VOC sensors are nothing new to this project, we’ve used them intensively in the uRADMonitor hardware units. Back in 2015, the Bosch BME680 was integrated into the uRADMonitor model D, followed by the model A3 using the same excellent sensor. Now the uRADMonitor AIR, our first wearable, relies on the BME680 again for a low power convenient solution operating on battery. The uRADMonitor model D was featured on the Bosch website in a dedicated article.

Working principle

These sensors work by initiating REDOX chemical reactions in a porous layer heated by a filament. This results in ionised local air and modified electrical resistance across the sensor, somewhat proportional to the air quality: A cleaner air will produce less such reactions, resulting in poor conductivity and increased resistance, while a polluted air will increase conductivity resulting in lower resistance.

VOC Sensor Operating Principle with Sensor Diagram and example response to VOC (ethanol)

The gas sensitive Metal Oxide Material (MOX) consists of highly porous and granular semiconducting material. The grains form a resistor with distinct conductive paths between the electrodes. Interaction of the VOC in the air with the MOX (combustion reactions driven by the heated plate) results in predictable modulation of the measured electrical resistance.

Size comparison, BME680(pile) vs MP503(singular)

Many metal oxides are suitable for detecting combustible, reducing, or oxidizing gases by conductive measurements. The following oxides show a gas response in their conductivity: Cr2O3, Mn2O3, Co3O4, NiO, CuO, SrO, In2O3, WO3, TiO2, V2O3, Fe2O3, GeO2, Nb2O5, MoO3, Ta2O5, La2O3, CeO2, Nd2O3.

Power consumption

The heated surface consumes power, but thanks to the micro-hotplate technology used in MEMS sensors Some of these are MEMs sensors (Bosch BME680 or the Sensirion SGP sensors), their power usage stays quite low, allowing a wide set of applications, including mobile / battery operated.

Humidity and Temperature

Environmental humidity is an important factor influencing the performance of metal oxide gas sensors. The reaction between the surface oxygen and the water molecules conduces to a decrease in baseline resistance of the gas sensor, and results in a decrease of the sensitivity. Water adsorption will significantly lower the sensitivity of metal oxide gas sensors.

Temperature and Humidity dependence

Temperature is also an important factor for the metal oxide gas sensors. A detailed study is available here.

Quantification limitations

Given the existence of many different gases / vapours / chemicals that generically are called VOC (volatile organic compounds), the question is how to estimate their nature and exact concentration in the intake air? Their molar mass / burn rate / ionising resulting compounds and multiple other properties and complicated chemistry, make it impossible to compute an absolute concentration level.

Metalic Oxide Sensor response to various VOC’s

It comes down to the question: Concentration for what? Any concentration indication would only be an approximation for a particular class of substances.

Data interpretation research

The uRADMonitor A3 is a multi-sensor Air Quality detector that also includes a MP503 MOX VOC sensor and reports the data in real time. Therefore it is both interesting and easy to compare the VOC MOX sensor output to the other sensors. In some cases, a perfect correlation between the MP503 VOC readings (electrical resistance) and the electrochemical sensor for formaldehyde (ppms) was observed:

Electrochemical formaldehyde sensor readings perfectly aligned on a MOX VOC sensor output

No difference in data, but big difference in sensor price and lifespan. This example can be proof for the potential of the low cost MOX sensor for detecting gases, assuming there is an additional software layer for properly interpreting the data. To get there, uRADMonitor has partnered with the University of Guelph of a research study on an algorithm for interpreting MOX sensor data:

More details on this study are available on ResearchGate:

Singh N. Thakor, Govind & Hu, Piaoyu & Motisan, Radu & Chiang, Yi Wai & Santos, Rafael. (2019). TESTING & VALIDATION OF MOBILE AIR QUALITY MONITOR FOR SENSING & DILINEATING VOC EMISSIONS.

10.13140/RG.2.2.13132.46727. Volatile Organic Compounds (VOC) are chemical compounds having high vapor pressure, and thus are volatile in nature at room temperature. There is a limit of VOC content in the air beyond which they can cause respiratory disorders, nausea, loss of coordination, and in some cases may lead to lung or kidney damage and many other health hazards. Due to the increase in industrialization and urbanization, there have been VOC emissions into the air at greater scales. This has prompted monitoring of VOC emissions through innovative sensing technologies, which give continuous information of the VOC pollutants in the air.

A Metal Oxide Sensor (MOX) is one of the sensors widely used for monitoring VOCs in the air. It consists of a porous layer heated by a filament that undergoes a redox reaction when it comes in contact with a VOC, changing the electrical resistance across the circuit in proportion to the concentration of the VOC. This transduction principle gives high sensitivity for monitoring air quality, and therefore can be used in continuous monitoring of industrial emissions. An important characteristic while considering this sensor for monitoring air quality is its selectivity, that is, its ability to discriminate between two compounds.A uRADMonitor A3 mobile air quality monitor, containing a MOX sensor (Bosch BME680), was tested and validated in this research for its ability to monitor VOC concentration and for determining the air quality index. Initially, point source emissions of a gasoline generator (Hyundai 1750W) were measured by the URADMonitor A3 and compared with a Horiba MEXA-584L device for validating the measurements. Subsequently, two URADMonitor A3 monitors were tested (in identical positions one at a time, and then in different positions simultaneously) for studying its sensitivity, accuracy, and precision. The electrical resistance values were noted, brought to a common baseline based on each sensor’s historical data, and scaled to a normalized internal. Linear and non-linear scaling approaches were compared. This normalized scaling approach allowed comparison of two units using the same VOC sensor, but is intended to also work also between different types of VOC sensors (e.g. BME680 vs MP503). The goal is to develop an improved method for determination of air quality index at a wide range of concentrations and for a wide range of VOCs.