Page 85 - Advanced Course

KNX ADVANCED COURSE

6.3 Air quality sensors

6.3.1 Terms and definitions

Various terms that are used to indicate moisture levels are given in the table below:

Table 14: Terms to indicate moisture levels, source: http://www.engineersgarage.com
S.No Term Definition Unit
Absolute Humidity Ratio of mass (vapour) to volume. grams/m
3
1
(Vapor Concentration)
Mixing Ratio OR Mass Ratio of mass (vapour) to mass (dry gas) grams/m
3
2
Ratio
Relative Humidity Ratio of mass (vapour) to mass (saturated %
3 vapour) OR ratio of current vapor

pressure to saturation vapor pressure.
4 Specific Humidity Ratio of mass (vapour) to total mass. %

Dew Point Temperature (above 0°C) at which the °C
5 water vapor in a gas condenses to liquid
water

Frost Point Temperature (below 0°C) at which the
6
water vapor in a gas condenses to ice

Volume Ratio Ratio of partial pressure (vapour) to partial % by
7
pressure (dry gas) volume
PPM by Volume Ratio of volume (vapour) X 10 to volume PPMV
6
8
(dry gas)
9 PPM by Weight PPMV X PPMW

6.3.2 CO2 concentration sensor

A CO 2 concentration sensor measures the concentration of CO 2 in a room and transmits it
to the bus. It can also receive an external CO 2 value via the bus and process it with its
own data to calculate an overall value (mixed value, e. g. room average). Measuring CO 2
concentration can be very useful in crowded areas, where the CO 2 concentration can
reach very high limits and ventilation needs to be activated on in order to refresh the air.
The CO 2 concentration value is measured and expressed in parts per million and is
transmitted using a 2 byte Group Object with encoding 9.008, DPT_Value_AirQuality,
which has a resolution of 0,01 ppm.

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