Gas Detection Methods and Principals

Catalytic bead sensors are primarily used for low-cost combustible gas detection. These sensors consist of a small sensing element, often referred to as a “bead,” which is made from an electrically heated platinum wire coil. This coil is coated with a ceramic base, such as alumina, and a final outer layer of palladium or rhodium catalyst dispersed in a substrate of thoria.

The sensor operates on the principle that when a combustible gas/air mixture passes over the hot catalyst surface, combustion occurs, generating heat that raises the temperature of the bead. This temperature increase alters the resistance of the platinum coil, which can be measured using the coil in a standard electrical bridge circuit. The resistance change is directly related to the gas concentration and can be displayed on a meter or similar indicating device.

For temperature stability under varying ambient conditions, the best catalytic sensors use thermally matched beads, placed in opposing arms of a Wheatstone bridge electrical circuit. One sensor, known as the “sensitive” sensor (S), reacts to combustible gases, while the other, the “inactive” or “non-sensitive” (N-S) sensor, compensates for external temperature or humidity changes. Inactive operation is achieved by coating the bead with glass or deactivating the catalyst.

Improved versions of catalytic sensors are resistant to poisoning from substances like silicones, sulfur, and lead compounds, which can rapidly deactivate other types of sensors.

To ensure safety, catalytic sensors are typically mounted in strong metal housings with a flame arrestor, allowing the gas/air mixture to diffuse into the housing and onto the sensor, while preventing flame propagation. Although this reduces the sensor’s response time slightly, the electrical output typically gives a reading within seconds of detecting gas. The response time is usually specified in terms of reaching 90% of the final reading, known as the T90 value, which typically ranges from 20 to 30 seconds.

TCD consists of an electrically heated filament in a temperature-controlled cell. Under normal conditions there is a stable heat flow from the filament to the detector body. When an analyte elutes and the thermal conductivity of the column effluent is reduced, the filament heats up and changes resistance. This resistance change is often sensed by a Wheatstone bridge

 circuit which produces a measurable voltage change. The column effluent flows over one of the resistors while the reference flow is over a second resistor in the four-resistor circuit. This detection method is suitable for measuring high concentrations of binary gas mixtures, such as methane and hydrogen. It is primarily used for detecting gases with thermal conductivities much greater than air. However, gases with thermal conductivities close to air (e.g., ammonia and carbon monoxide) or less than air (e.g., carbon dioxide and butane) are more difficult to detect.

The sensing element is exposed to the sample, while the reference element is sealed in a compartment. If the thermal conductivity of the sample gas is higher than that of the reference, the sensing element’s temperature decreases. Conversely, if the sample gas has lower thermal conductivity than the reference, the temperature increases. These temperature changes are proportional to the gas concentration present.

Infrared (IR) gas detectors are based on the principle that many combustible gases have absorption bands in the infrared region of the electromagnetic spectrum. Although infrared absorption has been used in laboratory settings for years, recent advances in electronics and optics have made it possible to use this technology for industrial gas detection.

Infrared sensors offer several advantages over catalytic sensors, including a very fast response time (typically less than 10 seconds), low maintenance, and simplified self-checking through microprocessor-controlled equipment. They are also unaffected by poisons, fail-safe, and can operate in inert atmospheres or under a wide range of temperature, pressure, and humidity conditions.

These detectors use dual-wavelength IR absorption, where light passes through the sample at two wavelengths: one at the absorption peak of the gas, and the other at a non-absorbing wavelength. The light is pulsed alternately and passes through the sample before being reflected by a retroreflector and compared by a detector. The difference in the signal strengths of the sample and reference beams is used to measure gas concentration.

This detector is suitable only for diatomic gas molecules and is not effective for detecting hydrogen.

Traditional gas leak detection methods typically use point detection, where individual sensors cover a designated area. However, open-path detectors utilize infrared and laser technology to form a broad beam that can cover distances of several hundred meters. These detectors are increasingly used as the primary detection method in applications such as loading/unloading terminals, pipelines, perimeter monitoring, offshore platforms, and LNG storage areas.

Older designs used dual-wavelength beams, one corresponding to the absorption peak of the target gas and another as a reference. The instrument compares the two signals transmitted through the atmosphere, detecting changes in the ratio to measure gas concentration. Newer designs use advanced filters and coaxial optical systems to eliminate false readings caused by environmental factors like fog or interference from radiation sources such as flare stacks.

