Monitoring

Photoionization Detector Use  

The PID is typically equipped with a 10.2 electron volt (eV) probe.

The PID detects the presence of organic gases as well as some inorganic gases. The basis for detection is the ionization of the gaseous constituent. The incoming gas molecules are subjected to ultraviolet (UV) radiation, which is energetic enough to ionize many gaseous compounds. Several different probes, with different UV energies (8.3, 9.5, 10.2, 10.9, and 11.7 eV), are available for the PID. The 10.2 eV probe is generally the most useful for environmental fieldwork as it is more durable than the 10.9 and 11.7 eV probes, and detects more compounds than the 9.5 and 8.3 eV probes.

A PID is also easier to use than a flame ionization detector (FID) and its lower detection limit is in the low parts-per-million (ppm) range. The response time is also rapid, with the meter reaching 90 percent of the indicated concentration in 3 seconds.

Since the PID is a nonspecific total vapor detector, it cannot be used to identify unknown compounds; it can only quantify them. In addition, it must be calibrated to a specific compound. It does not respond to all compounds, specifically methane or hydrogen cyanide. Only after the PID has been properly calibrated will it be used. A background reading will be taken prior to entering each area during the fieldwork, as vehicle exhaust and other common sources of gaseous pollutants may register on the meter. While operating the PID during the fieldwork, if a reading greater than 1 ppm in excess of the background reading for that area is encountered continuously for at least 1 minute, the area will be immediately evacuated until further guidance is received from the Field Team Leader. (The 1 ppm action level may be superseded, based upon exposure limits of known contaminants of concern). The PID will monitor the atmosphere continuously during non-invasive and invasive fieldwork. After daily use, the unit will be inspected for damage and cleanliness, and repaired or cleaned as necessary. The battery will be recharged overnight, since a low battery will cause the unit to malfunction.