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.