Metal Oxide IAQ Indoor Air Quality (VOC) Transmitter
1 Intended Use
The indoor air quality control IAQH-C with digital processing of the measuring values and temperature
compensation is used for measuring and controlling the air quality in indoor areas within environmental
conditions specified in the Technical Data.
The intended sites are all areas being directly connected to the public low voltage supply, e.g. residential,
commercial and industrial ranges as well as small enterprises (according to EN50 082).
The indoor air quality control IAQH-C must not be used in potentially explosive atmospheres.
2 Functional Description
2.1 VOC Sensor
The semi-conducting metal oxide sensor measures the electrical conductivity of the nanocrystalline metal oxide
coated on a heatable substrate. The typical operating temperature is between 300 and 400 °C. The doping of
the metal oxide with noble metals results in a positive sensibility to combustible gases like VOCs, carbon
monoxide and natural gas. The doping permits the adaptation to the demands of the measuring task. VOCs are
partially or totally burnt at the sensor surface by the oxygen of the metal oxide. The electrons released in the
semi-conductor by this process lead to an increase of the electrical conductivity. At the end of the combustion
process, the metal oxide returns to its initial state by incorporating oxygen from the air, with the conductivity
also adopting the initial value. The change of the conductivity is evaluated via the internal micro-controller and
output as a standard signal.
2.2 Air Quality Measurements
The VOC content in indoor areas is mainly determined by the persons present and their activities. See table 1.
When for example working with cleaning agents or when cooking, VOCs (Volatile Organic Compounds) are set
free, but also human respiration is a constant source of volatile metabolism products (VOCs). The air quality
sensor detects the increasing VOC level and calculates the proportional CO2value. The VOC/CO2correlation
was determined by taking measurements under real conditions. See diagram 1.
To this day, there aren’t any standard signals for the VOCs; therefore the IAQ air quality sensor reduces the
measured VOC values to CO2equivalents with the unit ppm. This grants the compatibility to existing CO2
ventilation standards.
Each time the IAQ air quality transmitter is switched on, it runs through a warm-up period of 20 minutes. During
this warm-up period there aren’t any measurements; the sensor outputs the signal of 80% of the measuring
range.
After the warm-up period, the sensor interprets the currently read VOC value as zero-point, independently from
the actual concentration. An internal algorithm continuously updates the zero-point by taking the lowest
measured VOC value. Therefore the ambient air should be of low VOC content after the warm-up period. This
can be obtained by shortly venting when starting the measurements with the 80% signal.
If the sensor isn’t started at low VOC concentrations, it can take a couple of days until the internal algorithm
has updated the zero-point so far that effective measuring results are available.
The natural sensor drift and ageing is corrected by the implemented control algorithms.
