Many fleet managers are under the impression that their maintenance facility is fully protected from fugitive methane gas since they have a matrix of combustible gas detection sensors installed. If you visit an existing CNG vehicle maintenance garage, the facility staff may direct your gaze towards the ceiling trusses to proudly show off the array of gas detection sensors that have been installed for the specific purpose of protecting their facility and its workers.
Like most monitoring devices, routine maintenance is necessary to keep the equipment operating at peak performance. In the majority of installations, routine maintenance, especially regular sensor calibration is required based on manufacturer instructions. In almost every scenario, there are certainly impacts on warranty coverage when there is a deviation from the manufacturers’ recommended maintenance policy.
Depending on the type of gas detection sensors that are installed, the recommended calibration interval can vary dramatically. For catalytic bead sensors, the interval is typically anywhere from 90-120 days, while infrared sensors can be in service anywhere from 6 months to a year before they require recalibration.
Why aren’t sensors being calibrated on a routine basis? Because, frankly, most maintenance workers and managers realize it’s a pain to access them, especially if their installation is not configured with sensor tubing connections to enable remote access to the sensor-transmitters. Coordinating access and use of a scissor-lift, or for the more challenging sensor locations, an articulating lift on a quarterly basis is not a chore that most facility managers look forward to.
After the lift has been secured, there’s still the impact of scheduling time and space to perform the calibration so that there is minimal impact on vehicle maintenance operations. For-profit maintenance shops are focused on maximizing space utilization and repair tech billable hours when the repair bays are open for service. Therefore, committing valuable space and staff resources to perform non-billable activity during normal business hours is typically not a priority. Based on the number of sensors, complexity of the array, size of the building, and various obstacles within the facility, a calibration session could take a few hours to a few days.
Just as important as the time, space use, and access issues, is the need to keep adequate calibration supplies on-hand. In addition to a regulator to control gas flow, for most sensors, you’ll need to have small cylinder(s) of O2 to provide “clean air” to purge the sensors. Also, in order to calibrate the span settings, you’ll certainly need methane (I.e., CH4) gas cylinder(s) to perform the appropriate combustible gas calibration.
You may be asking, “why do I need to calibrate the sensors so often?” If you think about the locations where most gas detection sensors are installed, it starts to make sense. For methane gas detection sensors, the transmitter assemblies are typically installed within 18 inches of the highest point within the structure. Most staff rarely spend much time in the elevated areas of the building for good reason; it’s often not practical for reasons described above, and it’s often not very comfortable due to much warmer (often very hot) ambient air temperature conditions. There can be a lot of air movement in those regions depending on the building layout and ventilation system design, carrying a lot of dust particles, lighter than air vapors, and fumes.
The gas detection sensors function similar to our nose. During the course of each day, various particles are drawn into the sensor opening. Over time the cumulative impact of the particles and vapors begins to erode the sensor’s sensitivity. The erosion of the sensor’s performance results in drift from the full range of detection sensitivity. As humans, we are able to quickly resolve particle accumulation in our nose by cleaning them away with a tissue. Sometimes, our body reacts to a dust accumulation or strong vapor by involuntarily sneezing away the undesirable contaminants.
If you operate a CNG vehicle maintenance garage, you may want to check your records to confirm when you last performed a calibration on your sensor set.