Pid Theory And Definitions - MSA ALTAIR 5X PID Operating Manual

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PID Theory and Definitions

To support the safe and effective operation of the ALTAIR 5X PID, MSA believes operators should
have a working knowledge of how the device functions, not just how to make it work. The information
presented in this section supplements the hands-on operational instruction provided in the rest of the
manual for the PID.
PID Theory
A photoionization detector (PID) uses an ultraviolet lamp to ionize the compound of interest. A current
is produced in proportion to the concentration of the VOC present, and the concentration of the
compound is shown on the device display.
Fig. 1 Typical photoionization sensor design
1 Molecules of Interest
2 High energy Vacuum Ultra Violet (VUV) radiation
3 UV lamp
4 Electrodes
Zero Gas
Zero gas is a reference gas used during calibration to zero the device. When a zero gas with no hydro-
carbon content is introduced to the device, the detector will still respond with a small signal. This signal
is a result of secondary background processes. During calibration, zero gas is applied to quantify the
background ionization current.
When only measuring concentration changes relative to a reference ambient environment, fresh air
can be used as the zero gas. When background hydrocarbon vapors are present, MSA recommends
using zero gas air.
5 Electrodes
6 Amplifier
7 DC Source
8 Microprocessor
ALTAIR 5X PID
PID Theory and Definitions
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