Sensors have been employed to collect signals from the environment, providing data to control systems for more than 2300 years, when the first noted system were developed by the Greeks to control the level of liquids using a floater, similar to those that are used today in water boxes to keep a water container at a constant level. With that it was possible to create a precise water clock, where time was measured by the constant dripping of water from the first container to another one, where the level changed proportionally to the water flow [1
Control systems use sensors to collect data from the environment where they are installed, actuators to react to the environmental changes until the system achieves the expected state, and a controller responsible to process the data collected by sensors, to adjust the response of the actuators and to inform users regarding the system’s status; as simple examples, it is possible to highlight the temperature control system on air conditioners and showers. The loop between the controller and the plant can be performed through a dedicated network, or through the Internet, although the last option cannot provide the best quality of service (QoS) [1
Wireless sensors networks (WSN) have been developed to enhance data collection from the environment and transferring process to databases, allowing remote monitoring of areas of interest and difficult access, for instance. Moreover, wireless actuators were also placed into networks known as Wireless Sensors and Actuators Networks (WSANs), working in a collaborative way to ensure automatic and intelligent decision making on certain events, reacting with environmental changes to provide the best user experience, without needing users’ interference [3
Currently, control systems have reached complex levels where buildings are being automated to make the best decision for users, self-driving cars and autonomous planes are being experimented on, relying on autonomous decisions based on data provided by numerous sensors placed on vehicles. Hereafter, these data will be provided not only by the embedded sensors on devices being controlled, but also by data collected on other devices and even in other networks. These significant data will be shared among the devices in the same context by the Internet, through the Internet of Things (IoT) paradigm, allowing even more crucial precise decisions [8
Mechanisms of sensing gases have been studied since the 19th century when the first method to notice the presence of unwanted gases at underground mines was performed by using canaries and observing their states. The presence of toxic gases is deadly for the birds and the workers would have enough time to leave the place without harm [15
]. More recently, the detection of other gases has been studied with the purpose of avoiding incidents and accidents, as fires and explosions involving flammable gas leakages as well as providing better results from industrial processes involving chemical reactions [16
Much research focuses on the sensing materials, always trying to improve gas measurements in terms of accuracy, precision, and response time. The sensors’ miniaturization is also a recurrent topic of research although these topics of study are mostly linked with the electrochemical transducers while techniques that make use of propagation characteristics have repeatability as a main theme of refinement [20
Several survey papers were published targeting particular characteristics of gas-sensing technologies. Chatterjee et al. [20
], Mirzaei et al. [21
], and Sun et al. [22
] researched metal oxide nanostructures for sensing gases. Chatterjee et al. [20
] studied hybrid gas transducers using graphene to sense toxic gases while Mirzaei et al. [21
] approached these nanostructures focused on sensing volatile organic compounds (VOC), and Sun et al. [22
] studied these nanostructures in general. In [23
], the authors covered the detection of gases by carbon nanotubes (CNTs) transducers. Optical fiber transducers to detect oxygen (O2
) and carbon dioxide (CO2
) are covered in [24
]. E. Llobet [25
] has studied nanomaterials as gas sensors. In [26
] and [27
] the authors have studied the use of polymers as gas transducers. The survey published Liu et al. [28
] covered gas-sensing technologies in general, suggesting the use of wireless technologies to transfer information, although no wireless sensor has been reviewed.
The purpose of this survey is to provide a deep review and analyze the state-of-the-art of the available gas-measurement systems, performing a comparison of their used technologies, identifying and analyzing further opportunities related to the integration of embedded sensors with communication systems, using the IoT to enable devices and applications to be developed. Then, the main contributions of this survey are the following:
An analysis of the gas-sensing technologies evolution;
A deep literature review on the most promising technologies to sense environmental gases through wireless sensors;
The analysis of the most promising wireless-based solutions for ambient gas monitoring;
The identification of open research issues on gas-sensing technologies and wireless gas sensors;
The lessons learned from this study on gas sensors are shared.
The rest of this document is organized as follows. Section 2
presents the most important gases, in terms of pollution monitoring, health issues, and accident prevention highlighting their main characteristics. Section 3
brings the opportunities in terms of IoT verticals and economic sectors offered by IoT-enabled multi-gas sensors. The evolution of gas-sensing technologies is addressed in Section 4
. Technologies to sense gases and wireless communications support are elaborated in Section 5
and Section 6
, respectively. The most promising solutions in terms of sensing methods and IoT-enabled solutions are discussed in Section 7
and open research topics are identified. Lessons learned are shared at Section 8
and, finally, Section 9
concludes the study.
