New small, low-power MOX VOC sensors: how might they be used for indoor air quality monitoring?

(original article written by Norwood Brown)

Alarming stories in the news media have served to heighten public concern over air quality. The Volkswagen emission tests scandal set off a storm of outrage over the effect of potentially harmful vehicle exhaust emissions on the health of city dwellers.

This has also heightened people’s awareness of the quality of the air we breathe both indoors and outdoors. The average human inhales about 15kg of air a day, 80% of it indoors. And while the quality of outdoor air is routinely monitored by public agencies, Indoor Air Quality (IAQ) monitoring is the responsibility of the building’s operator or occupier, if it is performed at all. Here, a new generation of small, surface-mount, low-power VOC (volatile organic compound) gas sensors has emerged, offering the potential for distributed, local IAQ monitoring by small and affordable devices, and therefore for more responsive operation of air-moving and air-filtering equipment in buildings.

This article explains the operation of these new VOC sensors and the differences between them and absolute single-gas sensors. It also shows how they can provide data which enables air management equipment to respond most efficiently and effectively to variations in indoor air quality.

How IAQ monitoring is performed today

Professional operators of commercial buildings today in general draw on one or two types of air quality data to control the operation of ventilation and air-filtering systems. Most commonly, they use absolute measurements of a single gas, usually carbon dioxide (CO2). They might also use occupants’ subjective judgements on the quality of the air.

Because humans exhale CO2, it is normal for the concentration of CO2 to increase over time in an occupied room, and the more people in the room, the more the CO2 concentration rises in the absence of adequate ventilation.

An excessive concentration of CO2 in indoor air is associated with feelings of drowsiness, loss of focus, impaired decision-making and a feeling of ‘stuffiness’. Commercial building management systems fitted with CO2 sensors today therefore regulate the operation of filtering and/or ventilation equipment in response to measured CO2 levels. The aim is to keep the indoor air fresh and comfort- able while minimizing the rate of thermal exchange, since the loss of artificially heated or cooled air wastes both money and energy.

In fact, the concentration of CO2 is a reasonable proxy measure for the density of human occupation of a given space. And since people are responsible for the production of VOCs as well as of CO2, through what are scientifically – and politely – termed ‘bio-emissions’, building operators today commonly assume that air-handling equipment configured to regulate the concentration of CO2 will also adequately regulate the concentrations of the many types of VOCs found in indoor air. 

Practical considerations underpin this assumption. The packaging, price and power-consumption characteristics of CO2 sensor components have been attractive enough for many years now to war- rant their integration into the circuit boards of mainstream building automation equipment.

The options for measurement of VOC concentrations were until recently much more limited. There are various methods for measuring and analyzing VOCs suspended in air: photo-ionization, flame ionization, colorimetric tubes, and wavelength absorption are reasonably portable methods. In the laboratory, gas chromatography coupled with mass spectrometry (known as GC-MS) is the favored method.

Unfortunately, these methods are unsuited to use in compact, localized, low-power air quality sensing equipment, either because they are too large or they consume too much power.

This is why the introduction of a new generation of metal oxide (MOX) VOC sensors, now available in surface-mount IC-type packages and using only milliwatts of power, is so exciting for the field of IAQ monitoring. These low-cost, compact and low-power VOC sensing devices may readily be integrated into everyday objects such as luminaires, thermostats, fans and remote controls for fans – even in mobile phones. Read full article