A supplier recently conducted laboratory testing on methane monitor sensors under various
environmental conditions and calibration procedures which could be found in underground coal mines.
These variations were found to impact the performance of the methane sensor. The test results showed that indicated gas levels could be higher or lower than the calibration gas depending on the scenario.
How did it happen?
The methane monitor test results indicated:
– Increasing air velocities over the sensor head showed an increase in methane level readings.
– Restricting air flow through the filter area showed a decrease in methane level readings.
– Incorrectly fitting gas calibration cups over the sensor could show higher or lower levels.
Power supply issues also affected correct methane concentration readings—if the monitor circuit resistance is too high the output value shown on any display may not accurately reflect the actual gas concentration at the sensor head.
Correct detection of methane concentrations is paramount in providing tripping functions which isolate power to coal mining equipment when methane levels exceed specific values. This monitoring is critical in providing the highest level of control available to prevent the ignition of flammable gas where coal mining equipment and plant is operational and potential ignition sources may exist.
This is evident from the requirements of the Coal Mining Safety and Health Regulation 2017, sections 229 – 249 and 251.
That the site senior executive initiates:
1. A review of current gas monitoring devices providing tripping functions on fixed and mobile plant and in fixed locations. Consider the impacts of ventilation velocities over the sensors and potential for the filter units on the sensors becoming blocked with dust or other matter. The review may find mitigating control measures are required e.g. increasing the frequency of calibration checks.
2. A review of gas calibration procedures and equipment being used, to identify the potential for errors to occur when conducting gas calibrations.
3. A review of statutory inspection procedures to ensure statutory officials conducting inspections verify and record the accuracy of the gas monitoring devices within their area of inspection responsibility.
4. A review of current gas monitoring devices to ensure that circuit voltage drop does not impact on the displayed output of such sensors.