Liquid and Component Monitoring

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There are many points at which particles may contaminate industrial processes. In this section of the AD030 learning center, you can find technical information about particle monitoring typically performed on a wide range of liquid samples.

Explore the following technical aspects of Liquid and Component Monitoring by clicking on these links:


Process water may contain particles present in the feed water or in-line contaminants resulting from corrosion and mechanical wear. The cleaning process of manufactured components contributes to particle contamination as well. Particles such as metal filings and joint compound and lubricating residues begin to accumulate in the fluid or feed water and eventually damage the final product.

The automotive and semiconductor industries have implemented quality control procedures for fluid particle monitoring and must be certified by many of their component suppliers. Particle contamination monitoring procedures are also in place for boiler feedwater and reactor cooling waters, which must be monitored for particle contamination prior to startup and at regularly scheduled intervals during normal operation.

Aviation fuels pick up particle contaminants all along the route from refinery to the aircraft. These particles commonly consist of fine sand, metal chips, metallic oxides, gums and resins commonproducts of corrosion, scaling, vibration, and abrasion generated during the process.

Hydraulic fluids normally pick up particles as a result of oxidation and polymerization of the fluid itself during operation at high pressure and temperature. These contaminants consist of organic gums, varnishes and acids combined with other particles that can occur as a result of erosion and mechanical wear.

Sample Collection and Filtration Methods

Indirect Sampling Method

When using batch (indirect) sampling, collect the liquid in a container. Next, use the Merck Patch Test Kit to filter and analyze it on site, or send it to a laboratory for subsequent filtration and analysis. When performing indirect sampling, you must make sure that both the collection containers and the laboratory filter holders are clean to avoid unwanted contamination. This technique requires specific equipment preparation and laboratory testing. Therefore, we do not recommend it for critical applications, such as monitoring fuels, component systems, process waters or boiler water systems.

Direct Sampling Method

The direct sampling method, in which you filter your samples directly at the fluid inlet from a fuel or water line, is a simpler and more reliable procedure. The direct sampling method eliminates the possibility of introducing extraneous contamination from the sample containers.

If the liquid is under pressure (excess of 10 psi), you can filter your samples directly at the sampling point using an Merck sampler device, which contains a disposable two-piece filter cassette. Plug the sampler/cassette assembly into a quick-release sampling valve installed at the outlet of the system or storage tank. The operating pressure (100 psi max.) forces liquid through the assembly. The initial volume is directed through the valve and bypass hose to flush the sampler. The sample volume passes through the filter cassette into a graduated container attached to the sampler base.

After sampling, you may analyze the test filter on-site using a colorimetric method (Patch Test) or send it to the laboratory for more detailed analysis (e.g. particle counting or gravimetric analysis).

Gravimetric Method

Gravimetric analysis consists of preweighing a filter before use and then weighing it again after the sample has been filtered. The difference in the two weights determines the approximate particle content. Alternatively, using another method of gravimetric analysis, pass the sample through two preweighed filters placed in the same holder. For convenience, Merck matched-weight membranes are available as disc pairs or in preloaded transparent polystyrene cassettes. Each cassette contains two superimposed filters matched in weight to within 0.1 mg. As the sample passes through both filters, all the contaminants are retained on the upper test filter.

The difference in the weights of the filters after drying is the weight of the contaminant in the sample. Using matched weight pairs eliminates the need to preweigh the filters and rules out any extraneous contamination of the test filter.