2. Background on Environmental Gases
Some gases are the key to ensure the functionality of systems and entire industries, as well as the presence of other gases being a problem in other fields, causing the loss of entire production lines, as in the food industry, or even cause the loss of lives and explosions. In this section, the most important gases in terms of pollution monitoring and control, health issues, and accident prevention are listed with their main characteristics.
is the most important gas for life, and is crucial in numerous fields. Patients under anesthesia, or recovering from surgery and from certain diseases need controlled O2
doses to keep them alive and fully recovered. The decrease of oxygen levels in enclosed spaces can be related with other gases leakage, which would lead people inside these spaces to asphyxiate, leading to an unconscious state, or even to death [29
]. Numerous industrial processes rely on the correct concentration of this gas to achieve the best results, mainly chemical and combustion, which without the correct percentages will not grant the best performance of systems. Engine control systems depend on the correct mixture to achieve the expected performances, whether it is lower fuel consumption or power and speed [29
Carbon dioxide (CO2)
is a colorless, odorless gas, generated by the oxidation and combustion of hydrocarbon, as well as by living beings in the respiration process. It is a key gas for the greenhouse effect, and the increase of its levels, in the presence of other gases, is related with atmospheric pollution. It is also the key gas on the oxygen production by the photosynthesis process. The accumulation of this gas in enclosed spaces can be responsible to suffocation; it can be deathly when the concentration of CO2
reaches levels above 3% [33
Carbon monoxide (CO)
is a result of the incomplete combustion of hydrocarbon fuels, due to the lack of oxygen or insufficient temperature. It is an odorless, colorless gas, mainly originating in enclosed or semi-enclosed spaces, such as closed parking garages, home heaters and fireplaces. It is well known that this gas has around 200 times more affinity to hemoglobin than oxygen, making the protein unable to carry the second gas to the body cells, leading to hypoxia, causing damage to body tissue. The poisoning symptoms are easily mistaken with fatigue. Depending on the exposition time and concentrations of this gas, it can be lethal or reduce lifetime, causing cardiovascular diseases and brain damage [19
Volatile organic compounds (VOCs)
are carbon-based organic compounds, in a vapor state at room temperature, generated by the combustion of fossil fuels, or natural emissions, where some compounds are toxic, affecting human health by causing irritation of the respiratory system and eyes, diseases as cardiovascular and respiratory malfunctions, or even cancer [39
Liquefied petroleum gas (LPG)
is a fossil fuel composed of a mixture of hydrocarbon gases, used in domestic situations and industry, to generate electricity, power heating systems, vehicular combustible and cooking. It is a highly flammable gas, capable of severe damage if a leakage is followed by an ignition; major explosions and fire incidents were reported in the literature, due to its gas leakage, in numerous countries, such as the 2011 Karakopru incident, where an entity plant was destroyed due to an explosion. The leakage of this gas can lead to great expense and the loss of many lives [17
is present at the atmosphere, in high altitudes, where it is fundamental to maintaining life on Earth, acting as a natural filter to ultra violet (UV) light emitted by the Sun, avoiding skin cancer on humans and allowing the agriculture, by filtering the UVc light. The increase of this gas in lower atmospheric layers is an indicator of air pollution and bad air quality, being one of the causes of lung dysfunctions, worsen respiratory diseases [41
Sulfur hexafluoride (SF6)
is an odorless, colorless, chemically stable, non-flammable gas used as electrical isolator in the electrical power industry, due to its capability of extinguishes electrical arcs in high tension. It is a non-toxic greenhouse gas, and it is not a risk if inhaled in proportions under 20%. The detection of this gas leakage is essential to prevent damage to high-power electrical equipment, and with that, avoiding failures on power distribution; the detection can be done by the SF6
itself or their sub-products, generated by electrical discharges [45
is a colorless, odorless, radioactive gas that can be emitted from soil and rocks like granite and its long-term exposition is related to lung cancer. It can be generated from the decay of radium (Ra) and uranium (U), and it has a half-life from approximately 3.8 days. Radon can be transported through water, or carrier gases such as CO2
