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The subject of Automatic Particle Counters (APCs)

APC’s have occupied the attention of users, designers and experts over the past 35 years or so, in an attempt to improve accuracy and traceability of the measurements from these instruments. At the last meeting of the ISO group responsible for drafting standards for the fluid power industry (ISO/TC131/SC6 Contamination Control), the project leaders gave an update of the project and explained how potential confusion can be averted. Mike Day of CMS Consultants Ltd was at that meeting as part of the UK team and has prepared this article.

APCs count  nd size the very small particles that can be present in a wide range of fluids in various industries e.g. fluid power, lubrication and fuel to name most, so that their numbers can be controlled. The size parameter is the µm. APCs were first introduced in the early 1970’s and have since proved to be indispensible for the control of particulate contamination in both service and associated process fluids. The principle is seen in Figure 1; the passage of a particle through the sensor reduces the amount of light received by the detector and this produces an output voltage pulse, the value of which is proportional to the size of the particle and is obtained by calibration.


As decisions are often based on the value of the data, it is essential that it is both correct and valid otherwise incorrect decisions will be made. This will waste valuable time as well as costing money. Important aspects in this is calibration and traceability of the measurement and the development of these are detailed below:

In the early models (1970s), the various channels of the were adjusted to ‘trigger’ when a voltage pulse was fed into the instrument which represented the passage of a spherical particle sized d µm. This was based upon the ratio of the projected area of the particle to the area of the detector, thus V = Ap*Vb/Ad,, where: A Is the area of particle, A the area of detector and Vb is the base voltage of the instrument. This was a theoretical calibration with little traceability.

Also in the 1970’s researchers at Oklahoma State University in the USA developed a calibration method for APCs based upon A.C. Fine Test Dust (ACFTD), which was then being used to test filters and it was a logical development to use this as a calibrant. Identical samples of a suspension of ACFTD were prepared and the number of particles were sized and counted using an optical microscope with the longest dimension as the sizing parameter. This was adopted as ISO 4402 [1] in 1977. Whilst this method offered standardisation, there was virtually no traceability to national standards of measurement and batch to batch control of the ACFTD was not good enough and lacked the consistency that was necessary for a reference material. Perhaps of greater importance was that the size vs. number relationship was shown to be incorrect in the ‘80s.

Also in the late 1970s, two calibration methods were developed using latex spheres. One used mono-sized spheres (ARP 1192) which had traceability to national standards and the other used distributed latex spheres (CETOP RP94H) which was a secondary calibration and loosely traceable to a mono-sized calibration standard. These two methods were dismissed by ISO

/TC131/SC6 as the carrier fluid was water with a different Refractive Index to the fluids used by the hydraulics industry and both gave variable results.

In 1997, ISO were informed that ACFTD would no longer be available and had to develop  an  alternative.  The  alternative chosen was SAE 5-80, a test dust of identical material to ACFTD, but with significantly less numbers  of  particles  below  5µm, which lessened coincidence errors in the APC. This was renamed ISO Medium Test Dust (ISOMTD) and formed part of a family of test dusts defined in ISO 12103 [2]. The particle size distribution (PSD) of calibration samples (SRM 2806) was certified by the National Institute is Science & Technology (NIST) in the USA  using a Scanning Electron Microscope  (SEM)  with  Image Analysis software, and traceability was obtained. The PSD of the dust was obtained by sizing in the particles in the sample on the basis of the diameter of a  sphere  whose  area  was  the  same  as the particle, hence the term equivalent spherical diameter’.

Unfortunately, this new measurement gave slightly different counts to the previous ACFTD calibration and, to lessen the effects on industry, ISO selected the measurement size that would give notionally the same particle count had the APC had been calibrated using ACFTD to ISO 4402. This is seen in Table 1 As the equivalent sizes were often decimal sizes, the nearest integer size was selected to simplify communication, as seen in Table 1. To distinguish between the ‘old’ and ‘new’ calibration data, the new size was labelled µm(c).

This new method of calibration has been published as ISO 11171 [3] and gives a more accurate and traceable calibration. This will enhance both the repeatability of measurement with the same instrument and the reproducibility of measurement between different instruments and laboratories. The standard also included procedures for determining the performance and accuracy of the APC.

The latest situation

The number of people using the NIST SRM 2806 calibration samples has increased subtantially the last 20 years and there is  a limit to the numbers of suspensions that can be produced in    any one batch. Thus, re-certification is necessary when a new batch is produced. As the first two batches (SRM 2806  and 2806a) were produced from the  same  material,  recertification was not necessary, and the original certification applied.

