Decoding gas sensors: Ultimate guide to cutting-edge gas sensors technologies

Gas sensors play a crucial role in industries by detecting environmental gases to ensure safety. They trigger visual and audible alarms to alert workers about hazardous gases and vapors, thus keeping the organization and employees safe. Choosing the right gas-sensing technology can be overwhelming. It's important to select the best sensor to ensure optimal safety and performance. This guide offers a comprehensive overview of various gas-sensing technologies, including application examples, principles of operation, and their advantages and disadvantages. It also provides information on how frequently you should calibrate a gas sensor.

What are the different types of gas sensors?

Gas sensors ensure a safe and sustainable working environment by optimizing processes, improving energy efficiency, and adhering to regulatory standards. Several gas sensing technologies are available, each operating on its distinct principle and with different strengths and weaknesses. Here's an overview of the various gas-sensing technologies.

Electrochemical sensors:

Electrochemical gas sensors detect toxic gases such as carbon monoxide (CO), oxygen (O2), ammonia (NH3), and hydrogen sulfide (H2S). They are essential in industries like medical and automotive and have numerous other applications.

Electrochemical gas sensors measure the target gas concentration through oxidation or reduction reactions. A standard electrochemical gas sensor consists of a 'sensing' electrode, a 'reference' electrode, and a 'counter' electrode, all housed with a liquid electrolyte (such as sulfuric acid) inside the sensor housing. The target gas reaches the 'sensing electrode' surface after passing through a hydrophobic membrane and a diffusion-limiting orifice. The 'oxidation' or 'reduction' reaction occurs at the surface of the 'sensing' electrode, resulting in a current between the 'counter' and 'sensing' electrodes. The magnitude of this current is proportional to the gas concentration.

Electrochemical gas sensors offer advantages such as compact size, high sensitivity, reliable detection, cost-effectiveness, and the ability to be integrated into various systems and devices. However, these sensors have certain drawbacks, including the short service life of electrodes due to extreme temperatures and low humidity. They are also susceptible to temperature fluctuations, leading to false alarms.

Figure 1: Construction of SGX Electrochemical Gas Sensor (Source: SGX Sensortech)

Pellistor gas sensors:

The Pellistor gas sensor is a catalytic sensor designed to detect flammable gases. It consists of a ceramic bead with a platinum coil coated in a high-surface-area catalytic layer. When it comes into contact with gases such as methane, the catalytic layer facilitates oxidation at approximately 500°C, generating heat that alters the coil's resistance. This change in resistance is then measured to determine the gas concentration. The sensor's lower operating temperature enhances stability and longevity by minimizing platinum evaporation.

Figure 2: Structure of Pellistor Gas Sensor (Source: SGX Sensortech)

Infrared gas sensors:

The Infrared (IR) detection technique is based on how much-infrared radiation is absorbed in specific wavelengths when it passes through a volume of gas. The detector comprises three parts: an infrared light source, an optical filter to select the proper wavelength and a single optical infrared receiver. When the gas flows into the gap between the receiver and the source, the hydrogen molecules in the gas absorb some of the infrared energy. The receiver detects this drop in received energy to measure the amount of hydrocarbon gas present. The infrared gas detector often uses two wavelengths of infrared energy: one for gas absorption and the other for reference wavelength. The Infrared gas sensor consists of an infrared source, detector, optical filter, gas cell, and electronics for signal processing. The sensor can detect gases such as CO2, hydrocarbons, and acetylene, making it suitable for indoor and outdoor environments in industrial, commercial, and residential settings. It has a measurement range of 0-1000 ppm, with an accuracy of two percent of the measurement range and a response time of under 40 seconds. Infrared gas sensors have advantages such as a wide gas detection range, high accuracy, selectivity, fast response, energy-efficient operation, and long lifespan.

Figure 3: Construction of Infrared Gas sensor (Source)

Photoionization Detectors (PID)

Figure 5 illustrates the construction of a PID sensor. The device includes an ionization chamber, an ultraviolet lamp, and positive and negative electrodes. The gas enters the ionization chamber and is exposed to UV light from the lamp, which releases electrons and generates cations. The positive and negative electrodes then draw the cations and electrons, producing a current that is proportional to the gas concentration.

The PID sensor is used to detect VOCs and gases in industrial and commercial settings. It has a measurement range of 0–30 ppm and an accuracy of ±10%. The sensor has a T90 response time of under 75 seconds. The advantages of PID include high sensitivity, discrimination against volatile organic compounds, fast response time, wide measurement range, compact design, suitability for fixed and portable systems, and compatibility with data logging and remote monitoring.

Figure 4: PID sensor Working Principle (Source)

MEMS Gas Sensor

MEMS gas sensors can detect various gases, such as Carbon dioxide (CO2), Nitrogen Oxide (NOX), Sulfur dioxide (SO2), and formaldehyde. They are suitable for various aerospace, defense, automotive, healthcare, consumer electronics, agriculture, and food industry applications. The MEMS gas sensor has a measurement range of 0-10 ppm and an accuracy of ±10%. It has a T90 response time of less than 15 seconds. The advantages of a MEMS gas sensor include their compact size, high sensitivity, low cost, and integration compatibility with electronic systems.

