A systematic approach to particle counting

April 1, 2004
Keep an eye on contamination to ensure long life and peak performance.
The CCS 2 contamination-control system monitors the cleanliness of hydraulic and lubrication oils. It displays and prints results, and permits monitoring and control via the Internet.

Lothar Nagel
Internormen Technology
Altlussheim, Germany

One important factor in ensuring reliable performance of hydraulic and lubrication systems is monitoring the amount of solid contamination in the fluid. Particle concentration (number and size of solid contaminants in a defined volume) is a valuable barometer of system health. It lets users make informed decisions on:

  • Filter efficiency.
  • Filter configuration.
  • Meeting the cleanliness requirements of sensitive components.
  • Changes in wear and system attrition.
  • Secondary contamination affecting the system, such as damaged cylinder seals.

Several different test methods provide reliable results. One is to extract bottle samples from the system and examine them in a laboratory. Another is to take online particle counts at specific intervals or install a permanent, online particle-counting system. All have advantages and disadvantages.

Bottle sampling eliminates factors that falsify automatic particle-counter readings, such as air or water. High-viscosity and extremely contaminated fluids can be diluted prior to testing. And should it be necessary, labs can perform a more-detailed follow-up analysis.

On the downside, there is a time lag between sampling and results, and laboratory costs continue to rise. Contaminated bottles and poor sampling procedures can produce inaccurate results. And samples that reflect dynamic operating conditions can be difficult or impractical to obtain. Samples taken from the reservoir limit results to basic conditions, rather than providing specific data for different parts of the system. For these reasons, online measuring methods that provide lab-quality results have gained favor.

Online sampling permits measurements at multiple points while a system is operating. Most outside influences can be eliminated, and results are available immediately. Online oil analysis protects systems with hydraulic components such as servo and proportional valves or high-pressure pumps by warning of problems before contamination reaches damaging levels. Permanent online counting can also monitor critical or weak parts of a system, such as the lubrication oil in a turbine bearing. But online systems cannot accurately count particles in oil containing free water or gas, or in extremely dirty or viscous oil.

One such online particle counter, the CCS 2 contamination-control system, monitors mobile and stationary high-pressure and high-viscosity systems. It includes an industrial PC, color monitor, and data-management software that can display results, create Excel spreadsheets, generate printouts, and download data via RS-232 connections to an external PC.

The unit determines contamination levels by automatically counting particles passing through a laser sensor (light-gate principle). A metering unit fills a small cylinder with oil, then an electrically driven piston forces oil past the laser sensor. This ensures constant volume and near-constant flow across the sensor during each test cycle. Eight different channels count particles ranging from ≥=4 to ≥=37 µm. System pressure can range from 22 to 6,090 psi.

The system calculates the number of particles per unit volume and categorizes results according to contamination classes such as ISO 4406:99, ISO 4406:87, and NAS 1638.

The CCS 2 offers several operating modes:

Continuous measurements are taken automatically every 20 sec, with the results of the last 100 measurements displayed and saved. This method is useful for dynamic and purification processes.

Single measurements average results of three consecutive tests. This method is useful for periodically monitoring hydraulic systems. It focuses on maintenance, for example ensuring that particulate levels have not exceeded a predetermined limit.

Cycling measurements involve a three-test average at preset intervals. This is suited for automatic long-term monitoring of hydraulic systems.

Network measurement is another option. Using an RS-232 interface and a connected miniature network server, such as the Internormen MWS 01 or MWS 02, data from the CCS 2 can be integrated into local networks or transferred over the Internet. Technicians can control measurements and display fluid-monitoring results using the Web and a browser. This permits instant remote diagnosis of hydraulic-system contamination, especially useful for sensitive machinery components, inaccessible systems, and even off-shore locations.

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