Portable gas detectors differ from gas analyzers in that where analysers are used to analyse a sample for a variety of specific compounds, a gas detector is used to monitor for, and warn of the existence of specific toxic or combustible gases and oxygen deficiency hazards. Typical measurement levels will be in the low parts per million level for toxic gas, while combustible gases are measured in percent LEL (Lower Explosive Limit), and Oxygen deficiency is measured in percent level.
Gas detection monitoring devices can be classified in one of three ways. A fixed system refers to a monitoring system permanently installed in the workplace (stationary). The detecting sensor may be hard wired, or use wireless signals to a central reporting station. Most will come with an auditory alarm system. The type of sensor used will be defined by the system, as well as the gas or gases to be detected. Fixed gas detection can be used indoors, as well as having outdoor use as a perimeter monitor with chemical manufacturing and petrochemical sites.
Portable gas detection refers to gas detectors which are worn or carried by an individual. Typically battery operated, portable monitors are used for toxic or combustible gas detection, as well as for oxygen deficiency monitoring in confined spaces.
Area monitoring detection offers the benefits of a multi-gas fixed system in a transportable unit. These units are designed for team protection or area surveillance for short-term work where fixed gas systems are not suitable.
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.
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.
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.
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.
Gas Detection - Other types
Laser Gas Detection:
Each gas atom or molecule has a frequency or wavelength in which they resonate or vibrate. The laser beam operates at a specific absorption wavelength for the target gas. When the laser beam strikes the gas molecule, it is induced to vibrate. The vibration affects the beam by changing this superimposed frequency, with the difference detected by a receiver in the instrument, after the laser beam is reflected back.
Gas Detector Tubes:
With Gas Detector Tubes, an air sample is drawn through the tube by way of a pump. The tube is filled with chemically treated reagents. If the sample has enough of a contaminant, it causes a change in colour of the reagent based on the level of contaminant. Gas Detector tubes can be direct reading, where measurement is obtained directly from a printed scale printed on the tube, or through the use of a separate concentration chart. Gas Detector tubes require no calibration, and are one time use only.
Dosimeter Badges are used to measure an individual's exposure in a hazardous environment. For gas detection, they use the principal of direct diffusion. The badge will indicate the presence of a specified gas by means of a highly specific chemical colour reaction in which the amount of colour change is directly proportional to the concentration of gas and the time of exposure. Dosimeter badges require no calibration, and are one time use only.