, methane (CH4
), Helium (He), and other gases. As this gas represent half of the radiation exposure to human beings, detection is crucial to avoid long term exposition, being a key factor in lung cancer prevention, mainly in underground miners, that suffer more contact with this radioactive gas [33
is an irritant, corrosive, colorless gas, with a strong odor, employed in the production of fertilizers and explosives, as well as in the textile industry. It can be found as a refrigerant gas and in hygienic products. Its leakage can cause atmospheric, soil and water pollution, and severe damages to the eyes and respiratory system, causing even the death of people directly affected by the gas escape [20
Nitric oxide (NO)
is a colorless gas that can be lethal if its concentrations reach certain ambient levels. In the atmosphere, it is one of the compounds responsible to the smog pollution, causing irritation to exposed people. It is employed in the semiconductor industry, as well as on medicine, as muscular relaxant and in the treatment of hypertension, due to its vasodilation property. This gas can be one of the sub-products of fossil fuel combustion, and it is also produced naturally, by the human body, being an important signal of inflammation if its concentration in human breath reaches levels above 50 parts per billion (ppb); it is used to monitor the inflammation conditions of asthma patients’ lungs. The oxidation of NO results in nitrogen dioxide (NO2
Nitrogen dioxide (NO2)
is a result of the combustion process of fossil fuels, as well as the oxidation of nitrogen. It is one of the pollutants responsible for the formation of acid rain, and it is toxic at low levels, 1 part per million (ppm) is the maximum recommended contact volume; its exposure is related with respiratory diseases, pulmonary malfunction and death. NO2
can be found in highway surroundings, as well as dense traffic areas; it can also be found indoors, as a result of the combustion of generators and heaters [39
Hydrogen sulphide (H2S)
is a colorless, flammable, corrosive and toxic gas, which is poisonous even in low concentrations. Exposure to this gas can lead to damage to the human nervous system. Its generation can be either natural, from volcanic activities or from the decomposition of organic compounds, or due to the combustion of fossil fuels or from sewage. This gas can be found in coal mines, petroleum exploration and in diverse industrial processes [71
is broadly used in the chemical and pharmaceutical industries, water treatment and in domestic cleaning products. With a strong odor, in the gaseous state, it is extremely toxic; the exposure limit in workplaces are around 30 ppb. The inhalation of low concentration levels (50 ppm) of Cl2
can cause severe damage to the respiratory system and levels of 1000 ppm are enough to be fatal to humans. Numerous accidents involving transportation and industrial leakages have been reported over the years. Moreover, it has been used as chemical warfare agent from the First World War, to the Syrian Civil War in the present days [74
Temperature control is vital in certain areas, like the food industry, hospitals and data centers. Refrigerators and air conditioning systems rely on refrigerant gases to accelerate the thermal exchange and keep the ambient on the adequate temperature [78
]. Not only can the malfunctioning of entire systems be caused by the leakage of refrigerant gases, but they can cause the increase of atmospheric pollution, the greenhouse effect and the destruction of the ozone layer. The first generation of refrigerant gases was composed of toxic and flammable gases, endangering workers that could be exposed to leakages. After that, chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and hydro-fluorocarbons (HFCs) were introduced; the last do not affect the ozone layer, by contrast with the first two [81
Monitoring and controlling the level of these gases can grant better work conditions, reduce pollution levels, enhance life quality, decrease health disorders and death in the limit, and prevent equipment malfunction on industry. This section presented a background on the most relevant gases that are considered in the study.
3. Internet of Things (IoT) Verticals Opportunities and Economic Impact
Environmental gas detection and measurement had become essential in diverse fields and applications, from preventing accidents (including life-saving), avoiding equipment malfunctions, warning about air pollution, and helping hospital patients [17
]. Considering this scenario, the development of a sensor capable to attend these demands being able to collect information about numerous gases is crucial to avoid all the losses caused by unwanted gases.
