Description:

A gas detector measures and indicates the concentration of certain gases in the air via different technologies. Typically employed to prevent toxic exposure and fire, gas detectors are often battery-operated devices used for safety purposes. They are manufactured as portable or stationary (fixed) units and work by signifying high levels of gases through a series of audible or visible indicators, such as alarms, lights, or a combination of signals. While many of the older, standard gas detector units were originally fabricated to detect one gas, modern multifunctional or multi-gas devices are capable of detecting several gases at once. Some detectors may be utilized as individual units to monitor small workspace areas, or units can be combined or linked together to create a protection system.

As detectors measure a specified gas concentration, the sensor response serves as the reference point or scale. When the sensor's response surpasses a certain pre-set level, an alarm will activate to warn the user. There are various types of detectors available and the majority serve the same function: to monitor and warn of a dangerous gas level. However, when considering what type of detector to install, it is helpful to consider the different sensor technologies. For information on other sensors please refer to our Types of Sensors article.

Gas Detector Technologies

Gas detectors are categorized by the type of gas they detect: combustible or toxic. Within this broad categorization, they are further defined by the technology they use: catalytic and infrared sensors detect combustible gases and electrochemical and metal oxide semiconductor technologies generally detect toxic gases.

Measurement of Toxic Gases

Electrochemical sensors or cells are most commonly used in the detection of toxic gases like carbon monoxide, chlorine and nitrogen oxides. They function via electrode signals when a gas is detected. Generally, these types of detectors are highly sensitive and give off warning signals via electrical currents. Various manufacturers produce these detectors with a digital display.

In its typical construction, an electrochemical detector sandwiches an ion conductor between a sensing electrode and a counter electrode. When a gas such as carbon monoxide comes into contact with the sensing electrode, the gas oxidizes through a chemical reaction with water molecules in the air. In this reaction, hydrogen protons flow through the ion conductor to the counter electrode while electrons flow to it by a conductive path. This current is measured to determine the level of toxic gas present.

Metal Oxide Semiconductors, or MOS, are also used for detecting toxic gases (commonly carbon monoxide) and work via a gas-sensitive film that is composed of tin or tungsten oxides. Heated to a high temperature, the film produces free electrons which flow easily through the material, generating current. In the presence of air, these free electrons combine with oxygen in the atmosphere, restricting the number of free electrons available that flow through the sensing material. As the air is displaced by another gas such as carbon monoxide, fewer oxygen molecules combine with the free electrons, allowing more electron flow through the sensing material. The fewer oxygen molecules available, the greater the current flow. The resistance of the sensor correlates to the amount of reducing gas in the atmosphere.

Generally, metal oxide sensors are considered efficient due their ability to operate in low-humidity ranges, as opposed to electrochemical sensors that require humidity in their operation. In addition, they are able to detect a range of gases, including combustibles. They are used in CO detectors, gas sniffers, refrigerant leak detectors, etc.

Measurement of Combustible Gases

Catalytic sensors represent a large number of gas detector devices that are manufactured today. This technology is used to detect combustible gases such as hydrocarbons and works via catalytic oxidation. The sensors of this type of detector are typically constructed from a platinum treated wire coil that serves as the detecting element. In addition, a second wire acts as a compensating element. As a combustible gas comes into contact with the catalytic surface, it is oxidized and the wiring resistance is increased by heat. No change occurs in the resistance of the compensating element in the presence of combustible gas; it serves to compensate for changes in ambient conditions. A bridge circuit is typically used to indicate the resistance change. Because these sensors need a certain level of oxygen present to combust the combustible gas, they will not work in zero- or low-oxygen atmospheres.

Infrared sensors or IR detectors work via a system of transmitters and receivers to detect combustible gases, specifically hydrocarbon vapors. Typically, the transmitters are light sources and receivers are light detectors. If a gas is present in the optical path, it will interfere with the power of the light transmission between the transmitter and receiver. The altered state of light determines if and what type of gas is present.

Specifically called non-dispersive infrared detectors, these systems often incorporate two detectors and two optical filters of different wavelengths placed in front of each detector. One filter is tuned to the target gas and exposed to the gas while the other filter is left unexposed and serves as a reference. Absorption of IR light occurs at the wavelength of the target gas, making these detectors highly gas specific. Another benefit is that these detectors do not lose their sensitivity to the target gas even after prolonged exposure. Units are available that monitor point sources (short-range) and open path sources (long-range).

Another method is PID, or photoionization detection, which is used to detect the presence of volatile organic compounds such as ketones and aromatics. PID detectors use high energy UV light to ionize gas which creates a current proportional to the amount of VOCs present in the atmosphere.

Common Gas Detector Applications

Although detectors are an essential application for home and commercial safety, they are also employed in numerous industrial industries. Gas detectors are used in welding shops to detect combustibles and toxics and in nuclear plants, to detect combustibles. They are also commonly used to detect hazardous vapors in wastewater treatment plants.
Gas detectors are very efficient in confined spaces where there is no continuous employee occupancy. Such spaces include tanks, pits, vessels and storage bins. Detectors may also be placed at a site to detect toxins prior to occupant entry.

Additional Gas Detector Information

Although gas detectors are generally a reliable technology, with some models capable of lasting up to five years, their proper function is generally dependent on user maintenance, battery inspection and calibration. Calibration is a safety procedure executed to ensure that detectors are measuring the correct level of gas. In addition, the life-span of gas detectors also often depends on the amount of gas vapors to which they are exposed. Contaminated sensors may not register dangerous gas levels, which is why frequent calibration is essential.