Most drivers in the city have encountered the scenario shown in the picture. So, on what basis can the police determine that you are suspected of driving under the influence of alcohol or drunk driving simply by letting you blow into their breath?
This brings us to the testing equipment used by the police in law enforcement – the breathalyzer. The core component of the breathalyzer is a gas sensor that can accurately measure the composition and concentration of alcohol. Law enforcement officers use it to measure whether the breath of a driver contains alcohol and how much alcohol, as a basis for judging whether the other party is suspected of drunk driving.
Definition of gas sensor
A gas sensor is a sensor used to detect the presence of a specific gas within a certain area and/or to continuously measure the concentration of gas components.
In safety protection applications such as coal mines, petroleum, chemical industry, municipal engineering, medical care, transportation, and homes, gas sensors are often used to detect the concentration or presence of combustible, flammable, or toxic gases, or the amount of oxygen consumed.
In manufacturing sectors such as the power industry, gas sensors are commonly used to quantitatively measure the concentration of various components in flue gas to determine combustion status and the emission of harmful gases. In atmospheric environmental monitoring, the use of gas sensors to determine environmental pollution levels is even more prevalent.
History of gas sensors
The first semiconductor sensor was invented in Britain in the early 20th century and was developed and applied in Europe until the 1950s when semiconductor sensing technology spread to Japan. In May 1968, Naoyoshi Taguchi, the founder of Figaro Technical Research Institute, invented the semiconductor gas sensor.
It can detect low concentrations of flammable and reducing gases with a simple circuit. This semiconductor gas sensor, named TGS (Taguchi Gas Sensor), is built into gas leak alarms. Many homes and factories in Japan and overseas have installed these alarms to detect leaks of gases such as liquefied petroleum gas, thus pushing this technology to its peak.
After discovering the various shortcomings of semiconductor sensors, Europeans began to research catalytic and electrochemical sensors. The theory of gas sensors was not introduced to China until the 1970s, and my country began to develop gas sensors in the 1980s, with the entire production technology mainly inherited from Germany.
Classification of gas sensors
Based on the type of gas detected, sensors are generally classified into combustible gas sensors (often using catalytic combustion, infrared, thermal conductivity, and semiconductor types), toxic gas sensors (generally using electrochemical, metal semiconductor, photoionization, and flame ionization types), harmful gas sensors (often using infrared and ultraviolet types), oxygen sensors (often using paramagnetic and zirconium oxide types), and other types of sensors.
In terms of usage, gas sensors are generally divided into portable gas sensors and stationary gas sensors.
Based on the method of obtaining gas samples, gas sensors are generally divided into diffusion gas sensors (i.e., the sensor is directly installed in the environment of the object being measured, and the gas being measured comes into direct contact with the sensor detection element through natural diffusion) and inhalation gas sensors (i.e., the gas to be measured is introduced into the sensor detection element for detection using means such as an inhalation pump. Depending on whether the gas to be measured is diluted, it can be further subdivided into complete inhalation type and dilution type, etc.).
Based on the analysis of gas composition, gas sensors are generally divided into single-gas sensors (which detect only a specific gas) and multi-gas sensors (which detect multiple gas components simultaneously).
Based on their detection principles, gas sensors are generally classified into thermal gas sensors, electrochemical gas sensors, magnetic gas sensors, optical gas sensors, semiconductor gas sensors, and gas chromatographic gas sensors.
Detection principles, characteristics, and applications of different gas sensors
Thermal gas sensor
Thermal gas sensors are mainly divided into two categories: thermal conductivity type and thermochemical type. Thermal conductivity type sensors utilize the thermal conductivity of a gas and measure the concentration of one or more gas components by observing the change in resistance of a thermistor. They have been used in industry for decades, and there are many types of instruments available, capable of analyzing a wide range of gases.
Thermochemical formulas are based on the thermal effect of the chemical reaction of the gas being analyzed. Among them, the oxidation reaction of the gas (i.e., combustion) is widely used. A typical example is the catalytic combustion gas sensor. Its main working principle is that at a certain temperature, the conductivity of some metal oxide semiconductor materials will change with the composition of the ambient gas.
Its key component is a Wheatstone bridge coated with a combustion catalyst, which is mainly used to detect combustible gases, such as CO, H2, C2H2 and other combustible gases in the air in gas generating stations and gas plants; CH4 content in tunnels in coal mines; methane content in leaks on oil extraction vessels; and petroleum vapor, alcohol vapor and ether vapor in the air in fuel and chemical raw material storage warehouses or raw material workshops.
Left image: Schematic diagram of the catalytic element (1-catalyst; 2-support; 3-Pt heating wire); Right image: Test circuit for the catalytic element
Main advantages: It has a broad spectrum of response to all combustible gases, is not sensitive to the influence of ambient temperature and humidity, has a near-linear output signal, and has a simple structure, low cost, accurate measurement, fast response, and long life.
Main drawbacks: low precision, high operating temperature (internal temperature can reach 700-800℃), posing a risk of ignition and explosion. High current consumption, and susceptible to poisoning by sulfides, halogen compounds, etc.
