Thermopile infrared gas detectors have many applications, from providing early warning systems for trace levels of atmospheric gases to analyzing several gases in an anesthetized patient in the operating room.
Dexter Research Center has a range of highly versatile thermopile detectors developed over forty years that can be custom-designed for each specific application. Thermopile detectors are passive radiation sensing voltage-generating devices, which require no bias or cooling and do not emit any radiation.
Thermopile Infrared Gas Sensors
Infrared (IR) gas detection is a well-established sensing technology. When exposed to infrared light, gas molecules absorb some of its energy and vibrate more vigorously: different gases absorb IR at specific frequencies. The amount of energy absorbed is related to the concentration of the gas, and results in a rise in temperature: the temperature increases in proportion to the concentration of gas present.
A thermopile converts this heat into electrical energy, generating an output voltage which offers information on the levels of gas or gases present. A thermopile is a range of miniature thermocouple junctions connected in series as differential pairs. These differential pairs consist of hot and cold junctions connected by alternating materials called arms, creating a Seebeck effect - where a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference - between the junctions. The voltage produced is proportional to the temperature gradient between the hot and cold junctions.
A Dexter IR gas detector is sensitive to changes in temperature as small as 0.1 °C and can operate between -40 °C to 85 °C without being affected by ambient temperature fluctuations.
Advantages of Thermopile IR Gas Detectors
In IR instruments, only the sample cell and related components are directly exposed to the gas sample stream: gases of interest, including carbon monoxide, carbon dioxide, methane hydrocarbons and refrigerants, are often corrosive and reactive.
In other types of sensor, such as those based on semiconductors, oxidation and catalytic technologies, the sensor itself is directly exposed to the gas, causing the sensor to stop working properly or fail entirely.
IR thermopile gas detectors are sealed against corrosion, making them robust, reliable, stable and long-lasting. And the remain active without a battery or external power source.
Dexter’s Design Capabilities
Dexter boasts a family of 20 models of thermopile and over 1,000 individual parts meaning they can be quickly customized based on customer’s specific application requirement, whether the quantity is one or one million.
Dexter’s detectors are designed in small transistor-type packages and before each package is hermetically sealed, air is removed and the package is backfilled with one of four gases (argon, nitrogen, xenon or neon). This provides one of the key thermal paths for energy loss from the active area and affects four important performance parameters: the output voltage, responsivity, signal-to-noise ratio (a measure of signal strength relative to background noise) and time constant (how quickly charge falls in a circuit). Different backfill gases have different molecular thermal conductivity, and this property affects the thermal resistance of the detector and package, which affects the output voltage, responsivity and time constant. Dexter’s four standard gas options have varying effects depending on the type of thermopile.
Dexter offers two distinct types of thermopile detectors with different performance characteristics: thin film-based (based on antimony and bismuth) and silicon-based (poly-silicon or silicon combined with gold or aluminum). Thin film-based thermopiles provide a higher signal-to-noise ratio than silicon-based thermopiles but will have a slower time constant than a silicon-based thermopile with equal output and are available with larger active areas. Silicon models are cheaper, and operate at higher temperatures of 125 °C compared to thin film models, which work best around 100 °C although some silicon models can be configured to work at 225 °C.
Dexter’s IR gas sensors can be used in a wide variety of applications, from continuously monitoring combustible, flammable and toxic gases, as well as falling oxygen levels, often as part of a safety system. They can be used as fixed ‘open-path’ gas detectors which send out a beam of infrared light, detecting gas anywhere along the path of the beam - widely used in the petroleum and petrochemical industries to detect leaks of flammable gases.
Detectors can also be portable and handheld, for example blood alcohol breathalyzers. Detectors can also be used to perform sophisticated gas analysis to monitor the critical levels of gases exhaled by a hospital patient under general anesthetic, or premature babies in incubators for example.
References and Further Reading
- What are IR gas detectors - http://www.enggcyclopedia.com/2011/11/infrared-gas-detectors/
- Effects of encapsulation gas on thermopile detectors - http://dexterresearch.com/?module=Page&sID=technical-library
- Introduction to thermopile detectors - http://dexterresearch.com/?module=Page&sID=technical-library and http://dexterresearch.com/?module=Page&sID=gas-analysis
This information has been sourced, reviewed and adapted from materials provided by Dexter Research Center, Inc.
For more information on this source, please visit Dexter Research Center, Inc.