The most common failure in catalytic sensors is performance degradation caused by exposure to certain poisons. It is therefore essential that any gas monitoring system should not only be calibrated at the time of installation, but also checked regularly and re-calibrated as necessary. Checks must be made using an accurately calibrated standard gas mixture so that the zero and ‘span’ levels can be set correctly on the controller.
Codes of practice such as EN 60079-29-2 outline the legal requirement for calibrating flammable gas detectors (%LEL) and also guidance on the calibration of toxic gas detectors (please note: toxic gas detectors will have a legal requirement for calibration in the future). Typically, checks should initially be made at weekly intervals but the periods can be extended as operational experience is gained. Where two alarm levels are required, these are normally set at 20-25%LEL for the lower level and 50-55%LEL for the upper level.
Older (and lower cost) systems require two people to check and calibrate, one to expose the sensor to a flow of gas and the scale of its control unit. Adjustments are then made at the controller to the zero and span potentiometers until the reading exactly matches that of the gas mixture concentration.
Remember that where adjustments have to be made within a flameproof enclosure, the power must first be disconnected and a permit obtained to open the enclosure. Today, there are a number of ‘one-man’ calibration systems available that allow the calibration procedures to be carried out at the sensor itself. This considerably reduces the time and cost of maintenance, particularly where the sensors are in difficult to get to locations, such as an offshore oil or gas platform. Alternatively, there are now some sensors available that are designed to Intrinsically Safe (IS) standards, and with these it is possible to calibrate the sensors at a convenient place away from the site (in a maintenance depot for instance). Because these sensors are IS, they can be freely exchanged with the sensors needing replacement on site, with no need to shut down the system first.
Maintenance can therefore be carried out on a ‘hot’ system and is much faster and cheaper than early, conventional systems.