PROGRAMMING OF VRAE
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4.4.1 Fresh Air Calibration
This procedure determines the zero point of the sensor
calibration curve. To perform fresh air calibration, the
calibration adapter and a bottle of “fresh” air (optional) are
recommended. The “fresh” air is clean dry air with 20.9%
oxygen concentration and without any organic, toxic or
combustible gases or impurities. If such an air bottle is not
available, any clean ambient air without detectable
contaminant can also be used. A charcoal filter should be
used if one is not sure of the ambient air’s purity.
If the unit is in VOL or Autorange mode, a second zero is
performed with the VOL zero gas, typically pure nitrogen.
This zeroing step affects only the VOL sensor and does not
modify the O
2
calibration performed in the first zero step.
In Autorange mode, the first step zeros the LEL sensor. In
VOL mode, the first step zeros the VOL sensor and the
second step is needed only if the VOL zero gas is different.
The calibration adapter links up the inlet port of the VRAE
Monitor to the regulator of the gas bottle (see Figure 4.4.1).
Figure 4.4.1 Set-up of Gas Calibration