The next batch (SRM 2806b) was certified using an improved SEM with automated analysis to improve both the speed of analysis and its accuracy. A 10% increase in particle size was observed and this caused a shift in the calibration curve, for example, 10 µm(c) particles became 11 µm(b) particles. This 10% shift in particle size has a significant impact on observed particle counts, and reported filter removal ratings. To compensate for this a size correction factor of 0.898 has to be applied to correct a 2806(b) calibration to a 2806(c) and µm(c),

i.e. µm(c) sizes =µm(b) x 0.898. Unfortunately, the latest version of ISO 11171: 2016) – which is still current – was somewhat ambiguous and it gives two options to obtain the calibration curve which will report different results on the same sample. This is in the process of being revised.

Note that it is inevitable that there will be differences in the sizing of these particles during certification because the angular shaped particles will adopt different orientations on the analysis membrane and thus present a different area to the detector. This is despite the extraordinary lengths that NIST go to in order to control variability.

ISO TC131/SC6 recently took a decision to ‘standardise’ on µm(c) as the measurement standard in ISO 11171 and all subsequent editions will feature this. SC6 have also instructed NIST to develop a ‘Consensus Standard’ which will simply give the numbers of particles per mL at the µm(c) size in the calibration suspension, thus greatly simplifying the issue.

What to do about your current calibration.

The actions necessary will depend on the  calibration  method used. If the calibration uses µm(c) then there are no actions necessary as this is the standardised unit now and in the future; both historical data can be compared and current specifications  can be used. If µm(b) calibration is used, it:

The actions necessary will depend on the  calibration  method used. If the calibration uses µm(c) then there are no actions necessary as this is the standardised unit now and in the future; both historical data can be compared and current specifications  can be used. If µm(b) calibration is used, it:

  • will give increased counts compared to µm(c) data
  • may lead to failure to meet µm(c) specifications may result in extra time and money being expended to determine if the increased counts are due to real issues
  • will give discontinuities with historical data
  • may cause investment in testing to validate revised specifications for entire size range
  • necessitate a repeat of the above factors when both subsequent batches of SRM 2806(x) and ISO 11171 revisions are released So, it is recommended that users of SRM 2806(b) calibration data convert the µm(b) sizes to µm(c) using the conversion factor in ISO 11171:2016, i.e. 0.898, to give the equivalent decimal µm(c) sizes. The calibration threshold values can be plotted against the decimal µm(c) values to obtain the µm(c) calibration curve for that calibration and the integer µm(c) sizes obtained by interpolation. Alternatively, a curve fitting routine can be used for more accurate interpolation. The ISO committee recommend the constrained cubic spline interpolation method developed by C.J.C. Kruger [4]. This curve fitting routine can be used to obtain the revised µm(c) calibration data and also to obtain the particle count data on the basis of µm(c) calibration  to see if the SRM 2806(b) data exceed earlier specifications.

Future calibrations

As stated above, NIST are working on the so-called ‘Consensus Standard’ so that future calibration samples will be certified to    the µm(c) base and calibration. Thus, it will be a simple matter     of setting the APC to record the certified numbers at the sizes required that are given on the NIST certification sheet. This will greatly simplify the process of APC  calibration,  overcome possible confusion caused by a recertification  process,  improve the supply of calibration samples and avoid running out  of samples. It will also greatly reduce the amount of time spent by members of the ISO Working Group that has to be involved with the validation of any new batch of suspension material.


  1. ISO 4402: ‘Hydraulic fluid power – Calibration of automatic count instruments for particles suspended in liquids – Method using classified AC Fine Test Dust’, International Standards Organisation, Geneva, Switzerland, 1991.
  2. ISO 12103-1: ‘Road vehicles – Part 1: Arizona test dust’, International Standards Org., Geneva, Switzerland, 2016.
  3. ISO 1 171: ‘Hydraulic fluid power – Calibration of automatic particle counters for liquids’, International Standards Org.,

Geneva, Switzerland, 2016

  1. C.J.C Kruger: ‘Constrained Cubic Spline Interpolation for Chemical Engineering Applications’, Korf Technology Ltd, Ontario, Canada, 2003,


Phil Keep has won the MCI’s Lifetime Contribution Award in recognition of his long and successful career in the industry.

MP Filtri’s Managing Director received the award from TV personality Penny Smith and former BFPDA council chairman Peter Wilson.

The award recognises Phil’s success in transforming MP Filtri UK from a fledging distribution company into an international market leader in the development of contamination control technology and the sale of hydraulic filtration products.

Under his guardianship, UK sales have increased more than one hundred fold – and by seizing key opportunities to enter new market niches, the company now designs, manufactures and sells a comprehensive range of innovative contamination monitors and particle counters.

