INFICON 2020ppbPRO Portable Photoionization Detector - Manual (Page 40)

34 ISP-074-576-P1D 34
A standard of isobutylene at a known concentration may be used for setting the
sensitivity. If the 2020ppbPRO 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 2020ppbPRO would display its
concentration directly.
The 2020ppbPRO responds more or less readily to other chemicals than it does to
isobutylene. Because it has a medium sensitivity to isobutylene, 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 2020ppbPRO. However,
all response factors are based on an isobutylene calibration.
Photoionization Detector
The 2020ppbPRO ’s PID is shown in Figure 3. The PID measures 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, nitrogen, oxygen, water vapor,
etc.) require a relatively high energy for ionization, 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.
Figure 3. Photoionization Detector
The ionized molecules in the detector cell are subjected to a continuous electric field
between the repeller electrode and the collector electrode. The ions move in the
electric field, generating a current which is proportional to the concentration of the
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Lamp Power
Supply
Lamp Driver
Circuit
Microprocessor
Light Sensor
Electrometer
+125 Volts
Sample In
Detector Cell
UV Lamp
Lampholder
High Voltage
Plates
Sample Out
UV Lamp
UV Light
Sample
Ionized Molecule
Collector Electrode
Repeller Electrode