Open-path detectors measure the total number of gas molecules in the beam, expressed in LEL meters, rather than the concentration at a single point.

Recent advancements in solid-state laser diode sources and digital signal processors have made it possible to detect toxic gases reliably using optical means. Unlike flammable gas detectors, which measure gases at percent levels, toxic gas detectors must detect concentrations at much lower levels (ppm). This requires a different measurement approach, where the instrument uses a laser diode light source to probe individual gas lines, enhancing sensitivity compared to open-path flammable gas detectors.

Open-path toxic infrared detectors are designed to detect gases at very low concentrations by using sophisticated modulation techniques, offering enhanced sensitivity compared to traditional methods.

Electrochemical sensors are commonly used to detect toxic gases such as PH3, CO, H2S, Cl2, and NH3. These sensors are compact, low-power, highly linear, and repeatable, with a typical lifespan of 1-3 years. Response times (T90) are typically between 30-60 seconds, with detection limits ranging from 0.02 to 50 ppm depending on the gas.

Electrochemical cells consist of three active gas diffusion electrodes immersed in an electrolyte. The target gas either oxidizes or reduces at the working electrode, altering its potential, which is measured by the associated electronic circuit. The current generated is proportional to the gas concentration.

These sensors are sensitive to environmental conditions, particularly temperature and humidity, and require a minimum concentration of oxygen for proper operation. Some designs use filters to enhance gas specificity, and their lifespan is typically 2 years, although it may vary depending on the gas, amount of electrolyte and type of electrolyte being used for that specific sensor.

Photoionization detector, high-energy photons, typically in the vacuum ultraviolet (VUV) range, break molecules into positively charged ions. As compounds enter the detector they are bombarded by high-energy UV photons and are ionized when they absorb the UV light, resulting in ejection of electrons and the formation of positively charged ions. The ions produce an electric current, which is the signal output of the detector. The greater the concentration of the component, the more ions are produced, and the greater the current. The current is amplified and displayed on an ammeter or digital concentration display. The ions can undergo numerous reactions including reaction with oxygen or water vapor, rearrangement, and fragmentation. A few of them may recapture an electron within the detector to reform their original molecules; however, only a small portion of the airborne analytes are ionized to begin with so the practical impact of this (if it occurs) is usually negligible. Thus, PIDs are non-destructive and can be used before other sensors in multiple-detector configurations.

The PID will only respond to components that have ionization energies similar to or lower than the energy of the photons produced by the PID lamp. As stand-alone detectors, PIDs are broad band and not selective, as these may ionize everything with an ionization energy less than or equal to the lamp photon energy.

PID Detectors are commonly used to monitor VOC’s (Volatile Organic Compounds). Many gases and vapors fall under the VOC banner and PID’s can monitor for over 700 VOC gas species. This makes a PID an ideal first responder for VOC leaks even though the PID cannot directly determine the gas type detected.

The PID will only respond to components that have ionization energies similar to or lower than the energy of the photons produced by the PID lamp. As stand-alone detectors, PIDs are broad band and not selective, as these may ionize everything with an ionization energy less than or equal to the lamp photon energy.

PID Detectors are commonly used to monitor VOC’s (Volatile Organic Compounds). Many gases and vapors fall under the VOC banner and PID’s can monitor for over 700 VOC gas species. This makes a PID an ideal first responder for VOC leaks even though the PID cannot directly determine the gas type detected.

Colormetric tape uses an absorbent strip of filter paper as both a gas collector and analyzer. This technology is particularly effective for detecting highly toxic gases, including di-isocyanates, phosgene, chlorine, ammonia, and fluorine.

The system employs a vacuum pump to draw sample gas through the porous tape, where it reacts with chemical reagents, forming a colored stain specific to the target gas. The intensity of the stain is proportional to the gas concentration, and by regulating the sampling interval and flow rate, detection limits down to parts per billion can be achieved.

An electro-optical system measures the stain’s intensity by reflecting light from the tape surface, and the signal is converted into a gas concentration using a calibration curve.

The chemically treated tape is wound on a cassette and required to be stored in a refrigerator for storage before being opened and installed by the customer/contractor.