Gas transmitters consist of an enclosure, sensor, and electronics that convert a signal from a gas sensor into an analog output signal. Electrical outputs for gas transmitters include analog current, analog voltage, and frequency. Analog current levels such as 4 – 20 mA are suitable for sending signals over long distances. Typically, a current is imposed on the output circuit proportional to the measurement. Feedback is used to provide the appropriate current regardless of line noise and impedance. Analog voltage outputs are simple, usually linear functions of the measurement. Frequency or modulated frequency outputs for gas transmitters include amplitude modulation (AM), frequency modulation (FM), sine waves, and pulse trains.
Gas transmitters differ in terms of the number of gases sensed, general gas types, and specific gas types. Suppliers rate products for two general types of gases: toxic gases and combustible gases. Toxic gases cause illness or death when inhaled or absorbed by the body in relatively small quantities. Combustible gases react with air or oxygen and burn over a wide range of concentrations. Within each category, there are many specific types of gases. Some gas transmitters are designed to sense aerosols, ammonia, arsine, bromine, carbon dioxide, carbon monoxide, chlorine, chlorine dioxide, diborane, fluorine, or germane. Others are designed to sense halocarbons, hydrocarbons, hydrogen, hydrogen chloride, hydrogen cyanide, hydrogen fluoride, hydrogen selenide, hydrogen sulfide, mercury vapor, nitric oxide, nitrogen dioxide, nitrogen oxides, oxygen, or ozone. Gas transmitters that sense phosphine, silane, sulfur dioxide, sulfur hexafluoride, and water vapor are also available.
Measurement type is an important specification to consider when selecting gas transmitters. There are five basic measurement types: percent lower explosive limit (LEL), percent by volume, trace, leakage, density, and signature or spectra. The lower explosive limit (LEL) or lower flammable limit (LFL) of a combustible gas is the smallest amount of the gas that supports a self-propagating flame when mixed with air and ignited. Zero percent (0 %) LEL denotes an atmosphere free of combustible gas. One hundred percent LEL (100 %) denotes an atmosphere which contains gas at its lower flammable limit. Percent by volume is expressed as %v/v, where v equals volume. Trace is expressed in units of concentration: parts per million (ppm) or parts per billion (ppb). One percent exposure equals 10,000 ppm or 10,000,000 ppb. Leakage is the prevailing fluid flow through a leak at existing conditions. Typically, the leakage rate is specified in units of flow, such as milliliters per minute (ml/min). Density is expressed as mg/m^3.
Gas transmitters include an analog or digital display and offer a variety of features. Some gas transmitters include audible or visual alarms, interchangeable probes, on-board data storage, a sampling pump, or a sensor array. Others are intrinsically safe (IS) or designed for use in a hazardous environment. Intrinsically safe (IS) is an adjective for equipment and wiring which is incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most ignited concentration. IS terminations and wiring may be brought into any hazardous location of any group classification for which it is accepted, without requiring explosion-proof housing or other means of protection.