The costs attributable to gas leakage are enormous: from the loss of production in the food industry, to reconstruction of buildings caused by flammable gas leakage followed by explosions, and the healthcare expenses to treat patients who suffered injuries by gas poisoning [17
]. In the United States of America (USA) alone, the annual costs linked to carbon monoxide intoxication is over $
1.3 billion, and the deaths as a consequence of this gas leakage are over 2000 per year [19
Other gases represent an imminent risk to the population and to the environment, being linked to cancer, cardiovascular diseases, cognitive disabilities and respiratory failure [19
]. Gas leakage can reach large proportions, affecting entire neighborhoods, or even cities. The magnitude of the environmental impact of such an incident can be catastrophic in terms of deaths and evaded area [17
It was estimated by the International Labor Organization (ILO) that 4% of the Gross World Product (GWP) is expended on labor accidents and the percentage is higher when taking in account work-related health problems [16
]. Another study has shown that, in a period of 40 years, accidents caused by gas leakage, fires, and explosions on oil sub-products storage tanks represented 90% of the accidents in this field, and human error were linked with 30% of all kind of incidents in the same industry. The same study showed that the average cost per casualty of the 10 most devastating incidents on this industry was around $
114 million, reaching $
330 million in the most expensive disaster up until publication [83
Together with the development of a multi-gas sensor, the information collected by these devices should be easily available to the users and also to other devices that could use the provided data to contribute to the best experience to the users of this system as well as to decreasing avoidable costs, making possible investments on demanding areas [7
The detection of gas leakage can be crucial to avoid all the health and environmental problems, not only by alerting people about incidents, giving them time to evade the area, but also by providing information to actuators that can act in order to stop the leakage and mitigate the consequences [17
With the paradigm of the IoT, it is expected the number of connected devices will reach 50 billion by 2025 with an estimated increase of $
1 to 2.5 trillion USD on the GWP by the same year [11
]. The sectors (IoT verticals) where the impact of the new technologies will be introduced and the use of a multi-gas smart sensor can be seen on Figure 1
A multi-gas sensor together with other devices integrated over IoT networks will have a positive impact on the sectors (IoT verticals) previously shown, allowing new investments by governments, companies, and people in sectors might demand more attention. The number of accidents related with the presence of determined substances should decrease, reducing the risks of specific activities, causing a positive effect on avoidable costs, and repairing the damage caused by such incidents [19
4. Evolution of Gas-Sensing Technologies
The attempt of sensing gases had become necessary when several pitmen lost their lives during underground mine explorations due to the lack of O2
or to the leakage and accumulation of other colorless, odorless gases, mainly CO2
, CO and CH4
. During the 19th century explorations, miners used to have canaries with them while working in the mines to signalize the presence of unwanted gases: while the birds were in their pits, there was enough oxygen for them to breath; in contrast, if the canaries had succumbed or passed out, the quality of the air was not good enough for the people inside to breath, and they should leave the mine immediately [15
Much has evolved in the last century, and gas sensing has become a key feature in numerous activities, such as medicine, sports, the industrial field, environmental monitoring and pollution control [28
]. Oxygen sensors were the first to be developed: in 1956, Leland C. Clark developed the first electrochemical oxygen sensor, known as the Clark Cell [31
]; in 1961, Peters and Mobius developed the Lambda probe to perform oxygen measurements in vehicle engines, helping with the admission control and fuel mixture to achieve the best performance, in terms of fuel consumption or in terms of power [31
], and it has been produced by Bosch since 1976. Both developed sensors are consumable, reacting with oxygen, in order to provide an output value representing the gas concentration in the environment [97
]. After the development of these expensive, and not so accurate sensors, the research for new technologies capable of granting more accurate measurements and more durable devices to this field have been taking advantage on many characteristics of the sensed gases; as an example, oxygen has magnetic characteristics, consequently, it can be measured by the attraction to a magnetic field. Other gases can be measured taking advantage on their ultrasonic properties or by optical spectrometry [28
