Gas instruments detect, monitor or analyze gases present in an environment. Detectors sense situations outside normal operating parameters and are set to alarm when these conditions are violated. Monitors are also set up to alarm, but their role is to determine which gases are in the stream being measured, and in what quantity they are present. Analyzers provide a breakdown of what is found, log the information, and can download it to a computer where further analysis and record keeping can be performed. Gas instruments and air instruments can sense sometimes just one gas or else can sense multiple types of gases with one instrument.

Measurement Types

When analyzing gases there are seven basic types of measurements that can be made. Percent LEL or lower explosive limit or lower flammable limit (LFL) of a combustible gas is defined as the smallest amount of the gas that will support a self-propagating flame when mixed with air (or oxygen) and ignited. In gas-detection systems, the amount of gas present is specified in terms of % LEL: 0% LEL being a combustible gas-free atmosphere and 100% LEL being an atmosphere in which the gas is at its lower flammable limit. The relationship between % LEL and % by volume differs from gas to gas. Another measurement is percent volume, which measures the amount of a specific gas within a sample. Trace measurement is usually given in units of ppm or ppb. Leakage and consumption rates can also be measured, as can gas density and signature or spectra, which is the spectral signature of the gases present.

Specifications

Specifications for gas instruments are first what type of gas the application requires sensing and second how many channels the instrument needs to sense through. This can be for multiple types of gas sensors, redundant sensors for the same gas, or for placing sensors throughout a location to get sampling at many different spots. Other factors to consider are response time, maximum distance from leak that the detector can detect gases, and flow rate through the sensor.

Types

Gas instruments can be detectors, monitors or analyzers. A gas detector will detect situations outside normal operating parameters. It is set up to alarm of this situation and covers gas leak detectors as well. A monitor is set up to alarm and determine which gases are present and in what amounts. An analyzer will analyze what is found, log the information and has the ability to download to a computer for further analysis.These instruments can typically be handheld, larger portable devices or permanently mounted instruments. They have four primary purposes: personal exposure monitoring, air quality monitoring, confined space monitoring, and process gas monitoring.

Features

General available features for gas instruments can include temperature and humidity measurements, external or internal sampling “sniffer” pump, interchangeable probes, alarm settings, controller functionality, self-calibration, data storage or logging, and usability in hazardous environments.

Standards

Gas detectors, monitors, and analyzers may be manufactured, tested, and used based on various standards. Example standards include:

• IEC 60079-29 - Performance requirements of detectors for flammable gases
• ISA 92.00.01 - Performance requirements for toxic gas detectors
• UL 1484 - Residential gas detectors

Image credit:

Det-Tronics