevented through proper maintenance. Other contamination forms can lead to fuel degradation and eventual fuel breakdown.

Diesel Fuel Contamination Timeline Illustration

To learn more about the water-driven microbial growth in diesel fuel: check out our article on Addressing Algae In Diesel Fuel.

Fuel degradation is classified as molecular fuel breakdown or changing of fuel chemical characteristics over time. Fuel can interact with its environment and change molecular form over time, leading to forms of contamination or being incompatible with your engine or the demanding equipment.

SAE Fuel Testing Standards

SAE International is an association within the engineering industry that develops international industry standards around sectors such as aerospace, automotive, and more.

SAE has developed technical standards that are universally understood such as horsepower ratings and aerospace engineering standards. 

J1488 & J1839

SAE J1488 is an emulsified water/fuel separation test procedure as recommended by SAE International.

J1488 is specified to be used when the water that is in the fuel has water droplets that are relatively small in size. This SAE standard lab test measures a fuel filtration system’s effectiveness at removing emulsified water.

The type of emulsified water that is targeted in J1488 testing is best remedied through water-absorbing filters or through using coalescing media.

SAE J1839 is a coarse droplet water/fuel separation test procedure. This lab test evaluates water emulsion where droplets are of larger size.

This type of emulsified water is best mitigated through centrifugal water separation and the use of hydrophobic filter media.

ASTM Testing Standards

The ASTM, originally known as the American Society for Testing and Materials produces technical standards for a variety of industries to enhance operability, efficiency, and effectiveness of many critical standards.

ASTM has various categories where their determined standards fall, and they also provide training and testing for those in the industry.

These standards are utilized in hundreds of industries, including manufacturing, construction, medical devices, nuclear services, water technologies, and more.

The ASTM updates their Annual Book of ASTM Standards each year, and ASTM has 143 technical writing committees that work on developing and updating these standards.

ASTM compliance has grown to be the standard in many industries, with many vendors, contractors, engineers, and architects exclusively requesting products and services to meet ASTM’s renown standards.

ASTM D2709

ASTM D2709 is a testing method for water and sediment in middle distillate fuels by centrifuge.

The “Water and Sediment” test is used to provide an indication of free water or sediment that is suspended within the fuel as a haze or cloudiness for No. 1 and No. 2 diesel fuels.

While the test doesn’t provide analysis for the type(s) of fuel contamination, it provides a basis for how much contamination is within your fuel supply. Further testing may be required after remediation to determine the source of fuel contamination.

ASTM D6469

ASTM D6469 is the standard guide for microbial contamination in fuels and fuel systems.

Microbial contamination within your fuel supply can lead to fuel degradation and more severe tank contamination. The microbes grow at the interface of water and fuel and can propagate into much larger issues.

In order to prevent this problem from spiraling out of control, it is important to identify the issue and handle it properly.

Many microbial tests require additional caution and more stringent methods to properly collect and isolate the fuel sample to prevent contamination from the environment (additional microbes getting inside the sample).

ASTM D7371

ASTM D7371 is the standard test method for determination of biodiesel content in diesel fuel using mid infrared spectroscopy.

Biodiesel consists of fatty acid methyl esters (FAME) and is blended with ULSD for use in most industries.

A 5% bio-blend can be sold as straight ULSD without notifying the customer (still falls under ASTM D975). Due to its production from fats and oils, it is particularly susceptible to oxidation (environmental breakdown), microbial growth, and has a higher affinity for water (holds more water in the fuel)).

For these reasons, it can be important to know the amount of the biodiesel within your tank. Backup power fuel sources sit for longer than normal amounts of time, leading to excessive time for fuel breakdown within the tank.

ASTM D7545

ASTM D7545 is the standard test method for oxidation stability of middle distillate fuels–Rapid Small-Scale Oxidation Test (RSSOT).

The RSSOT test is used to project the lifespan of a fuel supply (including biofuel blends and pure biofuel).

