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A gas detector is a safety device that detects and monitors various toxic, combustible, and flammable gasses in an area, and which can also register a decrease in ambient oxygen.

It can be portable or stationary, and are typically classified by their detection methods, which include catalytic, electro-chemical, point infrared, open-path infrared, semiconductor, thermal path conductor, and paper tape detection.

The first calibrated to known gasses that serve as a reference points. Their sensors can then be used to measure ambient gas concentrations in comparison to the calibration gasses. When gas concentrations exceed a pre-set limit, an alarm or signal is triggered, alerting the user to potential danger.

Pre-set limits often include a low-alarm level, indicating a gas concentration that poses a moderate danger; a high-alarm level, indicating a gas concentration that poses a large danger; a short-term exposure limit (STEL), the maximum gas concentration to which workers can be exposed continuously for a short period of time without serious risk of harm, and time-weighted average (TWA) limit, the maximum average exposure personnel can experience over the course of an eight-hour work shift without serious risk of harm.

Most gas detectors also feature LCD screens that provide a variety of information including gasses detected and their concentrations, pre-set exposure limits, and battery level.

The variety of industrial and governmental applications, such as in fire and rescue, mining, construction, petroleum refineries, tanneries, sewage treatment facilities, and paper mills. Portable gas detectors are typically used to monitor the air around personnel, are worn on clothing or on belts or harnesses, and are powered with rechargeable or disposable batteries.

They generally require periodic calibration, testing, and battery recharging or replacement, although some fixed-life detectors are designed to perform for a number of years without calibration or battery replacement before being discarded. Stationary gas detectors are typically powered electrically and mounted near the process area of a plant or control room. They also require periodic service, testing, and calibration.