Improve air sampling by measuring mass flow

How can contaminant concentration be stated unless the sample mass is known?

The answer: it can’t. Without an accurate measure of the mass of air collected as a sample, the relative concentration of a contaminant—whether it be chemical or particulate—cannot be stated as a part per million, billion, or trillion. Even for sampling applications where the contamination is expressed as a time integrated volumetric measure, ensuring that a constant sample flow has occurred requires accurate flow metering. Often air sampling systems have used passive flow methods based on orifices, needle valves, venturi meters, or rotameters to monitor the sampling rate. However, these passive devices are highly susceptible to errors due to temperature or pressure fluctuations which often occur in real-world situations due to weather or altitude. Even the best of these passive flow meters can vary by over 10% between typical daytime and nighttime temperatures—significantly distorting the sample weighting. Further, the measurement principle for all of these devices relies on critical dimensions in very small passageways that can easily become contaminated by the sample itself. Alternatively, a mass flow meter provides for accurate flow monitoring which is independent of pressure and temperature. These instruments also provide an electronic output. The output can be used for various purposes including regulatory documentation, event tracking, and flow totalizing. For applications where flow control is useful, a mass flow controller can be used to automatically maintain the sample flow at a constant rate throughout the sample period.

Objectives

• To ensure accurate air sample flow for collection or continuous analysis
• To provide a flow signal for documentation, event tracking, or flow totalizing
• To control the sample flow despite changes in temperature and pressure

Methods

With a variety of techniques and regulated methods, the systems that accomplish air sampling take many forms. This note will focus on two of the common forms, time integrated collection and continuous analysis, with possible extension to others.

Time Integrated Collection

In the figure to the right, the components of a time integrated collection system using a mass flow controller are shown. In this case, the canister is under vacuum to begin the sampling process, thereby allowing the air sample flow to be driven by the negative differential pressure. Regulating the flow at a constant rate is accomplished by a mass flow controller (MFC) throughout the sampling process. The MFC is an active closed-loop control system. It measures mass flow and actuates an internal control valve to automatically and continuously maintain the flow at the desired rate, regardless of the atmospheric pressure, the canister pressure, and the temperature. So as the canister fills and its pressure rises, the MFC’s internal controller opens the control valve to maintain the mass flow through its sensor and into the canister. This continues until the sample collection is completed or until the pressure in the canister has risen near enough to atmospheric pressure that the flow cannot be driven at the desired rate. Should this condition occur, it would be indicated by the output of the MFC.