The company broke the £10 million sales barrier for the first time in 2018, marking a major milestone in its growth, and is preparing the next phase of its expansion with a move to a purpose-built high-tech, state-of-the-art production facility.

The BFPA paid tribute to Phil’s outstanding contribution to the industry saying:

“Phil is a much admired and popular individual with both his friends and employees alike and is a key supporter of the FPIGS Golf Society. Never missing an opportunity to further the best interests of the industry, he also contributes to a range of domestic and international technical committees and has been a long-standing member of the BFPA Board of Directors.”

MP Filtri’s next-generation LPA3 portable particle counter is now available.

The culmination of an exhaustive three-year research and development programme, the LPA3 offers a market-leading feature set, outstanding speed and accuracy, and is fully portable – enabling operators to enjoy the functionality of the lab even when out in the field.

Featuring innovative optical and photodiode technology providing complete 8-channel measurement, the new LPA3 delivers a fast and accurate comprehensive hydraulic health check – while its real-time monitoring and proactive maintenance technology safeguards machinery, enhances performance and productivity, and reduces both costs and unplanned downtime.

Designed for ease of use, the LPA3 enables first time operators to get up and running in minutes – without the need for a dedicated training programme. The machine’s sophisticated functionality sets new standards in the industry but is surprisingly simple to master – saving customers time and money in training costs.

Key features and benefits of the new machine include:

  • A programmable 10.1” full colour touch-screen display
  • High-speed sample times up to 15 times faster than the LPA2
  • Perfectly portable weighing in at just 10kg
  • Greater storage capacity -results from up to 4000 tests retained in memory
  • Robust and durable co-polymer body case
  • Sophisticated yet simple to use software developed in-house specifically for the LPA3
  • Smooth learning curve – the LPA3 can be mastered quickly without the need for formal training
  • Instant results download via USB
  • Proactive maintenance technology – identifying risks before they impact performance
  • Online real-time monitoring
  • Long-life lithium ion battery
  • Optional thermal printer for an instant hard copy of results
  • Slashes costs and downtime, protects machinery and rapidly pays for itself.
  • Accuracy to ± 1/2 code for 4, 6, 14μm(c)
  • Compatible with MP Filtri’s range of bottle samplers

Not only is the LPA3 designed to thrive in a wide range of sectors and applications, the machine can

also be personalised for peak performance with operators able to tailor: sample volumes, flush sizes and the number of tests run concurrently. The LPA3 is even capable of completing the 100ml sample test in around one minute.

The LPA’s new full-colour display is a highly responsive touch-screen with no need for a stylus; and features a customisable home screen which displays key performance information at-a-glance.

Designed for professionals, the new LPA3 features a comprehensive range of reporting formats, so

wherever an operator is working and whatever standards used, the LPA3 has it covered. Reporting standards include: ISO 4406; NAS 1638; AS4059 (various); GBT14039; GJB420B and more

Discover more about the LPA3, here >

MP Filtri has launched its new DEH series of ATEX clogging indicators.

Designed to thrive in hazardous work environments, the indicators provide critical early warnings for operators, alerting them that filter elements need to be cleaned or changed.

Suitable for a wide variety of applications from oil, sea and gas to industrial production plants, the indicators have been designed to prevent machinery failures, reduce unplanned downtime, and improve safety.

MP Filtri’s New Product Development Manager Kris Perks said: “The indicators will save operators time and money by protecting complex machinery and extending its lifespan – as well as boosting the performance of the filters and minimising the frequency they will need to be replaced.”

The new DEH series features three different models each with a distinct connection type.

Key features of the new DEH Series include:

  • Construction from AISI 316 Stainless steel
  • 420 Bar (6,091 PSI) maximum working pressure
  • Approved for use in hazardous environments
  • ATEX, IECEX, UL, CSA, and TRCU EAC certification
  • Compact and reliable design
  • Fully tested to one million cycles at maximum working pressure


The full range of applications the DEH series is designed for includes:

  • Marine
  • Mining
  • Petrochemical
  • Offshore oil and gas
  • Saw mills
  • Paper mills
  • Car plants
  • Industrial plants
  • Storage silos
  • Hazardous environments

It is desirable to assemble any component or system with clean parts in a controlled manufacturing environment. However, this may not always be possible. It is sometimes necessary for the entire hydraulic system to undergo a cleanup process after final assembly to reach the desired roll-off cleanliness level. This article provides a theoretical calculation of appropriate flushing requirements. Adherence to established roll-off cleanliness levels will provide the OEM with a better product and fewer warranty claims.

Experts estimate that 75 percent of hydraulic component and system failures are caused by contamination. Contamination causes premature wear and lost efficiency which can result in catastrophic failure. Typically, sources of contamination can be characterised as:

Inadvertent contamination left in the system or a component during initial assembly or a system rebuild. Examples include weld splatter and cleaning rag fibres.