Gas sensors rely on a physical or chemical reaction with the gas that is trying to be sensed in order to generate a response proportional to the concentration of the gas, thus the speed of the reaction. Some of the sensors have a reversible reaction, while other have an irreversible one, the latter being expendable with 2 to 5 years of lifetime. The response generated by the presence of the target gas is either linear or non-linear, depending on the materials and target gases [28
]. It is fundamental to grant the quality of the air patients are breathing at a hospital, as well as to provide the correct mixture to divers, in particular to deep diving, where the gas mixture the diver must breathe is different according to the time and depth of the activity. Not only to grant the quality of air, but also to avoid gas leakage and its consequences such as poisoning or explosions, sensing gases is of great importance at the industry [17
Over time, other sensing methods were developed, exploring the propagation characteristics of the gases to perform measurements, and comparing with a reference to determine which gas is being sensed. The analyzed characteristics vary from signal attenuation, frequency shifting, propagation time, among others, and are performed by acoustic or optical sensors [28
]. Moreover, the miniaturization of gas transducers has taken place in the research topics [20
], as well as the development of wireless gas sensors, to monitor remote areas and easily collect and analyze data from the environment [34
With the IoT paradigm, gas sensors are becoming key devices to measure ambient gases, generate warnings related to the presence of unwanted gases and allow other systems, such as smart windows, smart curtains, automated exhaust systems, and automated heating ventilation and air conditioning (HVAC) systems, to automatically act in order to avoid damages from leakages [11
6. IoT-Based Wireless Gas Sensors
Over the last 20 years, the research and development of smart sensors for different purposes have become essential in many areas, such as environmental monitoring and pollution control, residential and industrial automation, protective equipment and assisted living devices [3
]. The necessity of remote monitoring of numerous parameters has led to the development of wireless sensors in numerous fields. Moreover, wireless sensors and the construction of WSANs facilitated the automation process, data collection, transfer, processing, and storage. More recently, these smart sensors have acquired the capacity of performing machine-to-machine (M2M) communications, simplifying the communication and interoperation between devices, granting numerous possibilities in control systems and automation [11
Many authors have focused on the proposal and creation of gas sensors to attend to specific target gases in particular conditions [32
]. As examples, Sieber et al. [32
] proposed an oxygen gas sensor for personal protective equipment without wireless transmissions; in [34
] the authors have developed a CO2
gas sensor for remotely monitoring the levels of this gas, transferring the data through the General Packet Radio System (GPRS).
Recently, systems for indoor air quality monitoring and even to control gas leakages based on the data collected by smart sensors have been proposed [93
]. They present different characteristics whether they are to do with the sensing mechanism or the implemented communication protocols. Many contributions towards wireless gas sensors and IoT-enabled gas sensors are only published on proceedings, and to the best of authors knowledge, almost no commercial solution is available on the market. In this section, the main characteristics and requirements of IoT-enabled gas sensors and the most relevant smart gas sensors proposed in the literature will be reviewed, emphasizing the main characteristics of the proposals.
6.1. Sensing Requirements for IoT-Based Gas Sensors
The demand on IoT environments might change in compliance with the applications and their specifications. In general, IoT networks are known for their low energy consumption, low power transmissions, and reduced number of data transfer through the network. Besides, it is essential that the devices on the network can operate for long periods, generate precise data and communicate with other devices on the network. Other applications may demand real-time data acquisition, exhibition of the collected information to users, long-range communications, and battery-based devices [11
To attend to these characteristics, it is essential to choose correctly the sensing technology for these applications, as well as the network characteristics, in order to avoid energy loss with poorly selected protocols on the communication stack, starting with the Layer 2 protocols, attending to the communication requirements in terms of mobility, security, area of coverage, and energy consumption.
For long-range communication, protocols such as Long Range Wide Area Network (LoRaWAN), SigFox, Narrowband IoT (NB-IoT), GPRS, and Long-Term Evolution for Machines (LTE-M) have been proposed in scenarios with low power consumption, some of those for low data transfer and other attending more frequent transmissions in short periods. These protocols are compatible with Internet Protocol (IP) networks, which allow M2M communications through high-layer protocols.
