The span gas is very important for performing correct calibration. The type of calibration gas depends on the type of sensor which are infrared, electrochemical and catalytic sensors.
The catalytic or pellistor sensors are the only sensors which rely on oxygen to function correctly. They need oxygen to work properly. In accordance with IEC 60079-29-1 the atmosphere must have minimum 10% oxygen to support correct catalytic oxidation. If the oxygen concentration is below the limit, the chemical reaction on the catalytic bead combustible sensor will be partial and measurement level incorrect. Due to this specific working mechanism the span gas used for calibration of catalytic sensors must be balanced with fresh air including %20.9 Oxygen and not any inert gas such as widely used N2.
Similarly zero calibration of the catalytic sensors must be done with clean air.
In case of IR sensors it is recommended to use N2 for zeroing and N2 balanced span gas for span calibration. Although the flammable gas sensors do not have problem in general when zeroed by clean air, especially CO2 gas sensors cannot achieve correct zero level with clean air.
Other important points are:
- For Flammable gas detector calibration use a calibration gas concentration of full scale between 25%LEL and 75%LEL to ensure that the required accuracy can be attained.
- For toxic gas detectors it is recommended to use %50 concentration of the full scale.
- It is necessary to use international traceable and certified gases.
- Gas must be exposed to sensor with a constant gas flow, use an appropriate flow regulator. Always apply the flow rate recommended by detector manufacturer for correct calibration.
- It is recommended to refer to the manufacturer’s instructions for the type of span gas to be used. Calibrations conducted with the incorrect span gas can lead to the sensor operating outside established tolerances and causing it to show inaccurate readings.
- Calibration and maintenance instructions should be included site maintenance system and all activities must be recorded near the gas detection display panel as good practice.
Calibration of the detectors are vital to take necessary actions on time. Hence it is important to understand specifications of detector and act according to its requirements.