Under high pressure and a high oxygen environment, fuel will break down more rapidly (oxidize).

This test is used to predict how well a fuel blend will do in the field over a longer duration. Fuel that scores low on the RSSOT test may want to be replaced instead of remediated.

For fuel scoring very low on this test, contaminants may build up very shortly after remediating the fuel supply. This test replaced older oxidation tests due to its ability to handle biofuel.

ASTM D4176

ASTM D4176 is a distillate fuel bar chart test that is used to determine contamination through “haziness” in a distillate fuel.

A fuel sample is taken from the fuel source, and is contained in a see-through glass jar for visual testing.

The ASTM bar chart has a series of ratings from 1 to 5 used to determine a pass/fail rating.

By putting the ASTM chart behind the fuel, the test can be completed by comparing the appearance of the fuel with the associated ratings on the chart.

The Cycle of Fuel Maintenance

Fuel Maintenance Cycle

When managing bulk storage fuel in often large volumes, it is important to prioritize fuel integrity, ensuring that the fuel is ready for use when a demanding system requires it.

To make this process easier to manage, we have simplified the fuel sampling and testing cycle into four revolving steps.

Fuel Sampling and Testing Cycle

Sampling Fuel

To gather a sample that best represents the current status of your energy source (the fuel), it is important to sample fuel directly from the storage tank.

Be sure to pull samples from problematic areas of the tank that are often hard-to-reach, where fuel stagnation can rapidly degrade fuel. 

Fuel contamination “falls out” of the fuel and towards the bottom of the tank, where it will collect. When using a fuel sampler, be sure to pull the fuel from the bottom of the tank to be sure you are sampling the fuel where the contamination is most abundant.

If your fuel system has multiple fuel sources, be sure to test all sources to be sure the fuel quality is consistent and/or that contamination has not spread farther from your original testing point.

Analyzing Fuel

When it comes to analyzing your fuel sample, there are many accepted tests and methods to gain an understanding of the current quality of your fuel samples.

There are tests, as mentioned in our SAE/ASTM section above, that can be used to turn your sampled fuel into relevant information regarding your current energy source.

For analyzation and testing, you can send your fuel samples to labs that specialize in fuel testing, where they will give you a breakdown of the different contamination levels that were found in your fuel. This method is thorough, however it can take up to a couple weeks to receive results.

There are other testing methods which can be carried out on-site, and can give fuel managers the insight needed to make critical operations decisions in 15 minutes or less.

Particle counters are used to translate fuel quality into an ISO 4406 standard, using laser technology to account for contamination particles of different micron sizes.

Proper Fuel Sampling & Testing

Fluid sampling pumps are often used to obtain fluid samples from hard-to-reach spots using flexible tubing. This allows for fluids to be drawn without the worry of cross contamination, as the fluid never comes into contact with the pump.

Fuel tank samplers, also known as “bacon bombs”, are industrial-strength stainless steel devices used to remove liquid samples from a bulk fuel storage tank. The device is lowered into a fuel tank until the sampler’s plunger makes contact with the bottom of the tank.

The plunger then opens, which admits a sample into the unit. To sample from any desired level in the tank, the plunger can be actuated by a pull-chain attached to the device.

Once fluid samples are obtained, they must be sent to a lab for testing. Getting results from the lab could take days, up to waiting periods of weeks.

For quicker results, Kolor Kut ® Water Finding Paste is a product used to instantly report if there is a presence of water in petroleum fluids such as gasoline, kerosene, diesel, and heavy fuel oil. The paste is applied to a rod and dipped into the tank, with the color of the paste changing instantly upon contact with water.

FUELSTAT ® PLUS is a simple fuel testing kit that provides results in less than 10 minutes. The objective of the test is to provide rapid screening of fuel samples to give a quick and accurate assessment of H. Res., bacteria & other fungi within the fuel.

