Gas Detector Technology

Gas detectors measure and indicate the concentration of certain gases in an air via different sensor technologies. They are typically installed to prevent hazardous exposure in industrial settings, such as wastewater treatment plants or oil and gas processing facilities. Sierra Monitor’s Sentry IT gas detectors are manufactured as stationary (fixed) units and work by signifying high levels of gases through a series of audible or visible indicators, such as alarms and lights. Some detectors may be utilized as individual units to monitor small workspace areas, or units can be combined or linked together via the Sentry IT Controller to oversee an entire facility.

As detectors measure a specified gas concentration, the sensor response serves as the reference point or scale. When the sensors response surpasses a certain pre-set level, a signal will be sent to an alarm to notify and warn the user. There are various types of detectors available and they all 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 that Sierra Monitor offers.

Catalytic Bead Sensors

A catalytic bead sensor consists of two alumina beads, each surrounding a platinum wire operating at approximately 450°C. One bead is passivated to not allow it to react with combustible gas, while the other bead is catalyzed to promote a reaction with combustible gas. Effects of changes in ambient temperature and relative humidity are nullified by placing the two beads in separate legs of a Wheatstone bridge circuit. When the catalyzed bead reacts with combustible gas, it heats up and increases its resistance and, in turn, increases the output of the Wheatstone bridge circuit.

The most important advantage of catalytic bead technology is the versatile range of gas detection. This sensor technology has the ability to detect most combustible gases including hydrogen. However, most catalytic bead sensors can be adversely affected by certain chemical compounds, reducing the sensitivity and reliability of the sensor. The degree can vary with the length of exposure and the type of chemical gas.

With most catalytic bead sensors susceptible to poisoning from silicates, Sierra Monitor’s poison-resistant sensor provides reliable zero stability and linear response to combustible gases over a wide temperature range. In addition, they require low maintenance, with a calibration interval of two times per year.

Learn more about our catalytic bead gas sensor module:

Infrared (IR) Sensors

Most gases absorb infrared light energy at defined wavelengths, providing an absorption signature for that gas. The more of the absorbing gas that is present, the more light is absorbed. Two light sources emit light of two different wavelengths through a gas sample. The beams are reflected back through the sample where the detector compares the signal strengths of the same and reference beams and calculates the gas concentration by the light absorbed.

Infrared sensors offer a large range of measurement, providing accurate and stable detection. No matter how high a concentration of a gas is, it cannot oversaturate a sensor. Infrared technology is made to detect light hydrocarbons, but does not respond to hydrogen. Although sturdy and reliable, gas sensors come at a higher upfront cost.

At Sierra Monitor, our long-lasting infrared sensors require low maintenance, with a calibration interval of one time per year. By lowering our operating costs, we have achieved a longer life cycle for infrared sensors. Operational in high combustible gas and/or low oxygen environments, our sensors are immune to catalytic bead poisons. In addition, we provide three selections of ranges for CO2/CO: Indoor Air Quality (IAQ) at 0 to 5,000 parts-per-million (ppm), 0-5% by volume, or 0-20% by volume. Each one of the ranges is made for different applications, such as monitoring a campus lecture hall or an industrial processing plant.

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Electrochemical Sensors

Electrochemical sensors, used to detect toxic gases such as hydrogen sulfide and ammonia, are fuel cell-like devices consisting of an anode, cathode, and electrolyte. The components of the cell are selected so that a subject gas, allowed to diffuse into the cell, will cause a chemical reaction and generate a current. The cells are diffusion-limited, meaning that the rate of the gas entering the cell is solely dependent on the gas concentration. The current generated is proportional to the rate of consumption of the subject gas in the cell.

Advantages to using an electrochemical sensor include allowing sensors to be specific to a particular gas in the parts-per-million range, low power requirements, high accuracy, and lower cost than other gas detection technologies. However, electrochemical sensors are sensitive to temperature, subject to interference from other gases, and have a shorter lifespan the greater the exposure is to the targeted gas.

Sierra Monitor electrochemical sensors provide improved reliability by allowing the gas to diffuse into the sensor through a capillary port, rather than diffusing through membranes. The result is an extremely stable sensor with very low temperature and pressure coefficients, and the capability to monitor gas as a percent by volume (oxygen) and ppm (toxics).

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Solid State (Semiconductor) Sensors

Solid state (semiconductor) sensors have a resistance that is affected by oxygen adsorbed on the surface of the sensor. Oxygen atoms capture electrons on the semiconductor surface, thereby increasing its resistance. The sensors can be impregnated with dopants such that the sensor's resistance changes when specific gases displace the adsorbed oxygen.

As solid state sensors are among the most versatile of all sensors, they are simple and robust and can be used in many different applications. Solid state sensors have the ability to detect both low ppm levels of gases, as well as high combustible levels. However, they are not very specific to a particular gas as often required for industrial applications, so they could potentially provide false alarms. Solid state sensors may be affected by fugitive volatile organic compounds (VOCs), and they must be located in areas without these gases present.

Sierra Monitor's solid state sensors use nanotechnology metal oxide semiconductors (NTMOS). They deliver fast and repeatable detection in extreme temperature and humidity applications where other technologies are not preferred. These sensors have established new standards in sensitivity, selectivity, and stability over traditional solid state sensors.

Learn more about our solid state gas sensors modules: