The Dos and Don'ts of VOC Sensors
Robin Mitchell for Mouser Electronics
The term volatile is often used to describe something as sensitive, dangerous, or explosive, but its scientific meaning is somewhat divergent. In scientific terms, volatile refers to a substance that has a high vapor pressure such that the substance will readily turn from a solid or liquid into a gas. The degree to which a compound is said to be volatile relates to an environment at room temperature and standard sea level pressure. As such, volatile organic compounds (VOCs) are any organic compound that has a high vapor pressure.
Although the term volatile doesn’t necessarily mean explosive or combustible, most VOCs are flammable, and many can be explosive at low concentrations. Because of this, the sensitivity of VOCs presents engineers with significant challenges in environments exposed to an ignition source, where VOCs can be ignited by electrical contacts, switches, and even static electricity caused by clothing.
VOCs are widely found in industrial processes and in nature. Examples include hydrogen sulfide in oil production and ethanol (alcohol) in natural fermentation (however, risks from ethanol are generally introduced during distillation and not through natural fermentation).
VOCs can also be sourced from synthetic means, and such VOCs are commonly used as refrigerants thanks to their high vapor pressure. The compression, cooling, and subsequent vaporization of a VOC can be used to achieve low temperatures, which makes VOCs ideal for applications such as heat pumps.
A wide variety of sensors are available to measure these VOCs, and each technology has distinct advantages and disadvantages. Implementing any VOC sensor technology requires significant consideration of the environment it will be used in as well as the nature of the manufacturing process.
Typical Applications for VOC Sensors
By far, one of the most critical applications for VOC sensors is monitoring explosive gases. An environment that carries the risk of the accumulation of VOCs will always require a gas detection system to alert anyone nearby. For example, oil and gas drilling and production require gas detection systems because the process’s release of hydrogen sulfide can be deadly to workers (through either an explosion or poisoning). VOC sensors are also helpful for detecting gas leaks. When attached to a portable wand or personal wearable, a VOC sensor can help engineers identify the potential source of a leak.
The presence of VOCs in the air also affects air quality, especially inside buildings. Facility managers can place VOC sensors in air quality systems where the build-up of key VOCs can indicate poor air quality. Such a system can then be tied to a building's air conditioning system and pump in fresh air.
Finally, VOC sensors are critical in monitoring vehicle exhaust gases. A vehicle that combusts its fuel correctly will produce only carbon dioxide and water, but an engine that does not perform optimally will produce VOCs (among other things). Test facilities can use a VOC sensor to check the performance and efficiency of an engine.
Challenges in Using Sensor Technology to Measure VOCs
As previously stated, VOCs can carry a severe risk of explosion or fire; thus, any sensor used to measure a VOC must do so without igniting the VOC. Sensors that expose electrical components to VOCs can generate a spark under fault conditions, and such a spark could ignite the VOC. Therefore, direct sensing methods that expose conductors must either incorporate fire arrestors (i.e., systems that prevent an ignited mixture from causing a cascade effect) or ensure that sparks cannot form between conductors.
Additionally, the potential of VOCs to be dangerous at low concentrations makes detection at these levels difficult. Trying to detect a compound at single-parts-per-million (ppm) levels presents a multitude of challenges for a sensor. VOCs are also highly reactive, which means that trying to detect a specific VOC is difficult if the sensor in question uses chemical binding (i.e., a sensor would only recognize the presence of a VOC not which VOC is present).
Types of VOC Sensors
Metal Oxide Semiconductor Sensors
One of the most common gas sensors on the market is the metal oxide semiconductor (MOS) gas sensor, which uses a direct sensing method whereby gases under detection make physical contact with the sensing material. To detect VOCs, MOS sensors use a small heating element that oxidizes the VOC. This oxidized compound then reacts with a metal oxide layer (usually tin oxide), which changes the layer's resistance.
While these sensors are often the cheapest and easiest to implement, they come with numerous challenges. MOS sensors use a small heater that takes time to heat up and become operational; this also means they cannot be switched on and off quickly. Second, these sensors can require up to 48 hours of settling time before they can be calibrated, creating challenges when working with a manufactured product.
Because MOS sensors react with organic and inorganic compounds, they offer little to no discrimination (i.e., they will detect the presence of all volatile compounds), leading to poor accuracy and low sensitivity. Additionally, using an onboard heater to oxidize VOCs presents an ignition risk. While many MOS sensors include cages to prevent ignition, damaged MOS sensors could be extremely dangerous for environments that frequently expect VOC leaks.
Photoionization Detection Sensors
Photoionization detection (PID) sensors use high-frequency light to break up VOC molecules, and the resulting broken molecules create an electric current that can be measured. PID sensors offer a high degree of accuracy, are sensitive to concentrations as low as 0.5 parts per billion (ppb), and react to changes in concentrations in seconds.
Selectivity on PIDs can be partially achieved by using a specific frequency of light, which will provide a known amount of energy to each molecule. This is defined by the Planck relation (E=hf), which states that the energy of an electromagnetic wave is directly related to its frequency. Because specific VOCs will have certain activation energies, a PID sensor will ignore VOCs under a particular energy but will react for those above this limit.
However, PIDs may not function properly in humid environments, and their applications are limited owing to an inability to detect small VOC molecules such as methane. Furthermore, PID sensors generally work with functional groups, but not hydrocarbon chains.
Electrochemical Sensors
Electrochemical sensors are like MOS sensors in that they oxidize a VOC to produce an electric current. While MOS sensors use a heating element to physically combust the gas, an electrochemical sensor uses a membrane that allows a VOC to diffuse (along with oxygen) and chemically combine at an activation site. This diffusion layer removes the explosive risk while allowing the sensor to operate at resolutions down to 10ppb.
Electrochemical sensors are cost-effective and have response times of around 30 seconds. Additionally, these sensors use a base voltage that allows for selectivity, which can be ideal for identifying a specific VOC. However, their construction means they have a short lifespan of less than two years (typical) and must be replaced frequently.
Intelligent Sensors
Some VOC sensors take things a step further by integrating artificial intelligence (AI) and machine learning (ML). These sensors use specially trained models that take sensed gases as an input and then use that input to help classify the VOCs.
By performing classification directly at the sensor, the sensor can provide the system with more detailed and valuable data that can be used for decision making. Bringing the intelligence to the system also has the benefit of offloading computing requirements from the rest of the system.
Flame Ionization Detection Sensors
Flame ionization detection (FID) sensors use a hydrogen flame placed between two electrodes. Under nominal conditions, the hydrogen flame produces no ions as the hydrogen is fully combusted into water vapor. However, any VOC that burns under the flame will produce ions, which can be detected via electrodes, and the size of the resulting current represents the concentration of the VOC.
FIDs are low-cost, require low maintenance, and are extremely rugged to ensure proper operation. Furthermore, FIDs are extremely linear in that the current produced is proportional to the VOC concentration. However, FIDs are destructive sensors; that is, the VOC being measured is destroyed. As such FIDs cannot be used where a VOC must be left unaltered. They also introduce an explosion risk as they rely on a hydrogen source that could leak if not installed correctly.
Photoacoustic Sensors
Photoacoustic sensors rely on the photoacoustic principle whereby absorbed light results in the production of sound waves. Essentially, a photoacoustic sensor uses an infrared (IR) emitter to heat a gas quickly, and the resulting soundwaves produced by the absorption of the IR light are detected with a small integrated microphone. Photoacoustic sensors are somewhat of a rarity, and those that do exist are typically used for the detection of carbon dioxide. However, research is under way to produce photoacoustic sensors for use in VOC applications.
Photoacoustic sensors are advantageous in that they can distinguish between different gases that produce different sound waves. Additionally, the use of a selective light source presents a theoretical possibility for greater selectivity as some gases will absorb specific wavelengths of light better than others.
Design Integration Considerations
When incorporating a VOC sensor into an application, engineers must consider a multitude of design options. Unlike most electronic components, VOC sensors can be extraordinarily sensitive to chemicals, temperature swings, and humidity in the environment. Additionally, many will have a plastic tab or cover to prevent damage to the sensor during manufacturing.
When incorporating a VOC sensor into a device, engineers must consider the temperature and humidity of the environment. Some VOC sensors will not operate correctly when exposed to temperature and humidity extremes, which can be disastrous in safety applications.
The gases that can be expected in the environment must also be considered. Some compounds (e.g., ammonia and nitrogen oxides) can trigger false alarms in VOC sensors. Furthermore, some compounds can even poison a sensor, meaning that its operation will no longer be reliable if exposed.
Because a VOC sensor requires exposure to the monitored environment, engineers must incorporate vents in the sensor enclosure. The sensor and vent should also be positioned in such a way that gas can flow through easily (i.e., convection); otherwise, air trapped in the device could result in false measurements.
Sensors that produce an analog output must be mounted away from noisy circuitry such as switch-mode power supplies and microcontrollers. Additionally, analog lines running from the sensor to a detector circuit must not allow high-speed signals to cross over the sensor lines (see electromagnetic compatibility and electromagnetic interference standard practices).
Another consideration is how the rest of the system will be using the output from the sensor. For example, if the sensor’s output will be used as an input to an ML classification model, then it may be a good choice to use a solution that already integrates ML capabilities into the sensor itself.
Finally, some VOC sensors have openings to allow gas to diffuse in and out of the sensor. Because these openings must be unobstructed, engineers should pay close attention to protecting them from blockages such as dust.
Choosing the Right VOC Sensor
MOS sensors are best suited to noncritical applications due to the potential (albeit unlikely) risk of explosion. For example, the simplicity and low cost of MOS sensors make them an attractive choice for air quality systems used to monitor homes. Additionally, MOS sensors are small enough that engineers can easily mount them inside low-profile enclosures.
PID sensors are ideal for industrial applications that require high levels of reliability and safety. With their ability to detect minimal concentrations of VOCs, PID sensors are ideal for early warning alarms. Their energy selectiveness allows them to ignore VOCs under specific energies; however, they will still react to VOCs equal to or greater than their energy setting.
Electrochemical sensors are ideal for use in explosive environments because of their use of a diffusion layer. Their ability to detect low concentrations also makes them suitable for detecting leaks in crucial infrastructure such as pipelines and rigs. Still, electrochemical sensors must be designed to be easily replaceable due to their short lifespan.
Some sensors, like the Bosch BME688 AI Gas Sensor, improve sensor design by integrating intelligent functionality into the sensor. BME688 is the industry’s first gas sensor with AI built into the chip, allowing for the detection and classification of gases for improved application-specific responses. This feature, combined with the sensor’s high sensitivity, low power consumption, and small footprint, makes it a desirable solution for applications like connected devices and smart homes.
Conclusion
VOCs present engineers with all kinds of challenges: They can be an explosion risk, are difficult to detect in small concentrations, and come in many forms.
A wide variety of VOC sensors exists, and each technology has its advantages and disadvantages. MOS sensors are ideal for low-cost applications but may underperform in humid environments and can be an explosion risk. PID sensors do not provide great selectivity but can respond to very sudden changes in VOC levels. Electrochemical sensors provide excellent selectivity but must be replaced frequently.
When implementing any VOC sensor technology, engineers must closely consider the environment it will be used in as well as the nature of the sensor’s manufacturing process.