Liqui-Cult Microbial Test Kits accurately detects and quantifies bacterial and fungal growth in a variety of fluids. Liqui-Cult tests for microbial growth in fuel samples over a period of a few days.

Through frequent fuel testing, the presence of contamination can be determined and action plans can begin to be determined. Depending on the scope of contamination levels and volume of fuel contaminated, some solutions may be more practical to implement than others.

Fuel Sampling & Testing Tools

To be proactive and catch fuel contamination early, fuel should be sampled and tested from a bulk fuel storage tank at least once every six months. Testing for different contaminants can be achieved in a number of ways, here are the most common tools for fuel sampling and testing:

Reacting to Fuel Results

Once fuel testing results are received, it is time to take the action needed to improve or maintain the quality of the stored fuel you are managing. 

In Part 4 of this article, we will review different fuel maintenance methods and equipment that are suited for both improving and maintaining the quality of bulk fuel storage tanks.

It is important to act quickly when receiving less-than-desirable fuel test results, as prolonged contamination issues can quickly elevate into bigger issues, and even lead to complete equipment failure if that fuel makes it to critical engine components.

Repeating & Prevention

Regular fuel testing is critical in being able to manage an energy source. If unnoticed, contamination in a fuel tank can compromise an entire fuel system, leaving room for vulnerabilities and unnecessary lapses in system integrity.

Fuel can fall out of desired spec in the matter of months to even weeks, so through persistent testing, you can be made aware of undesirable trends in your fuel quality and be able to make proactive decisions.

Keeping Diesel Fuel Clean

Fuel Filtration

When it comes to fuel filtration, there are recognized and preferred ratings that industrial professionals, equipment operators, and filter manufacturers prefer.

Three types of common types of filter ratings are absolute ratings, nominal ratings, and beta ratings.

Other than beta filter ratings (or ratios), an absolute filter rating is one of the preferred rating systems for those working in industries that utilize filters, as absolute filter ratings give operators and mechanical engineers a much more consistent result when compared to nominal ratings.

Absolute Ratings

Absolute filter ratings are absolute in that they are rated to the diameter of the largest particle that will pass through the filter media.

The absolute filter rating indicates the filter media pore size, and ultimately, what maximum particle size will be allowed to pass through the filter.

Filters that are “absolute rated” usually have (on average) a more consistent pore size, enabling them to achieve higher filtration efficiency, increasing their beta ratios as a result. 

Nominal Ratings

Nominal filter ratings are expressed in being able to filter a certain % of a given micron size, for instance, “90% of 25 micron” – meaning that the filter is able to stop 90% of all 20+ micron particles from passing through.

The nominal rating is considered by most to be an inaccurate and inconsistent rating system because of the many variables around liquid flow through a filter media.

Between the concentration of contamination and the system operating conditions like pressure and temperature, it is nearly impossible to rely on nominal ratings for accurate measurements of filter effectiveness.

Beta Ratios (ISO 16889)

Beyond both absolute and nominal filter ratings is the most preferred rating system in the fluid filtration industry, the beta rating.

Derived from ISO 16889, the Multipass Method for Evaluating Filtration Performance of a Fine Filter Element, the beta rating is the most accurate measurement of filter effectiveness, which bridges the gap between manufacturer claims and end-user results.

The beta rating system uses particle counts both upstream and downstream of the filter element to measure controlled quantities of contaminants in the fluid.

The beta ratio is then determined by dividing the number of particles of a certain size measured upstream of the filter by the number of particles of the same size measured to be downstream of the filter.

Referencing the chart below, you can see what particulate differentials correspond to what beta ratio.

Filter Types

Filter medias are especially important in that they are responsible for capturing contaminants in passing liquids, in this case fuel.

Through choosing efficient and effective filter media, fuel systems can be kept at an optimal state where risk of failure from fuel contamination is dramatically reduced.

However, the lack of optimized fuel filtration medias can also provide its own set of unique challenges, that could put the fuel system at risk.

There are two common types of filter medias primarily used in the filtration industry today, cellulose and microglass filters.


