All meshed up – revisiting an often maligned technique (Mar 09)
Mesh blockage as a particle monitoring tool often gets overlooked. More importantly, there have been significant changes in recent years. Martin Williamson of KEW Engineering brings the spotlight back onto this approach to trending solid particulate levels in machines.
The light extinction principle (Fig. 1) is referenced in the ISO standards. There is a number of suppliers providing units for either portable or laboratory use. Sample size flexibility (usually requiring <20mL) means it is the preferred laboratory technique. Typically, laboratories only offer particle counts on hydraulic and turbine fluids, and where conditions permit: Large, visible wear debris particles can block the sensor orifice (approximately 100µm in size); multi-phase hydraulic fluids (water based fluids, for example) are difficult, if not impossible to count; samples require de-aeration after re-agitation, or careful re-suspension of the particles to avoid aeration (aerated samples, eg. flushing fluids, cause counting errors on online instruments); water droplets in saturated samples cause counting (this can be overcome using solvent dilution); and very dark oils or ‘sooty’ diesel engine oils cause difficulties.
Mesh blockage units
Pall Corp. and Rockwell Automation Entek (RAE) have offered mesh blockage units and use two differing principles of measurement to achieve the same result. (Fig. 2.). The Pall PCM uses two screens to measure the number of particles greater than 5µm and 15µm (ISO 4406:1987 code). From the raw data, it gives an estimated NAS 1638 result.
The RAE Contam-Alert uses one measuring screen of an applicable size (5µm, 10µm or 15µm) for the sample tested. The unit extrapolates the measured data against its calibrated curve to provide an estimated particle count. An estimated ISO 4406:1999 or NAS 1638 result follows.
The RAE Contam-Alert is ideally suited to condition monitoring. It can be used online or with a small sample of oil < 50mL. The Pall PCM is better used online as it needs a greater sample volume, often >250mL. Its onboard pump and regulator means the unit can be used on a tank or on a pressure line up to 315bar. With automation, remote control by computer or PLC control is possible. This makes it ideal for research use on test beds or for permanent connection online on a large machine or a flushing rig. Mesh blockage units will not typically report results lower than ISO -/11/8 unless modified.
Best applications for mesh blockage
Roll-off Cleanliness Certification – To monitor the clean-up rates on flushing rigs. Flushing generates aeration, making it unsuited for light blockage units. The Pall PCM is best suited to this need. Condition Monitoring – The mesh blockage units allow sampling of all types of machinery and resevoirs irrespective of the lubricant condition or type. The RAE Entek unit is best suited to this need.
Mesh blockage – a renaissance
There were several negative issues that stood in the way of this technique that have been recently addressed. “Mesh blockage lacks an ISO standard.”Mesh blockage gained standards recognition in the UK as BS 3406-9:1997. More recent reference is given in ISO 21018-3:2008 Hydraulic fluid power – Monitoring the level of particulate contamination of the fluid – Part 3: Use of the filter blockage technique. However, particle counting only refers to: Microscope as referenced in ISO 4407:2002 Automatic particle counters (ie. light obscuration or extinction) as referenced in ISO 11500:2008. With mesh blockage, the correct terminology is particle monitoring or trending. The reason is the technique does not individually count the particles. It determines the approximate number that have been blocked by the mesh of a given size rating.
Accuracy
“It is a trending unit so any output of data in terms of ISO4406:1999 or AS4059/NAS1638 is an estimation, especially so where one mesh is used to measure the solid particulate levels.” This is not a negative issue. In condition monitoring, the analysis depends on trending, although ‘semi-quantitive’ mesh blockage does provide a reliable and consistent trend. Increases in the solids are an early warning. Particle monitoring should be used proactively with ferrous density testing. If using the mesh blockage technique as a trending tool to proactively monitor the cleanliness, then the benefits far outweigh the lack of absolute accuracy.
