CO, CO2, and VOC Gas Detection Sensor Types and Applications - Industrial Sensing

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Introduction

Environmental gas detection sensors are devices used to trace various air pollutants like carbon monoxide (CO), carbon dioxide (CO2), volatile organic compounds (VOCs), sulfur dioxide (SO2), and nitrogen dioxide (NO2) that affect human health. During the COVID-19 pandemic, it was discovered that high CO2 levels might diminish cognitive ability. Although a typical human nose can whiff over 1 trillion distinct odors, most of us still fail to identify the concentration or type of gas present in the surrounding atmosphere. This is where sensors come in. A gas sensor detects the imbalance in toxic gases' concentration to maintain a safe system and avoid unexpected threats. Different gas sensors detect distinct gases like Oxygen, Nitrogen, Methane, Carbon Dioxide, and VOCs. These sensors detect leakage of harmful gases and supervise the air quality in industrial facilities, offices and homes. This article discusses the health dangers of CO, CO2, and VOC gases, as well as related sensors, including their construction, types, function, and applications.

CO, CO2, and VOC Gases and their Health Dangers

CO (Carbon monoxide) is an odorless and colorless toxic gas. It is generated by burning carbon-based materials or fuel. Carbon monoxide usually comes from fumes produced when fuel gets burned in vehicles, engines, stoves, furnaces, lanterns, grills, and fireplaces. It may accumulate indoors and be toxic to animals and humans who breathe in that atmosphere. Breathing CO may cause headaches, nausea, and dizziness. Higher levels of CO may cause unconsciousness or even death. Exposure to high and even moderate CO levels over long periods has also been linked with growing heart disease risk. Individuals who survive acute CO poisoning may suffer from deep-rooted health problems.

CO2 (Carbon dioxide) is the fourth most plentiful gas in the atmosphere. It is a non-flammable, colorless, and odorless gas at room temperature and is formed by burning fossil fuels or decaying vegetation. It is also a consequence of normal cell function when we exhale. Exposure to CO2 may influence our ability to make decisions, maintain a superior quality of work, and to have effective communication. It produces various health effects, including headaches, dizziness, difficulty breathing, sweating, tiredness, high heart rate, high blood pressure, coma, asphyxia, and convulsions. Table 1 shows the CO2 level and related physiological response.

Table 1: CO2 level and related physiological response (Source: Amphenol)

VOCs (Volatile Organic Compounds) include any carbon compound (except carbon monoxide, carbonic acid, ammonium carbonate, metallic carbides or carbonates, and carbon dioxide) that engages in atmospheric photochemical reactions. VOCs are emitted as gases from a few particular solids or liquids. They include numerous chemicals, a few of which may cause adverse health effects in the short- and long-term. VOC concentrations are routinely higher indoors than outdoors. Common sources of VOCs include building materials such as paint, caulks, composite wood products, adhesives, carpet, and vinyl flooring. Activities like smoking, dry cleaning, cooking, and burning wood also spew VOCs. The common health effects of short-term exposure to VOCs include eye, nose, throat irritation, headaches, nausea, and asthma.  Chronic exposure may lead to cancer, liver and kidney damage, and central nervous system damage.

Gas Detection Sensors and Types

Gas sensors, also known as gas detectors, are electronic devices that identify and trace different gases. These devices are employed to detect explosive or toxic gases. They also measure gas concentration. Gas sensors fitted in factories and manufacturing facilities identify gas leaks. They detect carbon monoxide and smoke in residences, too. Gas sensors differ considerably in sensing ability, range, and size (both portable and fixed). They are frequently a component of a larger embedded system. The sensing process and physical configuration may vary considerably, contingent on the environments and intended functions between sensors. Gas sensors are generally classified into several types, such as Electrochemical Gas Sensors, Catalytic Combustion Sensors, Infrared (IR) Sensors, Photoionization Detectors, and Metal Oxide Gas Sensors based on the type of sensing element. We will discuss some gas sensors based on these techniques with their structures and operation.

Carbon Monoxide (CO) Gas Sensor

A CO sensor is a device that detects the presence of carbon monoxide (CO) gas to stop carbon monoxide poisoning. It is normally used for toxic gases like carbon monoxide (CO) from car exhaust or furnace leaks. The CO gas diffuses into the membrane at the top of the sensor and then reacts with the sensing electrode's chemicals to produce an electrical current. This current is measured and displayed. If CO molecules are absent, no reaction occurs, and there is no current production. Other types of CO sensors include:

• Biomimetic sensors: these use a gel that changes color with carbon monoxide absorption, and this color change initiates the alarm.

• Metal oxide semiconductors: The circuitry of the silica chip traces carbon monoxide and reduces electrical resistance. This change initiates the alarm.

Figure 1: AMPHENOL SGX SENSORTECH Carbon Monoxide (CO) SensorCarbon Monoxide (CO) Sensor

Carbon Dioxide (CO2) Sensor

A carbon dioxide sensor is a device used to calculate gaseous carbon dioxide levels. They are classified into two types: IR-based and electrochemical. CO2 is not very reactive and is impossible to detect using electrochemical cells. The IR sensors determine the quantity of gas by the amount of light the gas absorbs, rather than by a chemical reaction. Gases absorb specific light wavelengths, and particular gases absorb specific frequencies. The light emitted by a special "source" proceeds via a filter that screens out everything except an individual light wavelength set, typically inside the spectrum's infrared part. Figure 2 illustrates a CO2 sensor based on IR.

Figure 2: CO2 sensor structure and inner workings (Source: Amphenol)

The following steps describe its operation:

• Carbon dioxide gas molecules absorb infrared light per the Beer-Lambert law. Non-Dispersive Infrared (NDIR) sensor modules apply this principle.