For short-range communications, Bluetooth, Wi-Fi and ZigBee are the most common technologies proposed for sensors; energy consumption may limit the usage of some of these protocols in terms of IoT-enabled devices, as some devices should be able to operate on batteries for several years. Following the TCP/IP stack, the network layer protocol is dependent on the layer 2 protocol; in some cases, as the IEEE 802.15.4, it is necessary an adaptation protocol in between the layers 2 and 3, the IPv6 over Low Power Wireless Personal Area Networks (6LowPAN) that ensures the IPv6 addressing on low power networks using the IEEE 802.15.4 as a medium access protocol [11
The communication between devices with different layer medium access protocols must happen on WSN and IoT environments. It can easily be performed through the application layer protocol. The hypertext transfer protocol (HTTP) and the hypertext transfer protocol secure (HTTPS) can be modified to perform this bridge between lower protocols, although they were not developed to perform this kind of functionalities. Other protocols such as the constrained application protocol (CoAP) and message queuing telemetry transport (MQTT) were designed to allow M2M in different scenarios, as the first implements a request/respond scenario and the last attends a publish/subscribe scenario. Transport layer protocols are dependent on the application protocols [13
Traditionally, in WSANs all the devices communicate through the same protocols. For example, if a temperature sensor operates by Wi-Fi and HTTP, it only communicates with other devices operating with these protocols. Moreover, in these networks, the collected data are not always available for users through the Internet. The integration and communication among devices with different protocols should be supported in IoT-based smart environments. For instance, a gas sensor supporting IEEE 802.15.4 and MQTT should be able to communicate with a smart window with a GPRS module and operate with CoAP. This integration is possible through mediator software, called middleware, where data gathered by sensors are transferred and stored on this software, and it makes the necessary adaptations for these data being suitable for other devices under different communication protocols [13
In terms of energy consumption, IoT-ready sensors based on batteries should be able to operate for long periods, reaching the mark of years, mainly in outdoor environments. For gas sensors, it would be desirable to maintain the operation until the transducer lifetime expires. It would be possible to combine energy sources, including solar panels to recharge the batteries. Some gas sensing technologies consume less energy, as the transducers do not need a temperature compensation and can be turned off during the measurement intervals, as the acoustic and optic methods for detecting gases. Other technologies, such as electrochemical, MOS, and catalytics require a pre-heating time, and thus, turning off these transducers would influence negatively on the performance and capacity of delivering correct measurement values. For indoor environments, the proposed sensors could rely on the power grid energy, mitigating the necessity of batteries.
The stability of the transducers must be sufficient to provide accurate measurements, which in some technologies, such as MOS, electrochemical, and catalytic can be reached through periodic calibration. These calibrations must be performed according to the characteristics of the transducers; smart sensors can also analyze collected data to predict the necessity of calibration, as two discrepant measurements occur in sequence, the calibration must be performed, granting better accuracy to the measurements.
6.2. Gas-Sensing Solutions for IoT and Wireless Sensors and Actuators Networks (WSANs)
During the past two decades, authors have focused on proposing and developing wireless gas sensors for diverse applications. The primary approaches on wireless gas sensors were made for monitoring large areas with no specific wireless protocol implemented. Sensors were able to transmit the sensed information through satellite networks or the Industrial, Scientific and Medical (ISM) bands through simple modulation techniques [2
]. Later, systems to transfer sensed data through other technologies were proposed, using cellular networks for long range [34
]. Bluetooth, ZigBee and Wi-Fi have been used to short range communications [104
Recently, other systems have been proposed to attend the demand of remotely sensing gases in different ambiences, such as industrial, domestic or even outdoor remote areas [34
]. As new communication protocols are emerging, they are being used to cover wireless sensors networks, when the characteristics of these networks correspond to those of these protocols. Some authors have proposed sensors with more than one wireless protocol [104
], and different cellular networks have been used to perform the transmission, from the GPRS to the LTE [34
To attend to the demand for gas sensing on smart cities and remote area monitoring, sensors with different characteristics of the sensing elements, data transfer and energy consumption have been proposed, as in these scenarios, energy supply from power lines may not be available and batteries will not keep sensors functioning for long periods. A CO sensor using electrochemical transducers, employing techniques to reduce energy consumption or to grant the necessary power supply by alternative means, as solar panels and aeolic generators, was proposed by Baranov et al. [104
] to monitor urban areas with ZigBee, although no middleware integration nor online website was reported to provide data exhibition to the users.
Based on the second generation of mobile communications, different researchers have proposed gas sensors to attend the remote area-monitoring perspective. GPRS technology was used in [34
] and [112
]. The first presented a CO2
sensor for monitoring this gas concentration in remote areas with an optical sensing element and a Global Positioning System (GPS) module; the collected data is stored on a database, as well as on a SD-card and exhibit on a web page. In the second, authors have proposed a VOC, NO2
sensor with MOS transducers to collect information on urban air pollution, where data is stored on an online database. Based on the Global System for Mobile Communication (GSM), Sun et al. [108
] proposed and developed a CO and NO2
sensor with electrochemical elements to monitor air quality on urban areas, collecting data mainly during the 2015 Hong Kong marathon. Dong et al. [103
] proposed a natural gas wireless sensor with a MOS transducer, with deployments in different networks, allowing easy adaptation to divergent scenarios, through the GPRS, 3G, 4G, LoRaWAN and Bluetooth networks; although this work was focused on environmental monitoring and automation, the sensor was not integrated on a middleware with a dashboard, to facilitate the visualization of the collected data.
