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Priority to calibration

The malfunctions of catalytic sensors are often linked to a decrease in performance as a result of their exposure to certain poisons. It is therefore essential that all gas monitoring systems be calibrated during installation,and then regularly monitored and re-calibrated if necessary. The controls must be carried out using a standard gas mixture calibrated so that the zero and the set point can be properly configured on the control module.

Codes of good practice such as EN 60079-29-2 specify the legal calibration requirements for flammable gas detectors (%LIE) and provide guidance on the calibration of toxic gas detectors (note: toxic gas detectors will be subject to legal calibration requirements in the future). In general, checks must be carried out weekly, but it is possible to extend this period once the operation has been scheduled. When two alarm levels need to be set, the lowest level is usually set between 20% and 25% LIE and the highest level, between 50% and 55% LIE.

The control and calibration of older systems (and low-end systems) requires two people, one to expose the sensor to the gas flow and one for the scale of the control device. The zero and scale knobs are then set to the control module until the measurements match exactly those of the concentration of the gas mixture.

When adjustments are to be made in fireproof protection, the power should be cut off beforehand and permission to open the protection should be obtained. Today, many operator calibration systems are available. They allow calibration on the sensor itself. They significantly reduce maintenance time and costs, especially when sensors are difficult to access, such as on offshore gas or oil platforms. Some sensors now also meet intrinsic safety standards while offering the possibility of off-site calibration (for example, in a maintenance depot). These sensors are inherently safe and can therefore be directly replaced on site by other sensors, without having to turn off the system.

Maintenance operations can therefore be carried out on a “hot” system and thus gain in speed and economy compared to conventional systems.

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