Mesh blockage often gets overlooked as a particle monitoring technique. Several articles have identified mesh blockage (or pore blockage) in its more successful role: dealing with the problematic oil samples that elude the use of light extinction units.
This article reviews the techniques of light blockage and mesh blockage, bringing back to the spotlight this useful approach to trending solid particulate levels in machines.
The Success of Light Blockage
Historically, the light extinction principle enjoyed success due to its reference in the ISO standards. As the technique was readily available, a number of suppliers provided units for either portable or laboratory applications. Because of its flexibility in sample size (usually requiring less than 20 milliliters), light extinction is the preferred technique for most laboratories.
However, many laboratories offer a particle count only on hydraulic and turbine fluids, and where conditions permit. There are several reasons for this, namely:
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Large, visible wear debris particles can block the sensor orifice (approximately 100 micrometers [µm] in size).
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Multiphase hydraulic fluids (water-based fluids, for example) are difficult, if not impossible, to count successfully.
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Samples require de-aeration after re-agitation, or careful resuspension of the particles to avoid aeration. Aerated samples, such as in flushing fluids, cause counting errors on online instruments.
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Wet or saturated samples cause counting errors due to the water droplets present. This can be overcome by a method of solvent dilution as prescribed by oil analyzers (CSI’s 5200 Minilab is one such tool).
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Dark oils or diesel engine oils heavily laden with soot may cause difficulties.
In all of these cases, these problems can be overcome. However, this comes at a significant cost in terms of time and materials. Most laboratories offer a package of tests and generally question the value of particle counting when asked specifically to perform this test. Instead, most laboratories and users use light blockage for its perceived ease of use regarding sample volumes and its compliance with ISO standard references.
Mesh Blockage Units
Mesh blockage units had been available by Pall Filtration, Rockwell Automation Entek (the former Diagnetics Contam-Alert) and a number of other manufacturers in the late 1980s and early 1990s. These units utilize a mesh to measure the particles, but use two different principles of measurement to achieve the same result.
The Pall PCM series unit uses two screens to establish a measurement greater than 5 µm and 15 µm per ISO 4406:1987 code. Based on the raw data, it can compute an estimated NAS 1638 result.
However, the Rockwell Automation Entek unit uses one measuring screen of an applicable size for the sample tested. The unit extrapolates the measured data against its calibrated curve to provide an estimated particle count, and thus an estimated ISO 4406:1999 or NAS 1638 result. The screen may be a 5 µm, 10 µm or 15 µm rating, although the 10 µm and 15 µm versions are used most frequently.
From a user perspective, the Entek device is ideally suited to condition monitoring. It can be used directly online on an oil test port with a minimum pressure of two bars. Or, it can be used with a small sample of oil, typically less than 50 mL.
Because of the need for larger volumes of fluid for analysis, the Pall unit is better used online. With its onboard pump, the unit can be used either on a tank or on a pressure line. With the added benefit of automation, it can be operated remotely by computer or PLC control. This makes it ideal for research use on test beds or for permanent connection online on a large machine or a flushing rig.
With the exception of a Pall modified unit, however, mesh blockage units typically will not report results lower than ISO -/11/8.
Best Applications for Mesh Blockage 
Mesh blockage is often recommended for roll-off cleanliness certification and condition monitoring. Consider the following scenarios:
Roll-out cleanliness certification: The user needs a unit to monitor the cleanup rates on his or her flushing rigs. When flushing, a Reynolds Number in excess of 4,000 is required. This generates aeration, typically making it unsuitable for portable light blockage units.
However, while a mesh blockage unit will suffice, an ISO referenced measurement is still required for cleanliness certification once the machine is commissioned. This is one of the reasons why the Pall unit has had much success in the original equipment manufacturer (OEM) fluid power industry, particularly in the Oil & Gas sector, where OEMs must meet a cleanliness target of NAS Class 6. The Pall unit is the tool of choice because it can be remotely controlled or can operate unattended.
Condition monitoring: A reliability engineer wants to sample a range of machines and fluid types across the plant. The mesh blockage capability allows the user to sample all types of machinery regardless of condition (wet, aerated, dark, multi-phase fluids, or heavily contaminated oils with soot or wear debris). It also allows the user to sample a range of lubricants such as new oil in tanks and drums or used oil samples.
This works regardless of the type of the fluid, such as water-based glycols and diesel fuels and other fluids such as cutting-tool fluids and water-based washing fluids. For these applications, the Rockwell Automation Entek Contam-Alert unit is better suited, allowing smaller samples (less than 50 mL) to be measured on the bench stand.
References
1. Williamson, Martin. “The Low-down on Particle Counters.” Practicing Oil Analysis magazine, July 2002.
2. Spurlock, Matt. “Particle Counting or Ferrous Density … or Both?” Practicing Oil Analysis magazine, November 2006.
3. Spurlock, Matt. “Monitoring Large Particles in Gear Oils.” Practicing Oil Analysis magazine, March 2008.
4. Fitch, Jim. “The Agony of Diesel Engine Oil Particle Counts.” Practicing Oil Analysis magazine, March 2005.
5. Walker, Robert. “On-site Analysis Saves Cedar Bayou $1 Million.” Practicing Oil Analysis magazine, July 2002.
6. “Particle Measurement Options: Laser and Mesh Blockage.” Practicing Oil Analysis magazine, July 2005.
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