FAQ 11. How about cross-sensitivity and interfering gases?
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Cross-sensitivity and interfering gases
All gas detection sensors/technologies have the potential to respond to gases other than the target gas, but some less so than others.
Typically, sensors can be classified as selective (single gas) or non- selective (multi gas).
Selective: if they will only respond to one gas. This is, in our view, almost impossible, as most sensors will also see other gases under some circumstances.
Non-selective: This type of sensors responds to a number of gases, normally gases of the same group or family.
Selecting the correct sensor for the installation in question, calibrating for the target gas, and having a delayed alarm response can eliminate false alarms caused by cross-interference.
Murco sensor selection is designed for maximum sensitivity to a particular gas with minimum response to any cross-interfering gases likely to be present.
Different types of sensor behave differently:
CATALYTIC
Catalytic sensors are non-selective sensors, as they will respond to all combustible gases. This is used to advantage as most suppliers have established charts of cross interference or response levels of different gases relative to Methane and so Methane can be generally employed for calibration purposes. However sensors may be treated to respond preferentially to a particular gas such as ammonia. Some ammonia catalytic sensor will have zero response to methane and about 30 % to propane.
ELECTROCHEMICAL
Electrochemical sensor are generally very selective to the target gases and so do not suffer from cross interference once the correct sensor is selected.
INFRARED
Infrared sensors when first introduced were specific to a single gas and therefore not suitable for applications involving monitoring more than one gas. They were very selective and accurate – reading down to one part per million. Infrared was typically used where a high level of accuracy and specificity is required. This very precision in performance ensures that they are expensive.
However the specificity became a disadvantage in machinery rooms as phase out resulted in mixed gas installations needing a different model for each gas, which was a very expensive solution. Models were developed based on broad infrared wavelength monitoring that could detect a mixture of gases. This however reduced the specificity and accuracy.
An IR system will respond to any compound having a similar wavelength to the target gas.
SEMICONDUCTOR
Semiconductor sensors are non-specific sensors and will respond to most volatile organic compounds. By using filters they can be made more selective to particular compounds e.g refrigerants. Responding to a group of compounds such as refrigerants is an advantage in mixed gas applications as a separate sensor for each gas is not required.
They can be used to detect a large range of gases including all the CFC, HCFC, HFC refrigerants, ammonia and hydrocarbons.
In summary any sensor can experience cross sensitivity. This can be used to advantage when wishing to detect groups of compounds or using cross sensitivity for calibration. It is important to know the application so that the correct solution can be adopted.
Murco offers all types of sensors so that it can offer real solutions to its customer’s detection problems.
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