Micro Electromechanical System (MEMS) gas sensors, also known as MEMs Pellistors, detect gas molecules through electrochemical reactions on the surface of their electrodes. Gas molecules interacting with electrode materials result in varying electrochemical reactions, which in turn cause fluctuations in potential difference or current. These fluctuations serve as signals indicating the concentration of the respective gas. The operational process of a MEMS gas sensor involves several steps: preheating the device, absorbing the gas, detecting the gas, amplifying and processing the signal, and outputting the data.

Photoacoustic gas sensor

A photoacoustic gas sensor measures gas concentration by converting light energy into acoustic waves. Figure 6 demonstrates the working principle of a typical photoacoustic gas sensor. The process can be broken down into the following steps:

1. A high-intensity light source emits specific wavelengths.
2. These wavelengths cause the gas to interact and undergo thermal expansion.
3. This interaction generates acoustic waves.
4. An acoustic detector measures these waves and converts them into electrical signals.
5. Signal analysis determines the target gas concentration in the sample.

For instance, light from an infrared source passes through an optical filter tuned to the CO2 absorption wavelength (λ = 4.2 µm). CO2 molecules absorb the filtered light, producing a photoacoustic effect. An acoustic detector optimized for low-frequency operation detects the sound, and a microcontroller converts it into a CO2 concentration reading. The absorption chamber is acoustically isolated to ensure accurate sensing.

This sensor detects gases like CO2, CH4, and NO2 and is frequently used in industrial, environmental, and medical settings. The key specifications of photoacoustic gas sensors include:

• Measurement range: 0-5000ppm
• Accuracy: ±10%
• Response time: less than 75 seconds

The advantages of photoacoustic gas sensors include:

• High sensitivity, selectivity, and reliability in detecting specific gases.
• Ability to measure trace levels of gases.
• Non-destructive and non-contact measurement techniques.
• Compatibility with various gas compositions.

Figure 5: Working principle of photoacoustic gas sensor (Source)

A comparison table of various Gas Sensor types

Table 1: Gas Sensor Technical Specifications

Which Gas Sensing Technology is suitable for my specific application?

The choice of gas sensing technology selection depends on several factors, including the type of gas to be detected, the portability of the equipment, whether the sensor is fixed or mobile, whether single or composite gas, diffusion or pumping, and the various gas sensing principles involved.

1. Target gas: These sensors are designed to detect specific gases, such as carbon monoxide (CO), methane (CH4), and hydrogen sulphide (H2S).
2. Measurement Specifications: Consider response time, distance, and flow rate. These specifications are essential to consider when looking for gas sensors.
3. Sensor Output: Gas sensors measure gases in several ways:

• Lower explosive limit (LEL) or lower flammable limit (LFL): Measures the smallest amount of the gas that will support a self-propagating flame when mixed with air (or oxygen) and lit. In gas-detection systems, the amount of gas is measured in terms of % LEL, where 0% LEL is an atmosphere with no dangerous gases, and 100% LEL is an atmosphere where the gas is at its lower explosive limit. Different gases have different relationships between % LEL and % by volume.
• Volume per cent or per cent by volume: This is used for liquid mixes. It is calculated by dividing the volume of the solute by the total volume of the mixture, then multiplying by 100.
• Trace: Measures concentration in parts per million (ppm).
• Leakage: Given as a flow rate, such as ml/min.
• Consumption: Also called breathing, given in terms of ml/L/h.
• Density: Readings given in density: mg/m3.
• Signatures or Spectra: Provides a spectral signature of the gases present; the result is often a chromatogram.

When choosing gas sensing technology, several factors should be considered, including the type of gas to be detected, equipment portability, whether the sensor is fixed or mobile, whether it detects single or composite gases, and the gas sensing principles involved.

