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Introduction
A standard of isobutylene at a known concentration may be used for
setting the sensitivity. If the Multi-PID 2 is calibrated with isobutylene,
it displays concentrations in units equivalent to ppm of isobutylene. If
isobutylene were the only photoionizable chemical in the sample,
then the Multi-PID 2 would display its concentration directly.
The Multi-PID 2 responds more or less readily to other chemicals than
it does to isobutylene. Because it has a medium sensitivity to isobuty-
lene, this gas has been chosen as a reliable means of reporting an
average concentration of total ionizable compounds present.
Gases other than isobutylene can be used to calibrate the Multi-
PID 2. However, all response factors are based on an isobutylene
calibration.
Photoionization Detector
The photoionization detector (PID) is shown in Figure 3. The PID mea-
sures the concentration of photoionizable chemicals in the gas
stream from the sample inlet and produces an electrical signal for the
microprocessor.
A UV lamp generates photons which ionize specific molecules in the
gas stream. The permanent air gases (argon, carbon dioxide, nitro-
gen, oxygen, water vapor, etc.) require a relatively high energy for ioni-
zation, and are not ionized by the UV photons. Many of the
compounds considered pollutants, including most hydrocarbons, are
ionized.
The gas stream is directed into the PID through a small port at the
center of the UV lamp window and through a series of larger ports
around the perimeter of the lamp window. This arrangement permits a
high sample flow rate and short response time.