Hydrogen sulfide sensors detect toxic gas levels in various industries. They are crucial in petrochemical, wastewater treatment, and food processing plants, among others. The sensors use microelectromechanical fuel cells and potentiometry to detect gas in air, water, and sludge. Recent improvements in nanoscale engineered materials have increased sensitivity and response times. The American Conference of Governmental Industrial Hygienists has reduced acceptable exposure levels to the gas.
A hydrogen sulfide (H2S) sensor is a gas sensor that can be built to various design specifications to detect levels of hydrogen sulfide created during industrial and biological processes. Such sensors are very important in various industries due to the fact that hydrogen sulphide is an extremely toxic gas. Inhalation of 500-1000 parts per million (ppm) by volume almost always results in immediate loss of consciousness and death. Some hydrogen sulfide sensor units are single-use emergency components, while other H2S sensor models are built to repeatedly detect the gas and last for many years.
Many industries need the hydrogen sulfide sensor, but among the most common are the petrochemical industry, where it is a natural byproduct of crude oil and natural gas production, and municipal wastewater treatment plants. Related areas that produce hydrogen sulfide include fish farming or aquaculture, manure storage for fertilizers, and regions where volcanic gases or hot springs exist. Refineries and coking plants that convert coal to coke through a heating process in an oxygen-free environment are also places where a hydrogen sulfide sensor detection system is critical. Paper mills, steel mills, and tanneries also produce the gas, and because it’s a natural byproduct of the breakdown of organic matter by bacteria, it’s also a potential hazard in several types of food processing factories.
The ability to naturally detect dangerous but very low levels of hydrogen sulfide in the air can be challenging for several reasons. One reason is that it is a colorless and transparent gas, heavier than air, so it tends to settle at low levels in buildings where it may initially go unnoticed. While it smells like rotten eggs at low concentrations, the smell becomes sweet at higher levels, which can confuse the senses. Therefore, there are several methods to detect the gas in biological samples compared to concentrations in air or water.
A typical design for a portable sensor for continuous use is based on a microelectromechanical (MEM) fuel cell that can operate in a range of -22° to 122° Fahrenheit (-30° to 50° Celsius) and uses the principle of resistance. The MEMs sensor is built on a metal oxide (MOS) semiconductor material of microscopic tin oxide or gold metal films that respond to changes in electrical resistance as hydrogen sulfide gas passes through them. Such sensors have fast response times and can be accurate down to 25 parts per billion (ppb), but more often than not, they are designed to detect only higher levels of gas. They are inexpensive, however, and are commonly employed in harsh climatic conditions, such as in oil and gas exploration and drilling.
A hydrogen sulphide sensor designed to detect the gas in water and sludge is also based on the principle of potentiometry, i.e. changes in the electromotive force in the water. Water detectors can measure gas levels to less than 0.3 ppb and are often integrated into standard pH meters used in the wastewater treatment industry. However, to be accurate, they require frequent calibration, which is usually scheduled once a month. A frequent drift sensor problem occurs with the units needed to measure such fine levels, which is an indication that the displayed output reading is offset from the actual measured value. In a hydrogen sulfide sensor used in a liquid environment, a drift range of ±0.5 millivolt (mV) is standard, but drift can often reach up to 2mV in a month in readings.
Other types of hydrogen sulfide sensors are built into portable units carried by emergency services personnel that can detect other hazardous gases such as carbon monoxide. Similar types of units placed in structures are resistant to corrosion and explosives, which are two properties of hydrogen sulfide gas. They are capable of operating for two to five years with very low power consumption and no degradation in continuous sensing ability after being exposed to the gas.
The level of sensitivity and response times of less than one minute have been improved in recent years for the hydrogen sulfide sensor by incorporating nanoscale engineered materials. This supports new regulations in the United States as of 2010. The American Conference of Governmental Industrial Hygienists (ACGIH) has reduced acceptable exposure levels to the gas for an eight-hour weighted average from 10ppm to 1ppm and a level of short-term exposure of 15ppm up to 5ppm.
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