Contaminants internally generated during system operation, or caused by wear, corrosion, agitation, oxidation or fluid degradation.

Externally introduced contamination that enters a system from various openings such as breathers, worn cylinder wipers, improperly sealed access covers, etc.

This paper discusses built-in contamination, specifically particulate contaminants, and how to clean up the system following final assembly. Typically, particulate contaminants include weld splatter, dust, fibres, paint chips and other undesirable and potentially abrasive particles. Many of these particles are below the human visual threshold of 40 microns. Although they cannot be seen, they can be damaging to a system

Roll-off Cleanliness
The main purpose of roll-off cleanliness is to minimise damage to the various system components in their infancy. To underscore the importance of establishing roll-off cleanliness standards, the International Organisation for Standardisation (ISO) is developing new standards outlining the cleaning of components and systems. One draft standard, ISO/WD 16431, describes “roll-off cleanliness of an assembled hydraulic system upon release from the production area.” This title may change as the document is finalized, but it is obvious that the target is to provide the cleanest possible equipment to the customer.

Cleaning Methods
There are many ways to clean a system, and it is up to the manufacturing group of a company to decide which method(s) to use. The ultimate goal is to reach the desired cleanliness level at the most reasonable cost and minimum time interval. Some methods of achieving this are:

  1. Let the system run through its normal operating cycle and allow the system filter(s) to clean the fluid. The system will operate at low pressure during the cleaning/flushing process. The main advantage of this method is simplicity. However, one disadvantage is that the system filter(s) might not have sufficient dirt-holding capacity to last through one cleaning. Several element changes may be necessary to clean a dirty system. This method may also damage system components if the initial contamination level is too high.
  2. Use a filter cart, sometimes referred to as filter buggy or kidney loop (following the idea of kidney dialysis). This mobile, self-contained unit filters the fluid off-line using its own pump, motor and filter. It is designed to operate at a low pressure, usually less than 100 psi. The best way to use this device is to attach its suction and return hoses to the reservoir with fluid fittings and let it run while the system is running at a low pressure. Oil returning to the reservoir from the return line will now be filtered through the filter cart. This off-line process supplements system filter(s) and decreases cleanup time. This method may also damage system components.
  3. Design an off-line filter that can be attached to the system at system pressure. It can be connected to the system in such a way that it becomes the power supply. The equipment can be cycled using hydraulic power from the off-line system. The main system does not need to be run except to remove oil from the lines that are not in circulation. The cart flow is not required to be as large as the system flow. The idea is to cycle the system for flushing purposes but not necessarily as fast as normal operating speed. This method minimises damages to system components.

It is not economically feasible to remove all contaminants from a system. Most systems operate trouble-free with a small amount of contamination present. The amount of contamination that can be tolerated in a system depends upon the sensitivity of the most critical component. System reliability continues to improve, however, as ideal conditions are reached. Diminishing returns on increasing effort is the limiting consideration. This threshold for the contamination level is established by the component manufacturer and ultimately by the system builder.

The size and type of filter used are important in making calculations for cleaning a system. The analysis presented here makes use of the following assumptions:

  1. Contaminants are uniformly distributed in the fluid.
  2. During roll-off cleaning, no additional contaminants enter the system and no contamination is ingresses.
  3. The filter exhibits uniform efficiency throughout its working life.
  4. The filter does not go into bypass. If it does, the element is replaced. In order to avoid filter element change during the roll-off cleanup, the filter must be adequately sized. It has been observed that contamination may inadvertently be added to the system during element changes.

Generally after a hydraulic system has reach the required cleanliness level, the system has been running for a significant time and  at working temperature.  The hydraulic media and the particulate contamination are Homogeneous. (Substance in which components are evenly mixed, particles and hydraulic media). So when the hydraulic system is shut down, the particles settle out in the reservoir, hydraulic components and system pipework. When start up is then initiated the particle and hydraulic media are not Homogeneous. This can lead to initial high levels of contamination for a significant length of time until the hydraulic application is back up to temperature and the particles and hydraulic media are once again Homogeneous.

Real applications will vary from this idealisation to some degree, but the variation is not expected to significantly affect the results.

Proper roll-off cleanliness procedures protect equipment in its infancy and provide for fewer warranty claims. The end-customer is provided with a high-quality system with clean components that meet his initial use needs. Roll-off cleaning however, is only the starting point for trouble-free system operation. The final responsibility in controlling contamination lies with the user. Users must maintain proper filtration and practice responsible contamination control in the system to keep the hydraulic fluid clean.