Visibility monitoring for ventilation control belongs to today’s standard measurement technology in the area of road tunnels. SIGRIST has been a leading provider in this area for many years. With VisGuard, which was introduced in 2001, both an in-situ installation in the tunnel tube and an extractive sampling installation are possible.
The evaluation of the visibility monitors after the fire catastrophes in the Montblanc, Tauern and Gotthard tunnels showed that here, too, they can make an important contribution, since they were the first measuring devices to be actuated by fire. This is because cold smoke is measured, too. The current trend in safety technology is towards early smoke detection and placement of the visibility monitors for early smoke detection every 100 to 200 m, as prescribed in the current German RABT. Using digital data output (e.g. with Profibus) or two separate analogue measuring ranges, readings can be output in the lower measuring range (visibility measurement) as well as for dense smoke (fire detection).
The effectiveness of this system has been demonstrated in a number of fire tests, some of which we carried out (Ofenauer Tunnel, Austria; Etterschlag Tunnel, Germany) and some of which were officially carried out by VSH in Sargans, Switzerland. After 30 to 60 seconds, depending on the distance of the sensors to the fire and the wind velocity in the tunnel, the threshold value of 0.030 E/m was exceeded. At the same time it could be demonstrated that 0.030 to 0.050 E/m is a meaningful threshold value for triggering a smoke alarm. With this value, not even detectors located in the immediate vicinity of smoke sources are actuated on the one hand, yet the value is high enough above the visibility value of approx. 0.005 E/m, which is normal in tunnels. The advantage of VisGuard is that besides the usual measuring range of 0 to 0.015 E/m a second broader measurement range or a threshold value outside the measurement range can be output.
Since the market launch in 2001, there have been over 600 devices installed throughout Europe, many of which serve the purpose of fire detection, particularly in Germany (Ilverich Tunnel and Weserauen Tunnel each have 30 devices).