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Application, hazardous zone or not

A hazardous area can be defined as any location where there is risk of an explosion. Every hazardous area is different and each has specific requirements depending on the nature of the atmosphere and the elements that are present. Where there is potential for an explosive atmosphere, special precautions are needed to prevent fires and explosions. Electronic equipment, including gas detection gear, needs to be purpose designed for use in hazardous areas to prevent a spark occurring and igniting any flammable substances. More information on hazardous zones and classifications can be found here.

Quantity of gas detectors and mounting height

The quantity of sensors depends on the application. Should there be a large number of potential leak sources (typically in oil and gas package equipment) a good starting density is a spacing of 8 metres between detectors or working with an area of coverage of 75 to 100m2 for each detector. For carparks detection systems, the older standard AS1668.2 recommended a sensor cover radius of 25 metres, however the latest revision has removed this recommendation.

The sensor mounting height depends the type of application and on the density of the gas relative to air. Heavier than air gases should typically be detected close to the floor, lighter than air gas sensors should typically be placed on or near the ceiling, and gases which have a density close to that of air should have sensors installed in the "breathing zone" approx. 1.25 metres from the floor. This is a good default location for sensors, as many gases often disperse well in air.

Air is a mixture of gases, typically:

• Nitrogen 77.2 %
• Oxygen 20.9 %
• Water Vapour 0.9 %
• Argon 0.9 %
• Carbon Dioxide 0.03 %
• Other Gases 0.07 %

Consideration should be given to accessibility for calibration when locating sensors. For example, a sensor mounted 10 metres off the floor will be difficult to service. Sensors should be placed near the source of the gas if possible. Sensors should not be placed near ventilation fans or openings to outside. They should be placed in areas where there is good air circulation, but not in the path of rapidly moving air. More information about gas and specfic weight compared to air can be found here.

Cross reference to other gases

Cross-sensitivity refers to the response of a sensor to a gas other than the target gas. It is important to be aware that a sensor is not always gas-specific. Choosing the sensor to be calibrated for a particular gas does not always mean that it will respond only to this particular gas. If mixtures of other gases are present, then the gas reading becomes indeterminate as it is not possible to discern the contribution to the reading from each individual gas.

Therefore, cross-referral to other gases increases the flexibility of a gas detector but also is a disadvantage as it works as an interference. Often, “false” alarms can be sourced back to cross sensitivity of the sensor. Cross sensitivity can sometime be avoided by using different sensor techniques.

Accessible for testing and maintenance

Another important consideration is sensor accessibility for testing and maintenance. Some sensors may be fitted at ceiling height and not always accessible for ongoing calibration. Alternatives may be suction based system, were a sampling pump brings the gas sample to an accessible location. Bionics Instrument offers a range a suction detectors for remove gas sampling.

Sensor technology

There are many sensors used to detect gases. Some gases can be detected by some sensors and not by others, and some sensors are more accurate or gas-specific than others. Types of sensors include Electrochemical, Catalytic, Infrared, Photoacoustic, PID and others Solid State (also known as Metal Oxide Semiconductor. Most gases can only be detected by one or two sensor types. Then consideration is given to required accuracy and specificity, sensor life expectancy, and finally cost.

• Electrochemical : Accurate, repeatable, gas specific, defined cross sensitivities
• Catalytic: For many combustible gases, accurate, long life
• Infrared: Gas specific, best way to detect CO2, accurate & stable, long life
• PID: Able to detect low volumes of VOC’s, cross sensitive
• Solid State (MOS): Low cost, long life, not as accurate as other types

Environmental circumstances

Environmental circumstances: When locating gas detectors, consider the possible damage caused by natural events, such as rain or flooding. For detectors mounted outdoors, use the weather protection assembly. In addition, use a detector sunshade if locating a detector in a hot climate and in direct sun. Detectors should be installed at the designated location with the detector pointing downwards to ensure that dust or water will not collect on the front of the sensor.