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 (The 1 ppm action level may be superseded,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.