“The mesh blockage units do not compare with the results from my laboratory, and it frequently gives a result of better than ISO 12/9.” The RAE unit’s calibration was set against the ISO 4402 (now ISO 11171) as used for light blockage counters, using ACFTD (Air Cleaner Fine Test Dust). Owing to this, and that most users took bottle samples, it was more likely to compare with laboratory samples. Inconsistent results were still possible if the screens had not been properly cleaned, or the user followed the incorrect test procedures. With regular validation checks and training, this risk could be eliminated.
Pall’s unit was factory calibrated against real-world samples measured as per ISO 4407 (Microscope). The recorded data was analysed for a correlation factor. This factor is set in the firmware, and validated during the build and subsequent annual servicing. The Pall unit requires large volumes for analysis; thus it produces much cleaner results than small samples sent to a laboratory. Naturally, users tended to question the Pall instrument. Any light blockage unit used in the same way as a mesh blockage unit online, with similar volumes of sample fluid would produce similar results to the Pall PCM. If using the mesh blockage technique as a trending tool to proactively monitor the cleanliness, then accuracy is not an issue. Instead, good repeatability and reproducibility of the results is needed which the mesh blockage units offer.
Commercial issues and costs
“The technique is commercially controlled by two companies.” Now that both patents have expired, the technique will attract more suppliers to consider it. “Servicing is costly and takes too long.” Pall’s supplier and servicing company, Industrial Monitoring Systems (IMS), is based in Bradford, UK. RAE’s facilities are in Columbus, OH, in the US. IMS has been building and servicing the PCM units for Pall since its launch in 1994. IMS can now be contacted directly for Pall PCM servicing. IMS aims to improve the service arrangement, with better turn-around times. By operating directly, the cost of the service has also been cut significantly.
“Considering it’s not a particle counter the cost of the units are similar to light blockage units.” The RAE test kit offers Viscosity and Ferrous Density in addition. Pall’s PCM 400 Series also offers Viscosity and Relative Humidity. When considered as three results, the cost is significantly less than a comparable portable light blockage counter. In addition, IMS has recently designed a lower-cost unit for Pall, badged the PCM 200. While lacking the ease of portability and features of the bigger PCM 400, it does provide a simpler and lower cost unit for potential online application.
If you haven’t given mesh blockage much thought, then I would recommend that you should take a closer look. However, if you are still using the older RAE and Pall PCM 100 Series units for condition monitoring, I would suggest you take a look at the newer units on the market that offer more for your analysis on site. If nothing else, make sure your unit’s service and validation is up to date.
References:
NAS 1638 ‘Cleanliness requirements of parts used in Hydraulic systems’, Aerospace Industries of America, Washington D.C., USA, 2001.
ISO 11171 ‘Hydraulic fluid power – Calibration of automatic particle counters for liquids’, International Organisation for Standardisation, Geneva, Switzerland, 1999.
ARP 598 ‘The determination of particulate contamination in liquids by the particle count method’, SAE, Warrendale, Pa., USA, December 1986.
ISO 4406 ‘Hydraulic fluid power – Fluids – Method for coding level of contamination by solid particles’, International Organisation for Standardisation, Geneva, Switzerland, 1999.
AS4059 ‘Aerospace fluid power – cleanliness classification for hydraulic fluids’, SAE, Warrendale, Pa., USA, April 2001.
ISO 4402 ‘Hydraulic fluid power – Calibration of automatic count instruments for particles suspended in liquids – Method using classified AC Fine Test Dust’, International Organisation for Standardisation, Geneva, Switzerland, 1991.
ISO TR16386 ‘Hydraulic fluid power – Impact of changes in ISO fluid power particle counter, contamination control and filter test standards’, International Organisation for Standardisation, Geneva, Switzerland, 1999.
Suppliers:
Pall Corp
Rockwell Automation Entek
Servicing:
Pall Units – Contact Industrial Monitoring Systems
Diagnetics/Entek units – Contact RA Entek
Light Blockage.
Mesh blockage.
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