INFICON 2020ComboPRO Portable Photoionization Detector - Manual (Page 70)

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The initial prompt asks if the user wishes to calibrate the 2020ComboPRO. The user
can choose to calibrate the unit or simply start the monitoring activity. The display
will next alert the user that tube life is used up, and to then continue. The user is
then directed to insert a pre-filter tube and to then continue.
For analysis of benzene concentrations, press the sample key. An air sample is
drawn into the benzene pre-filter tube and 2020ComboPRO for 45 seconds. At the
conclusion of the sample, the benzene concentration will be presented on the
display, in addition to STEL and SMAX readings.
Calibration Technical Description
Periodic calibration is required to compensate for the 2020ComboPRO output
changes due to inlet filter restriction, ionization chamber contamination, lamp
cleanliness, pump wear and other factors.
During calibration, the 2020ComboPRO is first exposed to zero air. A small (zero)
signal is generated. This zero signal is stored by the microprocessor.
The 2020ComboPRO is next exposed to span gas. This span gas signal is stored by
the microprocessor. The microprocessor subtracts the zero signal from the span gas
signal and divides the difference by the user-entered span gas concentration. The
resulting sensitivity is stored in the selected Memory Slots with the zero signal and
the alarm levels. This number is then multiplied by the response factor and
displayed.
The 2020ComboPRO readings are always relative to the calibration gas. After
calibration with isobutylene, the 2020ComboPRO will respond directly in units
equivalent to isobutylene. Most volatile organic compounds will be detected by the
2020ComboPRO. It cannot distinguish between isobutylene and other ionizable
compounds. A reading of 10 ppm indicates all ionizable compounds that are present
have generated an ion current proportional to 10 ppm of isobutylene. The reading is
actually 10 ppm isobutylene equivalent units. The 2020ComboPRO readings give an
indication of the total ionizables present and their concentration relative to the
calibration gas.
Calibration Introduction
The 2020ComboPRO must be calibrated in order to accurately display sample
concentrations. First, a supply of zero air, which contains no ionizable gases or
vapors, is used to set the 2020ComboPRO’s zero point. Then, a calibration gas,
containing a known concentration of a photoionizable gas or vapor, is used to set the
sensitivity.
Clean indoor ambient air may be substituted for a cylinder of zero air. Due to the
2020ComboPRO’s sensitivity, outdoor air is usually unsuitable for calibration
unless the activated charcoal filter (F1760176) and filter tube holder (A1790500) is
used.
Commercially sourced zero air should not have more than 0.1 ppm total
hydrocarbons (THC).