In these experiments the straight wind tunnel at VTT was used [5]. The flow
velocity can be adjusted to a constant value between 0.2 and 0.8 m/s. The test smoke
is produced in a controlled way by an aerosol generator described previously [5]. The
aerosol is fed into the tunnel at a variable rate in the range of 0.1–2 dB/m/s. The
smoke density is measured both with a standard ionisation chamber (MIC, measures the smoke density parameter y) [6] and an infrared smoke densitometer (SICK,
measures the optical smoke density m).2 The aerosol feeding is controlled by a data
acquisition and control unit connected to a microcomputer. The experimental
arrangements are described in more detail in Ref. [4]