Air Monitoring Equipment

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Filter Holders

Merck 47 mm stainless steel filter holders are designed for sampling large volumes of particles in air, such as those found in a large, dust-containing environment where protective clothing would be worn. These filter holders are available in an open configuration for maximum unimpeded flow in open atmosphere sampling, or a closed system with an inlet dispersion chamber for optimum particle distribution on the filter. In situations requiring extremely precise and/or sensitive particle detection, the filter holder must be precleaned and the filter precounted for background particles.

A 37 mm monitoring cassette is best for monitoring low particle volume in a clean environment. These cassettes have been precleaned during assembly with the average surface particle background count supplied with each lot.

To eliminate the need for preweighing test filters, matched-weight cassettes are also available for gravimetric analysis. Each of these cassettes contains two superimposed filters matched in weight to within 0.1 mg. These filters are available in 47 and 37 mm disc sizes.

While there are a variety of different sampling methods, the two basic air sampling procedures are personal sampling to determine individual worker safety and area sampling to determine the safety of the entire work environment.

Flow-Limiting Orifices

The flow-limiting orifice is a simple way to control flow rate. Insert the specific orifice into the threaded outlet (vacuum-connected) end of either a 47 mm filter holder (stainless steel) or an aerosol adapter (stainless steel) when using a 37 mm contamination cassette. When you apply the required level of vacuum, air flows through the filter and orifice at a constant rate. The amount of vacuum required to maintain the correct flow rate for each orifice available is listed in Table 1. The applied vacuum must be equal to or greater than the specified level. The orifice is available in a two liter/min configuration for a constant flow rate, or a set of inlet pressures to monitor a range of flow rates.


The filters required for each air monitoring application depend on the contaminant. Pore size, filter compatibility and analytical method all play a part in filter choice. All Merck filters display high particle collection efficiency over a broad range of particle sizes.

The most recommended filter is a 0.8 µm MF-Millipore™ cellulosic filter (filter code AA), which has been shown to retain essentially 100% of all airborne particles (>99.99%). Also, the binderless glass fiber (depth) filter has been shown to have a retention efficiency of >99% for a 0.3 µm aerosol of dioctylphthalate (DOP). We also offer PVC, PTFE, nylon, and silver filters to fit specific applications.

For published NIOSH, OSHA and ASTM procedures refer to the online guide to regulated analytical methods to find the recommended filter.

Gas Lines

Gas Line Filter Holders are the best choice for
in-line gas monitoring
The advantage of this holder is that you can remove the filter for further analysis without removing the holder from the line
Merck:/Freestyle/BI-Bioscience/Filters-Particle-Monitoring/A030 Images/particle-monitoring-09v2.jpg Merck:/Freestyle/BI-Bioscience/Filters-Particle-Monitoring/A030 Images/particle-monitoring-10v2.jpg
In-line sampling from compressed gas bottle with the 25 mm gas line filter holder.
Removal of filter for analysis from gas line filter holder.

Click for details on other filter holders

Sampling Frequency

Gas lines should be checked monthly or quarterly, depending on usage. You should also check gas lines during each line transfer to prevent cross contamination. For troubleshooting, monitoring for short periods (few hours) may be adequate. At the other extreme, you may change and analyze filters every three months on clean gas streams. A gas line filter prevents particle contamination in a clean outlet stream, which can be critical for highly sensitive instrumentation, such as gas chromatography or atomic absorption spectroscopy.