In simple terms, a small amount of a harmless gas is added to the room air or an air volume flow and the air is ‘labelled’ with it. After a period of time has elapsed or at a different point downstream in the ventilation system, the remaining concentration of the tracer gas in the air is measured and conclusions are drawn about the room air exchange rate or the volumetric flow rate.
As an example, we consider two rooms with the same interior volume, one room with a large window and the other room with a small window on an outside wall. When the windows are closed, the same amount of tracer gas is released in both rooms. Assuming that the gas is evenly distributed in the rooms, we measure the same concentration of tracer gas in the room air in both rooms. If both windows are opened, there is an air exchange between the uncontaminated outside air and the room air labelled with the tracer gas, e.g. due to thermal equalisation currents. The room air in both rooms is diluted with outside air and the concentration of tracer gas decreases. In the room with the larger window, however, the concentration falls more quickly, as more outside air enters the room over the same period of time. The rate at which the tracer gas concentration in the air falls can therefore be used to calculate the incoming outside air volume.
Of course, the principle does not only work with window ventilation. Any type of leakage in the building envelope, including mechanically operated ventilation systems, which also introduce uncontaminated air into the room and extract room air labelled with tracer gas, can be investigated experimentally.
The prerequisite for an exact calculation of the volume flows is a very precise measurement of the tracer gas concentration. The centrepiece of the overall system is therefore a photoacoustic infrared spectrometer that fulfils these high accuracy requirements and can determine tracer gas concentrations with sufficient accuracy to below one ppm (millionth of a volume). In addition, the room air to be measured can be extracted by an automated extraction system via long, thin hoses at several points in the building and fed to the measuring cell. The amount of tracer gas released can also be specified very precisely. The experiments can thus be carried out unattended over long periods of time, which is sometimes necessary due to stochastic effects (think of gusty winds that massively influence the exchange rate of window ventilation).
The tracer gas system is currently being used in the CoolAir research project, for example. Research Burgenland is working with scientific and corporate partners to investigate the efficiency of natural night ventilation via windows and skylights to reduce overheating in summer. The focus is on listed buildings in which it is not possible to retrofit air conditioning or centralised ventilation systems.
TECHNICAL DATA:
- Detectability limit of the photoacoustic infrared spectrometer of approx. 0.01 ppm
- Parallel use of two tracer gases (tetrafluoroethane R134a and sulphur hexafluoride SF6)
- Automated extraction at 6 and injection at 3 different positions
LOCATION:
Pinkafeld
CONTACT PERSON:
DI Florian Wenig BSc
florian.wenig[at]fh-burgenland.at