Electrochemical gas sensor
Electrochemical gas sensors utilize the electrochemical activity of the gas being measured to electrochemically oxidize or reduce it, thereby distinguishing the gas composition and detecting the gas concentration.
Schematic diagram of an electrochemical gas sensor
Electrochemical gas sensors are divided into many subcategories:
1. Galvanic cell gas sensors (also known as Gavoni cell gas sensors, fuel cell gas sensors, or self-generating cell gas sensors) operate on the same principle as dry cell batteries, except that the carbon-manganese electrodes are replaced by gas electrodes. Taking an oxygen sensor as an example, oxygen is reduced at the cathode, and electrons flow through an ammeter to the anode, where lead metal is oxidized. The magnitude of the current is directly related to the oxygen concentration. This type of sensor can effectively detect oxygen, sulfur dioxide, chlorine, etc.
2. Constant potential electrolytic cell type gas sensor: This type of sensor is very effective for detecting reducing gases. Its principle differs from that of a galvanic cell type sensor; its electrochemical reaction occurs under forced current, making it a true coulometric analysis sensor. This sensor has been successfully used to detect gases such as carbon monoxide, hydrogen sulfide, hydrogen, ammonia, and hydrazine, and is currently the mainstream sensor for detecting toxic and harmful gases.
3. Concentration cell type gas sensor: When an electrochemically active gas is placed on both sides of an electrochemical cell, a concentration electromotive force is spontaneously generated. The magnitude of the electromotive force is related to the concentration of the gas. Successful examples of this type of sensor include automotive oxygen sensors and solid electrolyte carbon dioxide sensors.
4. Limiting current type gas sensor: One type of oxygen concentration sensor utilizes the principle that the limiting current in an electrochemical cell is related to the carrier concentration to prepare an oxygen (gas) concentration sensor, which is used for oxygen detection in automobiles and oxygen concentration detection in molten steel.
Main advantages: small size, low power consumption, good linearity and repeatability, resolution can generally reach 0.1ppm , and long lifespan.
Main drawbacks: Susceptible to interference, sensitivity is greatly affected by temperature changes.
Zirconia oxygen sensors are a relatively recent development among electrochemical composition analysis sensors, first appearing in the 1960s. Their working principle is based on the concentration cell principle, measuring the oxygen content in the gas to be analyzed by measuring the concentration electromotive force caused by the difference in oxygen concentration between the gas to be analyzed and the reference gas.
Due to its advantages such as simple structure, reliable operation, high sensitivity, good stability, fast response speed, and convenient installation and use, it has developed rapidly. It is commonly used for oxygen analysis of multi-component gases such as sulfuric acid, air separation, and boiler combustion, as well as for determining the oxygen content of molten metals.
Magnetic gas analysis sensor
Among magnetic gas analysis sensors, the most common is the magnetic oxygen analyzer, which utilizes the high magnetization of oxygen to measure its concentration. It operates on the principle that oxygen in the air can be attracted by a strong magnetic field. It has the widest oxygen measurement range and is a highly effective oxygen measurement sensor.
Commonly used oxygen analyzers include thermomagnetic convection oxygen analyzers (which can be further subdivided into velocity thermomagnetic and pressure-balanced thermomagnetic types according to their structure) and magnetomechanical oxygen analyzers.
Main applications: Used for oxygen detection, with excellent selectivity, and is the core of a magnetic oxygen analyzer. Typical applications include oxygen control and interlocking in industrial production such as fertilizer production, cryogenic air separation, thermal power plant combustion systems, and natural gas-to-acetylene production, as well as environmental monitoring of emissions of waste gas, tail gas, and flue gas.
Optical gas sensor
Optical gas sensing technology is one of the latest but fastest-growing technologies. Commonly used types in industry include infrared gas analyzers, ultraviolet analyzers, photoelectric colorimetric analyzers, chemiluminescence analyzers, and light scattering analyzers.
Infrared absorption spectra of various gases
Infrared gas sensors work by utilizing the infrared absorption spectrum characteristics or thermal effects of the gas being measured to determine its concentration. The commonly used spectral range is 1–25 μm, and common types include DIR dispersive infrared and NDIR non-dispersive infrared. Infrared gas sensors can effectively distinguish gas types and accurately determine gas concentrations, including the detection of carbon dioxide and methane. Infrared detectors use a light source that requires no modulation, have no moving mechanical parts, and are completely maintenance-free.
Commonly used ultraviolet analyzers include non-spectral ultraviolet analyzers and ultraviolet fluorescence analyzers. The former is similar to infrared absorption principle, based on the selective absorption of ultraviolet light by the measured gas. Its absorption characteristics also obey Beer's Law, and the ultraviolet wavelength range used is 200-400nm.
The latter, such as the ultraviolet fluorescence SO2 analyzer, is a dry analyzer. Its working principle is based on the fact that SO2 molecules accept ultraviolet energy to become excited SO2 molecules, and produce characteristic fluorescence when returning to a steady state. The intensity of the emitted fluorescence is proportional to the SO2 concentration.