Research focused on gas sensors for indoor monitoring has started with the development of new devices with the capacity of sensing at least one target gas with good precision and alert the users by enabling an alarm, which become a commercial product, installed in many houses, apartments and offices.
Wireless gas sensors focused on indoor monitoring were proposed in the literature by some authors, with different characteristics. Peng et al. [145
] presented a ZigBee-based optical gas sensor to monitor VOCs in enclosed environments, displaying the collected information in real-time in an online dashboard. A system to detect and withhold CH4
leakages in industrial environments was proposed by Somov et al. [111
], based on catalytic gas sensors and with the ZigBee stack to perform the system’s communication.
Focusing on indoor environmental quality control, a photoacoustic-based CO2
sensor was proposed by [106
], employing a Z-Wave transceiver to make the communication with a gateway, responsible to transfer data to an online dashboard with periodic data collection and transmission. Suh et al. [107
] presented a portable dual gas detector for H2
S and CO based on Wi-Fi and Bluetooth, operating with MOS transducers and communicating with smartphones; the sensed data being transferred to an online spreadsheet. To avoid accidents related to gas leakages, a CO and LPG gas sensor was proposed based on the MOS technology connected through the 6LoWPAN over the IEEE 802.15.4 [146
]. Sensed data is transferred to a local MQTT broker and stored on a local database to validate the proposed wireless communication.
To improve the life quality of incontinence patients, Perez et al. [147
] proposed a MOS sensor to detect NH3
, methylmercaptan (C3
SH), and dimetylsulfide ((CH3
S). These gases are present in urine and feces, which allows caretakers to be aware of these patients need through smartphone alerts. The proposed system is connected to a smartphone through BLE, allowing alerts on these devices. It is also small and hence able to be used by these patients as a gadget.
Jelicic et al. [93
], Kumar et al. [110
] and Choi et al. [105
] have proposed multi-gas wireless sensors based on ZigBee. The first proposed a sensor to detect and VOC with MOS transducers deploying a solution that is capable for detecting people on the environment to perform the gas sensing, which helps to decrease energy consumption. The second presented a SO2
, CO, CO2
, and O2
sensor with electrochemical transducers with periodic calibration, to monitor greenhouse gases. The last developed a CO, CO2
, and CH4
sensor operating with MOS, electrochemical and optical transducers, to monitor air pollution in diverse environments.
7. Discussion and Open Issues
In the previous two sections, the authors presented the most important sensing technologies for ambient gas detection and the most promising IoT-enabled sensing solutions. This section brings the discussion towards the best solutions for sensing gases and the most promising wireless communication solutions in the related literature, as well as the suggestion of open research topics.
7.1. Sensing Technologies
shows the main characteristics of the sensing technologies reviewed in this survey paper. To the best of the authors’ knowledge, until the date of this publication, no authors have reported the approximately lifetime of carbon nanotubes transducers for sensing gases.
The studied solutions for gas monitoring in connected environments mostly apply the most antique technologies to sense gases, as the electrochemical, MOS and catalytic, as these technologies are the most available on the market. These technologies also consume less energy than acoustic and optical gas sensors, which is not the best option for battery-based sensors. Polymers have a short lifetime and carbon nanotubes were not found in commercial solutions, being unviable for IoT-based solutions.
Despite the possibility of interference from other gases and environmental factors, such as temperature and humidity, sensors based on chemical reactions can have their precision and accuracy increased with the employment of recalibration processes along with filters for other gases.
Some authors have employed the combination of two sensing technologies like acoustic and optic methods, resulting in a better detection capacity without environmental interference and with no need for periodic calibration, reducing the complexity of the proposed solutions [148
]. Moreover, these technologies do not need a temperature compensation and do not require the employment of heaters and a pre-heating time, which also reduce the energy consumption of these solutions.
7.2. Wireless Gas Sensors
summarizes the main aspects of the most promising systems, in terms of sensing technologies, wireless protocols, and their focus. It includes a brief description of each proposal highlighting their strengths and weakness.