Cellulose filter media is a widely used filter type that is made of larger diameter fibers than microglass.

Because of the media quality, cellulose filters are cheaper than other media types but that initial cost saving often comes with a greater cost of its own.

Due to the larger fibers in the cellulose filter media, the contaminants that are captured in the filter can severely restrict fuel flow and develop into a rapid pressure drop in the system.

This leads to more frequent filter changes, and less effective filtration overall.

Because of this, cellulose filter medias are seen to be much less efficient when compared to microglass media filters.


Microglass filter medias are made of synthetic media that is of a smaller diameter than cellulose media.

This smaller diameter media allows the microglass filter to capture more contaminants with less severe impact on fuel flow and pressure.

The increased surface area of the microglass filters also provide more surface area for the filter media to be able to remove contaminants from the fuel.

These microglass filters translate into higher efficiency filters for the fuel system, allowing optimal fuel filtration as well as an extended filter lifetime.

What is Fuel Polishing?

Fuel polishing is a fuel filtration technique used across many industries to increase and maintain fuel quality in stored fuel. Through these fuel filtration systems, various forms of contamination are removed and prevented.

Mobile Fuel Polishing

These polishing systems can be mobile units built onto carts or skids, or these systems can be mounted (sometimes in an enclosure) that is plumbed into the bulk fuel storage tank.

Graphic depicting periodic fuel polishing with a mobile fuel polishing system

The mobile polishing systems are advantageous when having to maintain a number of different fuel tanks without having to incur the financial cost of installing multiple fixed systems. Mobile systems come in a number of different sizes and flow rates, and we recommend you visit our Mobile Fuel Polishing page to view the different systems that are available.

Mobile fuel polishing may seem like the perfect solution especially if you have multiple tanks, however that is not always the case.

As these units are not fixed onto the fuel storage tank, these systems must be hauled out on a scheduled basis to maintain fuel cleanliness. The problem arises not when fuel is cleaned to the desired fuel cleanliness standard, but rather when fuel is again left to sit untreated.

This causes the fuel to fall back out of the desired cleanliness specifications where it must be polished again. This creates a cycle, which is illustrated below, that gives chance that the fuel fails to maintain the quality requirements if stringent polishing cycles are not maintained.

Graphic comparing periodic fuel polishing against automated fuel maintenance as solutions for preventing harmful levels of fuel contamination

Now, you can see where this fuel polishing cycle could turn into something that proves to be quite taxing, especially in situations where multiple fuel tanks on a specific site need to be treated on a regular basis.

Automated Fuel Polishing

Automated fuel polishing systems can be beneficial to fuel storage in facilities where frequent access for mobile polishing isn’t preferred or practical.

Our Automated Fuel Maintenance and Enclosed Fuel Maintenance systems are engineered to allow for the scheduling of periodic fuel polishing so that fuel is constantly being cycled and polished. This eliminates the concern for fuel to fall out of the desired fuel cleanliness and quality standard.

Graphic depicting the installation of an enclosed and compact fuel maintenance system for automated fuel filtration

For facilities that are reliant on backup power systems, this is immensely important. Mission critical facilities, such as hospitals and data centers, cannot risk electrical downtime in the event of a power outage.

With these facilities having large volumes of stored fuel to power the backup generators, it is important that fuel quality is maintained to ensure quality fuel is delivered to the backup power system at a moment’s notice. Any fuel quality issues could render the backup generator inoperable, putting critical systems at risk.

Levels of Fuel Filtration

Fuel polishing systems have a number of components necessary to ensure that fuel is being cleaned and contamination is removed in an appropriate manner.

Micron filtration removes clumps of fuel and other particulate that could harm equipment. By passing fuel through micron filters, contaminants such as dirt, grime, and sludge can be caught by the filter and removed from the fuel.

With water separation, free water is captured and removed from the fuel to prevent the proliferation of microbial growth. By capturing the water, fuel polishing systems effectively remove the conditions to which “the diesel bug” thrive.

