Particulate matter sample ports

To prevent the formation of reaction products from the interaction of the ozone-air sample with Alters, they arc intentionally not used at probe inlets (see Table 6-4). Some of the newer instruments, however, require Alters at the inlet of their sampling ports to prevent the particulate matter in the ambient air from fouling reaction-chamber cells or from clogging the gas-flow controllers. When the same type of Alter also precedes the calibration and zero gas sampling ports (which has not always been the practice), the problem is minimized to the extent that similar events occur during the calibration and sampling. 

Sample ports are also a key issue. While the EPA accepts five pipe diameters before and two pipe diameters downstream of the sample port, experience has shown that the recommended eight pipe diameters before and two diameters after the port improves testing accuracy. The proper lengths are important to flow measurement, but they are also critical to obtaining representative dust samples. Turbulence in gas flow will result in mass emission test results that are not representative. The particulate matter will be maldistributed after an elbow and the heaviest particles will be biased to the outside wall. Even if appropriate gas rates are collected, the amount of dust may be biased to the outside wall but collected at too small a rate. 

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