In terms of wireless transmission protocols, in short-range communication the ZigBee protocol is the most applied to the studied proposals. This protocol uses the IEEE 802.15.4 physical and MAC layers, although it does not allow the use of IPv6 on the final devices, as the pure IEEE 802.15.4 deployed with the 6LoWPAN adaptation layer. As this protocol can be used in mesh networks, the energy consumption and communication range on the network can be greater than Wi-Fi applications with one single access point, which is more suitable for IoT applications.
Regarding long-range communications, cellular technologies were more explored in wireless gas sensors. The second generation of mobile communications was applied in four solutions, as these networks are still operating and the cost to use these networks is small, compared to the costs of deploying a new network to operate wireless devices, as the LoRaWAN. The monitoring process of remote areas, such as big farms, where the coverage of legacy mobile networks is poor, protocols as LoRaWAN may be more suitable for these scenarios.
Most of the studied solutions do not support M2M communications and were not integrated though an IoT platform or middleware solution, which are important features on smart environments and IoT-ready solutions. These aspects allow devices to act in favor of users in determined situations, such as stopping gas leakages, opening windows to help with the ventilation, or even warning users with alarms. The M2M communication feature can be achieved through IoT platforms that store and forward data to other devices with the same context of the sensors. Moreover, these platforms can also exhibit the collected information for the final users.
7.3. Open Issues
In the sequence of the presented discussion, the following open issues are identified and proposed for further research studies:
Gas sensing could provide valuable data to diverse applications, using the IoT paradigm, offering important data for decisions taken by smart devices. They can provide better experiences to users.
The improvement of sensing characteristics, miniaturization of transducers and combination of sensing technologies are topics with great potential for research.
Creation of customized multi-gas smart sensors since, to the best of the authors’ knowledge, there are no these kind of solutions in the literature.
Proposals following a plug-and-play approach based on IoT focusing on the end-user empowerment to properly configure these devices according to their needs.
Performance evaluation, demonstration, and validation of available gas transducer proposals in real environments since they only were studied through theoretical and laboratory prototype approaches.
8. Lessons Learned
The review of the state of the art of gas-sensing technologies has demonstrated the need for more research into ameliorating the sensing characteristics of the transducers to achieve better levels of reliability. The use of these devices can grant better results in numerous fields, as well as grant security to people.
Many researchers mistake metal oxide semiconductors for electrochemical and catalytic sensors; the principle of operation and their sensing characteristics are similar, although the fabrication techniques, sensing methods, as well as the types of materials to construct the transducers differ in quite a few aspects.
As presented above, ZigBee is chosen by many authors due to its easy deployment characteristics and lower power consumption, compared to Wi-Fi and Bluetooth. Nevertheless, the standard IEEE 802.15.4 with the 6LoWPAN should be considered as a relevant alternative in future solutions given its promising characteristics.
Despite the potential of some sensing techniques, no further investigations on these topics were found. Commercial solutions are more focused on using the most researched and antique technologies such as the metal oxide semiconductors and electrochemical transducers, even knowing that these sensors can lead to mistakes due to interference from other gases. Decreasing the final prices of these transducers could help to make gas sensors ubiquitous systems.
The opportune scenario of the IoT will lead to the development of smart sensors to attend different contexts, as household users, industries, environmental monitoring, as well as remote electrical distribution stations; the data collected by these sensors will be publicly available through the Internet. This system could be linked to public and health authorities in order to develop public policies according to the risks of each region, decreasing the costs of public health and improving the quality of life. Other smart devices may receive the sensed data to make the best decisions in a smart environment to protect and provide the best experience to the users.
A wide smart network can provide important data, as well as actions through these data, to reduce costs in numerous governmental and private sectors, allowing investment in other areas of necessity. In addition, the development of one unique device, capable of collecting data from numerous gases of interest, with the capacity to be a modular device, allowing users to use only the transducers that correspond to the target gases of a given application, is crucial to make gas-sensing technology more affordable. One important characteristic of the device is the fact that a user without technical knowledge should be able to install the chosen transducers as well as deploy the sensor in the network used to transfer the data.
More research should be conducted in order to improve transducer’s sensitivity, accuracy and selectivity, together with the miniaturization of these devices, which would make easier the deployment and mobility of such devices, along with the use of them in personal protective equipment. Moreover, the combination of more than one technology to sense gases could result in better sensing characteristics, mitigating drawbacks from the isolated sensing techniques. Furthermore, it is necessary to make these technologies more affordable; reducing the costs of production of these devices is fundamental to make the benefits of these sensors ubiquitous.