Ultraviolet fluorescence can continuously and automatically measure the SO2 content in the atmosphere without damaging the sample. Its sensitivity can reach 0 to 2 × 10⁻⁷ in the measurement range, its stability can be achieved with a drift of ±2% of full scale over 24 hours, and its repeatability can reach ±2% of full scale. Moreover, the influence of coexisting background gases on the measurement is small, and it has significant advantages such as long service life and low maintenance workload.
Photoelectric colorimetry is based on Beer's Law to achieve automatic photoelectric colorimetric measurement. It is applicable to the analysis of SO2, NO, hydrocarbons, halogen compounds, etc.
Chemiluminescence analyzers work by utilizing the photothermal generation principle associated with chemical oxidation reactions. Commonly used chemiluminescence analyzers include ozone analyzers (which use the photons emitted by the chemiluminescence reaction of O3-C2H4 to measure ozone) and chemiluminescence NOx analyzers (which utilize the strong oxidizing effect of O3 to cause NO to undergo a chemiluminescence reaction with O3 to achieve measurement).
Light scattering analyzers use the interaction between a light beam and particles in a gas to produce scattering (forward scattering, side scattering, and back scattering) to measure the turbidity or opacity of a gas. They are one of the most commonly used analytical instruments in environmental emission monitoring.
Semiconductor gas sensor
Semiconductor gas sensors measure gas concentration based on the conductivity, current-voltage characteristics, or surface potential changes that occur when a detection element made of metal oxide or metal semiconductor oxide interacts with a gas, resulting in surface adsorption or reaction and causing carrier movement.
Schematic diagram of the surface charge layer model
When metal oxide semiconductors are heated to a certain temperature in air, oxygen atoms are adsorbed onto the negatively charged semiconductor surface. Electrons on the semiconductor surface are transferred to the adsorbed oxygen, and the oxygen atoms become oxygen anions. At the same time, a positive space charge layer is formed on the semiconductor surface, which leads to an increase in the surface potential barrier, thereby hindering the flow of electrons.
Inside the sensitive material, free electrons must pass through the bonding sites (grain boundaries) of metal oxide semiconductor microcrystals to form an electric current. The potential barrier generated by oxygen adsorption also exists at the grain boundaries, hindering the free flow of electrons. The resistance of the sensor is due to this potential barrier. Under operating conditions, when the sensor encounters a reducing gas, the oxygen anions undergo an oxidation-reduction reaction with the reducing gas, resulting in a decrease in their surface concentration and a decrease in the potential barrier, which in turn reduces the size of the sensor.
Based on their working mechanism, gas sensing elements can be categorized into surface-controlled (using gas adsorption on a semiconductor surface to generate changes in conductivity), surface potential-controlled (using changes in surface or interfacial potential after gas adsorption on a semiconductor), and volume-controlled (based on the working principle of changes in conductivity caused by changes in volume when a semiconductor reacts with a gas). They can detect percentage concentrations of combustible gases and also toxic and harmful gases at the ppm level.
Main advantages: simple structure, low price, high detection sensitivity, fast response speed, etc.
Main drawbacks: small linear measurement range, significant interference from background gases, and susceptibility to ambient temperature.
Gas Chromatography Analyzer
Gas chromatographs are analytical instruments that separate and determine the concentration of each component in a gas sample based on chromatographic separation and detection technologies, thus serving as comprehensive analytical sensors. They are already being used in power plant boiler testing.
During operation, a certain volume of gas sample is periodically taken from the injection device and carried by a pure carrier gas (i.e., mobile phase) at a certain flow rate. The sample flows through the chromatographic column, which contains a solid or liquid called the stationary phase. By utilizing the different absorption or solubility of each component of the gas sample by the stationary phase, the components are repeatedly distributed between the two phases, thereby separating the components and allowing them to flow out of the chromatographic column in sequence and enter the detector for quantitative determination.
Based on the detection principle, gas chromatographs are further divided into two types: concentration detectors and mass detectors.
Concentration detectors measure the instantaneous change in the concentration of a component in a gas; that is, the detector's response value is directly proportional to the component's concentration.
Mass-type detectors measure the rate at which a component in a gas enters the detector; that is, the detector's response value is proportional to the amount of that component entering the detector per unit time. The most commonly used detectors include the TCD thermal conductivity detector, the FLD flame ionization detector, the HCD electron capture detector, and the FPD flame photometric detector.
Main advantages: High sensitivity, suitable for trace and micro-volume analysis, capable of analyzing complex multiphase gases.
Main drawbacks: Periodic sampling cannot achieve continuous sample injection analysis, the system is relatively complex, it is mostly used for laboratory analysis, and it is not very suitable for industrial field gas monitoring.
Currently, there are gas chromatographs that use computer-controlled instrument systems for operation and data processing, and can provide alarms for components exceeding limits, as well as automatic instrument fault detection functions.
As health issues receive increasing attention, air quality, indoor air quality, and in-vehicle air quality monitoring data have become data that people want to see anytime, anywhere, and gas sensors will undoubtedly play an increasingly important role in this process.
As a converter that transforms the volume fraction of a gas into a corresponding electrical signal, gas sensors have a wide range of applications. In real life, gas sensors play a significant role in civilian, industrial, and environmental monitoring fields.
