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D e t e c t i o n
User Manual
Photo-ionization detector NEO
D e t e c t i o n
13
6.3.1 CALIBRATION
Calibration Overview
Although the instrument will typically hold an approximate calibration for
several weeks, when used for worker health and safety it should be calibrated or
bump tested before each day’s use. The interval between span calibrations can
be extended based on the user’s experience for how long the calibration holds
under their operating conditions. For best accuracy the calibration gas chosen
is the same as the gas to be measured. However, this is often inconvenient, and
isobutylene has become the standard surrogate calibration gas because it has
good response and is inexpensive, stable, and non-toxic. A correction factor (CF)
can then be applied to make the NEO display in true concentration units of the
gas being measured. This CF is selected from the instrument’s gas library of over
200 compounds, or entered manually.
Calibration Set-Up
Span gas from a cylinder is conveniently supplied to the instrument using a regulator with fixed flow
rate of about 0.5L/min (500 cc per min.) to match or slightly exceed the flow rate of the instrument
pump. Alternatively, the span gas can first be filled into a gas bag (e.g., Tedlar® bag) or delivered through
a demand-flow regulator to match the pump flow precisely. Another alternative is to use a regulator
with >500 cc/min flow but allow the excess flow to escape through a T connector or an open tube.
Zero Calibration
This procedure determines the zero point of the sensor calibration curve. Connect the instrument to a
clean air source free of VOCs such as from a cylinder, gas bag, or ambient air filtered through a charcoal
tube (VOC Zeroing Tube). Most outdoor air is sufficiently free of contaminants that it can be used for
zero calibration except for measurements in the low ppb range. The air source should have an oxygen
concentration of 20.9% (or the same as in the gas to be measured) because oxygen levels have some
effect on the PID response.
Select ‘Zero Calib’ by pressing the Right key to enter the submenu. The message ‘ Please Apply Zero Gas’
is displayed. Start the zero calibration by pressing the Up key, and a 30- second countdown begins.
Note: To abort the zeroing and proceed to Span calibration, press the Left key at any time during the
process. A confirmation message ‘Zero aborted!’ will appear, followed by the Span calibration menu.
When complete, this message “Zero Calibration Done!” is shown, and the display automatically moves to
the Span Calibration menu.
