July/August 2019

HYDRAULICS APCs count and 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. Traceability 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 d = A p *V b /A d, , where: A p Is the area of particle, A d the area of detector and V b 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 26 HYDRAULICS & PNEUMATICS July/August 2019 www.hpmag.co.uk Calibration of automatic particle counters – the latest situation The subject of Automatic Particle Counters (APCs) has 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. An example of a modern portable particle counter (Photo courtesy of MP Filtri). Figure 1. Principle of APCs