Gas Detection - Sensors
Catalytic bead sensors:
Consists of a wire coil encased in a ceramic or glass material. The material is coated with a catalyst, based on the specified type of material to be detected. The sensor is electrically heated to a temperature that allows it to burn the gas being monitored. When the gas burns on the active sensor surface, the heat of combustion causes a temperature rise, which changes the resistance of the sensor. The resistance change is measured electrically and is the source of the signal. Mainly used for LEL detection With portable detectors, the sensors can last for a long time because they are used sporadically in portable applications. Catalytic sensors will require periodic calibration.
Electrochemical sensors:
The target gas diffuses into the sensor through a porous membrane to the working electrodes where it is oxidized or reduced. This reaction results in a current that passes through the external circuit, and is proportional to the gas concentration. Used in toxic gas monitoring, and will require calibration. Electrochemical sensors consist of electrode arrays with two, three or more electrodes, which are called auxiliary electrode. reference electrode, and working electrodes. The electrodes of an electrochemical sensor provide a surface at which an oxidation or a reduction reaction occurs to provide a mechanism whereby the ionic conduction of an electrolyte solution in contact with the electrodes is coupled with the electron conduction of each electrode to provide a complete circuit for a current. In a typical electrochemical gas sensor, the gas to be measured typically passes from the atmosphere into the sensor housing through a gas porous or gas permeable membrane to a working electrode where a chemical reaction occurs. For example, you can use an electrochemical sensor for hydrogen fluoride to detect a leak in excimer lasers using krypton fluoride gas mixtures. When the gas reacts with moisture in the atmospheric air, the chemical reaction of krypton fluoride converting into hydrogen fluoride will occur.
Infra-Red Sensors:
Optical detectors for combustible gases that illuminate the gas using an infra-red light beam using filament or LED, to detect the gas - typically used to detect CO2 and Methane. Stable and with a long life, will require periodic calibration.
Photoionization sensors:
The PID lamp is filled with a low-pressure inert gas, which when energized with energy in resonance with the natural frequency of the gas molecules produces an electric current, which is the signal output of the detector. The greater the concentration of the component, the more ions are produced, and the greater the current. PID detectors will require calibration.