Gas Sensors and Air Quality Management
How to accurately calibrate and monitor air quality in underground mines.
Gas Sensors
Clean air is key to safe working conditions underground. A good ventilation system is an excellent way to mitigate potential harmful situations due to toxins in the air. Another important step is to actually measure the concentration of toxins present. This information can be used to help automate ventilation systems and only operate them as needed, increasing efficiency and reducing costs in the ventilation system. This can be done by employing gas sensors.
Toxic gases are typically measured with electrochemical sensors. The sensor has a porous membrane which gas will diffuse through. The gas then reaches the electrode, where the gas is oxidized or reduced. This electrochemical reaction creates an electric current which is measured by an external circuit. These sensors are designed such that the gas supply to the electrode is limited by diffusion, and the resulting current output is linearly proportional to the gas concentration. This linear output is an advantage of electrochemical gas sensors over other types, as the linear output allows for precise measurements of low concentrations, and the calibration process is much simpler.
Calibrating Gas Sensors
When installing a new gas sensor, it should be closely monitored for the first 30 days after installation. During this time, any immediate problems with either the sensor placement or the sensor design will surface. If it is operating normally for 30 days, this is a good indication that the sensor will operate as expected for the remainder of its life. During these 30 days, the sensor should be checked weekly. After this time, a monthly calibration should be adequate to insure each sensor is performing as required.
Zeroing the Sensor
The first step for calibrating a gas sensor involves zeroing the sensor. This can be somewhat complex. Many analytical procedures use either pure nitrogen or pure synthetic air to zero the sensor, because these are typically readily available. However, a better practice is to use ambient air in an area that is known to be clean. Ambient air contains traces of gases other than nitrogen and oxygen and contains small amounts of water vapour. Therefore, it is more practical to zero the sensor with ambient air.
Span Calibration
Once the sensor is properly zeroed, a span calibration is required. The best accuracy can be achieved by using a mixture of the target gas balanced in the background environmental air. This can be difficult to achieve, and it is much more practical to purchase calibration gasses from suppliers. Calibration gasses can be purchased as low-pressure containers or high-pressure container. Highly pressurized gas containers require some sort of regulator assembly to safely remove the gas from the bottle. However, these bottles can be designed to be much smaller and hold much more volume than low pressure containers.
Another method for span calibration is cross calibration. This involves using one gas to calibrate sensors that detect a different gas. All sensors are subject to interference by other gasses. Using this fact, gasses that may be easier to handle can be used to calibrate sensors that detect more difficult gases to handle. For example, a sensor calibrated to detect 100% LEL hexane can be calibrated using 50% LEL methane gas. Hexane gas is liquid at room temperature and it is difficult to make an accurate mixture and keep this at a high pressure. When calibrating a sensor using cross calibration, the sensor is calibrated using a calibration gas, and then a response factor is applied to the sensor. For example, if a pentane sensor is calibrated using methane, the output for pentane will be half of what the output for methane is. Therefore, a response factor of 0.5 is applied. This method only works for linear sensors, such as catalytic sensors. There are some issues with this calibration method. Every sensor is different, and the response factors may not be identical. Catalytic sensors must be at the specified heater voltage, otherwise the response factors are not applicable. It is good practice to periodically check the calibration of the sensor with the actual target gas.
Preventing False Alarms in Gas Sensor Readings
False alarms can be costly, and if they occur often, they can desensitize workers to actual alarms, and potentially put them in a dangerous situation. Proper monitoring and calibration is key to avoiding false readings. Setting up a regular maintenance schedule can help identify problems with gas sensors and reduce the amount of false readings.