1. Target gas: These sensors are designed to detect specific gases, such as carbon monoxide (CO), methane (CH4), and hydrogen sulphide (H2S).
2. Measurement Specifications: When looking for gas sensors, consider response time, distance, and flow rate. These specifications are essential.
3. Sensor Output: Gas sensors measure gases in several ways:
   • Lower explosive limit (LEL) or lower flammable limit (LFL): Measures the smallest amount of the gas that will support a self-propagating flame when mixed with air (or oxygen) and lit. In gas-detection systems, the amount of gas is measured in terms of % LEL, where 0% LEL is an atmosphere with no dangerous gases, and 100% LEL is an atmosphere where the gas is at its lower explosive limit. Different gases have different relationships between % LEL and % by volume.
   • Volume per cent or per cent by volume: This is used for liquid mixes. It is calculated by dividing the volume of the solute by the total volume of the mixture, then multiplying by 100.
   • Trace: Measures concentration in parts per million (ppm).
   • Leakage: Given as a flow rate, such as ml/min.
   • Consumption: Also called breathing, given in terms of ml/L/h.
   • Density: Readings given in density: mg/m3.
   • Signatures or Spectra: Provides a spectral signature of the gases present; the result is often a chromatogram.
4. Consider the gas concentration range: Determine the expected concentration range of the target gas in your application. Gas sensors have specific detection limits and dynamic ranges. The selected sensor must cover the required concentration range.
5. Assess environmental conditions: The gas sensor’s operating environment must be considered.
   • Temperature: Temperature affects gas sensor performance. Sensor technologies have ideal temperature ranges. Extreme temperatures can cause sensor drift, reduced sensitivity, or failure. Consider the gas sensor's operating temperature range when choosing a sensor technology.
   • Humidity: Some electrochemical sensors require specific humidity levels to operate optimally. For example, an EC gas sensor functions best at 20°C and 60% relative humidity. Humidity below 60% can cause the sensor's electrolyte to dry out, affecting the response time. Conversely, humidity above 60% can lead to water absorption, diluting the electrolyte and affecting the sensor's characteristics. In high humidity, catalytic beads or infrared sensors may be more effective.
   • Other Environmental Parameters: Pressure, vibrations, dust, and airborne chemicals can influence sensor performance. Some sensor technologies are more durable than others. For instance, solid-state sensors like MEMs Pellistors are less susceptible to vibrations than fragile catalytic bead sensors. Assess the ambient circumstances and select a sensor technology that can function reliably.
6. Evaluate power requirements: Gas sensors may have different power requirements, including voltage levels and power consumption. Ensure the selected sensor matches the power supply capabilities and energy constraints. A battery for low-power and long-term monitoring applications can power gas sensors. Some gas technologies, such as MEMS Pellistors, can be powered by a pulse power supply.
7. Response time and stability: Assess the required response time for your application. Gas sensors can have varying response times, ranging from a few seconds to minutes. Also, the sensor's stability and reliability over time should be considered.
8. Cost considerations: Gas sensors can vary in price based on technology, performance, and features. Determine the budget for your project and choose a sensor that meets your requirements without exceeding your cost limitations.

Application Example of Monitoring Air Quality Using Gas Sensor

The SGX Sensortech MICS-4514 is a metal oxide semiconductor gas sensor that monitors air quality and identifies potential health risks associated with nitrogen dioxide (NO2) and ozone (O3) gases. It is integrated into air quality monitoring systems in urban areas, providing real-time data on concentration levels. The collected data can help local authorities and environmental agencies assess air quality, identify pollution hotspots, and develop strategies to mitigate the impact of NO2 and O3 on public health. Continuous sensor monitoring helps detect short-term spikes or long-term trends in gas concentrations. This detection can guide decision-making processes, including implementing traffic control measures, adjusting industrial emissions, or alerting the public about potential health risks. The MICS-4514 sensor supports proactive air quality management in urban environments, leading to healthier living conditions and improved environmental sustainability.

Figure 6: Different types of interaction between atmospheric gases and a MOS sensor (Source)

Calibration of Gas Sensors

There are three methods of calibrating gas sensors: span (2-point), single-point, and automatic background calibration. A bump test is recommended to check the target gas's accuracy and response within an acceptable range. If a sensor fails a bump test, immediate calibration is necessary.

Gas detectors should be bump-tested monthly and calibrated biannually for environments with hazardous gases and frequent or prolonged human exposure. Detectors in workplaces, schools, and hospitals must also be calibrated biannually, while those in underground parking facilities should be calibrated annually. Sensors used for scientific experimentation must be calibrated before each test. Similarly, in the case of greenhouse monitoring, it should be calibrated after each growing season.

Gas detectors need regular maintenance, including inspection, bump testing, and calibration. Issues are often found during bump testing or calibration, indicating problems that arise between these activities. Over time, gas sensors' electronic output decreases, and calibration readjusts the output to account for the sensor's declining lifespan. The sensor can "drift without regular calibration," leading to inaccurate readings.

There are three methods of calibrating gas sensors: span (2-point), single-point, and automatic background calibration. A bump test is recommended to check the accuracy and response of the target gas within an acceptable range. If a sensor fails a bump test, immediate calibration is necessary.

Gas detectors should be bump-tested monthly and calibrated biannually for environments with hazardous gases and frequent or prolonged human exposure. Detectors in workplaces, schools, and hospitals should also be calibrated biannually, while those in underground parking facilities should be calibrated annually. Sensors used for scientific experimentation must be calibrated before each test. Similarly, in the case of greenhouse monitoring, calibration should be done after each growing season.

Conclusion

When choosing gas-sensing technology, engineers must consider several factors: power consumption, accuracy, specific gases, and the environment. Different gas sensing technologies have their advantages and limitations. This guide aims to simplify the sensor selection process. Avnet collaborates with suppliers and manufacturers to offer gas sensors and associated accessories.