What is in this article?:
Today's emphasis on pollution prevention and waste stream minimization has made the benefits of contamination control programs for hydraulic fluids even more compelling.
How can you check on the status of the hydraulic fluid in an active system? Not too long ago, you had to take samples of fluid from the system, send them to a laboratory, and wait for a report. Today, a variety of on-line monitors is available for in-house measurement of particulate levels and water concentration. Some monitors can be interfaced with fluid-purification devices for automated operation. When the monitor detects that preset threshold concentration limits for particulates and/or water have been reached, the contaminated fluid is directed into the purifier - with no operator intervention required.
On-line particle counters or contamination monitors are used most commonly to measure particulate levels in fluids in installed systems. Particle counters provide an estimate of the particle size distribution, i.e., particle size vs number of particles, for several preset size ranges (usually six to seven). They operate on the principle of light obscuration by particles in the fluid-sample stream. Portable models, Figure 1, can be moved to multiple locations or installed at a single location for continuous on-line sampling. The contamination level typically is quantified in terms of the fluid cleanliness code (ISO 4406) or fluid cleanliness classes (NAS 1638 or ISO 11218).
Contamination monitors operate on the principle of mesh blockage by particles in the fluid-sample stream. They provide an estimate of the fluid cleanliness level in terms of cleanliness codes or cleanliness classes. Contamination monitors are the preferred choice for systems where the fluid properties prevent the use of light-obscuration particle counters: dark fluids that transmit light poorly or fluids containing air bubbles or emulsions.
Typical on-line water monitors, Figure 2, are adaptations of devices that measure relative humidity. They indicate the percent saturation of water in the fluid - free water is 100% saturation - and the temperature. Note that the saturation level for water in hydraulic fluids depends on the specific fluid composition (both base stock and additive package), the actual condition of the fluid, and its temperature. The same type fluid, formulated by different manufacturers, could differ in water saturation levels. Likewise, the saturation level of the same fluid could differ over a period of time. Thus, correlation of the absolute water concentration, i.e., ppm concentration, with percent saturation, requires determination of the absolute water concentration in the specific fluid in question. In view of the above, it is most convenient to specify water concentration limits in terms of percent saturation.