Specialist advice should be sought for applications in extreme air velocities.
3.6.6 Operational Restrictions
Methane gas concentrations above the specified range may cause incorrect operation due to
limitations of the catalytic bead. The catalytic sensor will latch the reading to 5% v/v when the gas
concentration exceeds 5% v/v and will require the user to externally verify the ambient gas
concentration is within the measurement range prior to resetting the latch using the digital latch
command.
After a catalytic sensor has exceeded its measurement range, it is recommended that a bump test be
performed to verify the integrity of the apparatus.
Field bump tests and calibrations using portable gas applicators are susceptible to external air
velocities and pressurisation. These phenomena lead to incorrect concentrations of gas being applied
to the detector during calibration, which may result in under reporting the environmental concentration
of gas. The GG2 Calibration Cup and procedures detailed in this manual have been designed to
prevent errors in the calibration process. Use of any calibration cup not detailed in this manual is
prohibited.
3.7 NDIR Sensors
3.7.1 Methane and Carbon Dioxide Gas Sensors
NDIR Gas Sensors work on the infrared gas absorption principle. An infrared light source illuminates
the sensor through a gas pathway with an optical filter that selects the appropriate wavelength for the
gas being detected; the presence of this gas reduces the amount of infrared energy reaching the
detector. This difference in infrared energy with and without gas is used to measure the amount of gas
present, and is converted to a digital value by the integrated sensor electronics. The GG2 Detector is
temperature compensated and provides a linear output.
3.7.2 Humidity
NDIR sensors cannot operate in a condensing atmosphere. In such an environment, a thin film of water
can form across the membrane, effectively sealing it and stopping the passage of gas into the sensor.
On evaporation of this water, the sensor usually resumes normal operation. Above 90% R.H. the
sensor accumulates water vapour and may form condensation to block the infrared path used for gas
detection. Provided the exposure to these extremes of humidity has not been for a long period, the
sensors will recover when exposed to R.H. in the range 15% to 90%.
3.7.3 Pressure Effects
The NDIR sensors do not exhibit a permanent response to changes of pressure but respond to
pressure on a directly proportional basis.
3.7.4 Air Velocity
NDIR sensors are insensitive to typical air velocities.
3.7.5 Operational Restrictions
Field bump tests and calibrations using portable gas applicators are susceptible to external air
velocities and pressurisation. These phenomena lead to incorrect concentrations of gas being applied
to the detector during calibration, which may result in under reporting the environmental concentration
of gas. The GG2 Calibration Cup and procedures detailed in this manual have been designed to
prevent errors in the calibration process. Use of any calibration cup not detailed in this manual is
prohibited.
3.8 Enclosure
3.8.1 Stainless steel housing
The GG2 Detector circuitry and sensor elements are incorporated in a stainless steel housing. The