Not only that, but by removing the water from the fuel it keeps the engine and fuel injection system from receiving water that could pose harm to the integrity of the equipment.

AXI International’s LG-X Inline Magnetic Fuel Conditioners are a proprietary part of our fuel polishing systems that use a magnetic field to achieve a number of things.

Through running fuel through the magnetic chamber, metallic particles and fragments are captured and thus prevented from making their way into critical engine components. These metals could be comprised of various ferrous metals or even rust.

Rust is typically a sign of the presence of water within a fuel tank. Rust can only develop where there is water, and if fuel was previously clean but rust was found during the polishing cycle of a fuel tank, chances are that there is water present as well.

The magnetic field is also responsible for breaking down clumps of diesel fuel known as agglomeration where the fuel molecules in diesel fuel, over time, naturally pull together creating thick clumps of fuel. By passing these clumps through a magnetic field, the intermolecular bonds are weakened, allowing clusters to fall apart and return to a more fluid-like state.

Fuel Additives

Fuel additives can also prove to be beneficial for those concerned with fuel contamination issues. However, with such a wide variety available on the market, it could be hard to decide which additive is best suited for your unique needs.

Fuel stabilizers as a fuel additive work in a manner that prolongs the stability of fuel in storage. These fuel stabilizers are often used in circumstances where fuel is expected to sit for an extended period of time without any fuel maintenance.

By dosing the fuel tank appropriately, this fuel additive prevents fuel from oxidizing and experiencing a chemical breakdown.

Combustion catalysts can be used to not only enhance engine performance, but also provide for a more complete burn of fuel being supplied to the combusting cylinder, which results in reduced carbon deposits. This, in turn, reduces engine emissions as less unburnt fuel is released from the exhaust system.

By increasing power output, combustion catalysts can often result in a healthier engine response.

Corrosion inhibitors in certain fuel additives prevent corrosion on metal surfaces, which prolong engine life and equipment operability. This reduces the amount of “surprise” equipment maintenance that is needed due to the failure of certain parts within an engine’s mechanical system.

The corrosion inhibitor is comprised of compounds that attach to component surfaces and form a film that acts as a lubricant which reduces engine wear and extends the lifetime of mechanical components.

We recommend AFC Fuel Additives as the go-to fuel additive to add to your fuel maintenance schedule. As the only fuel additive offering all of these features and benefits within a single formula, AFC Fuel Additive is the smart choice for your equipment. With a concentrated formula, just eight ounces of AFC Fuel Additive is able to treat 320 gallons of fuel. AFC is also available in 1 Gallon (treats 5,000 gallons), 5 Gallon (treats 25,000 gallons), and 55 Gallon (treats 275,000 gallons) quantities.


Through gaining an understanding of what diesel fuel contamination is, what causes it, how to test for it, and how to treat and prevent it, we hope to give you greater in-depth knowledge on just how critical your fuel quality is.

From lawnmowers to tractor trailers, fuel quality is something that affects everyone logistically- for it could be the reasons behind your car not running and your generator failing. Sometimes, the application is small, and fuel is simply replaced before damage is done and you are on your way again.

But, in many cases, this can be a costly solution especially when there are thousands of gallons of fuel at risk. And in the worst-case scenario, this fuel can not only be contaminated, but also further contaminate and cause detrimental mechanical issues within the equipment the fuel was being supplied to.

Engines and equipment rely on quality fuel to operate as designed, and when that standard of fuel isn’t being supplied (which is often the case), gradual wear and breakdown of components could lead to costly repairs, particularly in and around the fuel injection system.

To ensure fuel quality and mitigate the effects of contamination, it is recommended to instill fuel maintenance systems and procedures. At the general consumer level, this could mean using a fuel additive when you fuel up your vehicle. At the business operational level, this could mean installing automated fuel management systems to polish bulk fuel and prevent contamination from proliferating.

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