18
Calibration
Calibration Overview
Because the NEO BENZ has two independent operating modes, both VOC and
Benzene channels must be zero and span calibrated separately. 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.
VOC Mode Calibration:
For best accuracy the calibration gas chosen is the same as the gas to be measured. However, if this
is inconvenient, isobutylene can be used as a 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 several hundred compounds, or entered manually as a user custom gas.
Benzene Mode Calibration:
This mode always uses benzene for calibration gas, typically 5 ppm. Because the filtering tubes
affect the zero and span readings, both these calibrations must be done using a benzene filtering
tube in the holder. If zeroing is done first, the same tube can be used for span calibration.
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.
Note on Benzene
: Because flowrate and timing are critical to benzene tube
calibrations, use only a flow-matching system such as demand-flow regulator, gas bag, or T
connector for benzene calibrations. Do not use a fixed-flow regulator.
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, enter the temperature, insert an opened benzene tube,
and press Right again. The message ‘Please Apply Zero Gas’ is displayed. Press the Up key, and a
countdown begins, timed between 30 seconds and 180 seconds, depending on the temperature.
Up
Calibration
Zero Calib
Span Calib
Set Cal. Gas
Set Span Value
Set Span 2 Value
Current Temp.
+
2 5 °C
×
Please Insert Tube
Press Right Key
To Start
Up
VOC Mode
Right