• An infrared source emits energy via a gas sample cell (waveguide). A tuned optical filter eliminates all but the target wavelength for the gas of interest (~4.3 µm for CO2) from interaction with the thermopile detector. The thermopile then changes the amount of filtered infrared energy to an electrical signal—the greater the gas concentration (CO2), the lower the infrared energy that reaches the detector.

• A microprocessor appraises the detector's electrical signal with sensor calibration constants and other parameters, offering a signal-conditioned gas concentration output (ppmv).

CO2 sensors are employed in HVAC applications to supervise air quality. They are also used to monitor fermentation, aerobic respiration, photosynthesis, and other carbon dioxide consuming or producing processes.

Amphenol offers a variety of CO2 sensors, including the T3000-Series (Figure 3). It is a Non-dispersive infrared (NDIR) gas sensor designed for measuring CO2 in harsh environments. This series of sensors are tested to operate in salt mist, ammonium, ethylene, SO2, and O2-saturated environments.

Figure 3: Amphenol T3000 Series CO2 SensorCO2 Sensor

Volatile Organic Compounds (VOCs) Gas Sensor

A VOC sensor is a device that computes ambient aggregation of comprehensive "reducing gases" correlated with poor air quality, like alcohols, amines, aldehydes, organic chloramines ketones, organic acids, and aliphatic and aromatic hydrocarbons. These gases all burn and cause the rise of VOC sensor ppm output. Two common techniques are used to detect VOCs: photoionization and flame ionization. Another type of VOC sensor is based on metal oxide semiconductors. In  a photoionization-based sensor sensor, instead of absorbing light, a light source is used as an ultraviolet (UV) spectrum to ionize electrons off of gas molecules. After the gas gets ionized, it proceeds via two charged plates, splitting the gas ions and the "free" electrons. With gas ions flowing towards the plates, the current generated between the two plates can be measured. Such current corresponds to the ionized molecule population. This current is then measured and displayed.

Figure 4: Telaire MiCS-VZ-89TE CO2 and VOC Sensor ModuleCO2 and VOC Sensor Module

Figure 4 shows a Telaire MiCS-VZ-89TE Integrated Sensor Module from Amphenol that combines MOS sensor technology with intelligent detection algorithms to supervise targeted VOCs and CO2 comparable variations in tight spaces. This dual signal output is utilized to administer ventilation on-demand, thus saving energy and also reducing cost-of-ownership. VOC sensors are used for indoor air quality, health, and safety. Outdoor applications include urban air monitoring networks, roadside air monitoring, and industrial perimeter monitoring.

Applications

Amphenol provides a range of CO, CO2, and VOC sensors based on technologies discussed previously. We will now design a few electronic circuits using these sensors for real-world applications.

Circuit with SGX-4CO Industrial Carbon Monoxide (CO) Sensor

The SGX-4CO is an electrochemical-type CO gas sensor with three electrodes, as shown in figure 5. It provides an output current of 70 ± 20nA / ppm corresponding to the gas concentration exceeding the  0 – 2000 ppm range.  As this is an electrochemical gas sensor, it needs a potentiostat (a bias circuit) to continue the accurate bias potential among the reference and sensing electrodes. A trans-impedance amplifier, also known as a current-to-voltage converter, is needed to change the small electrochemical currents into a useful voltage, which can then be measured.

The analog-to-digital converter (ADC) samples the trans-impedance amplifier's output and produces a digital reading of the voltage level. The microprocessor uses this to calculate the actual gas concentration. The microprocessor may drive several outputs, depending on applications. These could include an LCD, a 4 – 20 mA interface, many alarms, or other outputs as needed. Figure 5 shows a block diagram of a typical gas detection system using an electrochemical gas sensor. At some point in the system, zero settings and repeated setting adjustments are required. This can be implemented in the hardware at the trans-impedance amplifier, or in software within the microprocessor.

Figure 5: Typical Gas Detection System with CO gas sensor (Source: SGX Sensortech/Amphenol)

Circuit with IR11 Series Infrared Carbon Dioxide (CO2) Sensor

The SGX IR11 series is a CO2 sensor established on the Non-Dispersive Infrared (NDIR) principle to detect and track gases' existence. It provides a voltage output and works with a 3v - 5v DC power supply. These sensors are appropriate for reliable gas levels in general safety applications, where the sensor size is constrained and needs a flameproof enclosure for hazardous environments. These sensors are interfaced to a suitable circuit for power supply, output amplification, and signal processing. Sensor outputs require linearization and compensation for ambient temperature variation using algorithms in the system firmware. Figure 6 shows a block diagram of a typical gas detection system using an infrared gas sensor:

Figure 6: Gas Detection System with IR based CO2 gas sensor (Source: SGX Sensortech/Amphenol)

The IR gas sensor accommodates a lamp pulsed at low frequency by a lamp drive circuit. Pyroelectric detectors (pyros) are used to detect any infrared signal variations. The active pyro is sensitive to changes at IR wavelengths, typically absorbed by the traced gas. The reference pyro is sensitive to changes at a nearby IR wavelength, which is not absorbed by the gas being detected. The output signals are roughly sawtooth-shaped and amplified, and filtered. A bandpass amplifier is used to pass only the fundamental frequency and reduce any pyro noise at other frequencies. The amplifier outputs are approximately sinusoidal in shape.

The ADC samples the maximum and minimum of the amplifier outputs to ascertain the peak-to-peak level, and then a microprocessor is employed to calculate the actual gas concentration. The microprocessor may drive some outputs depending on the application, including an LCD, a 4 – 20 mA interface, multiple alarms, or other outputs as required.