Environmental detection


General Information

Today the high population density in urban areas require railways to be installed underground, thus reducing noise and environmental impact, improving safety and enabling better traffic solutions.

But kilometre long railway tunnels require specific measures for the environmental control, especially if diesel engines share these tunnels with passenger trains.

Environmental control purpose

There are two different situations inside a railway tunnel:

  • Normal operation with emissions from Diesel engines
  • Incidences due to explosive/toxic gas accumulations or fire inside the tunnel

Normal Operation

Among other security concerns the environmental control system has to ensure visibility conditions inside, allowing a normal railway service, and monitor toxic gas levels. Those two conditions will determine the air management inside tunnel.

The most important parameters are:

  • Carbon Monoxide (CO) (measurement range from 0 to 300 ppm CO)
  • Visibility/Opacity (measurement range from 0 to 15x10-3/m (extinction))

Event/Incidence Operation

The main purpose is to get a fast response against high level of explosive/combustible gases and assure an early fire detection inside the tunnel. In those cases important parameters change to:

  • Carbon Dioxide (CO2) (measurement range from 0 to 5000 ppm CO2)
  • Explosive/Combustible gases (range from 0 to 100% LEL)
  • Ambience temperature
  • Relative humidity
  • Fire with smoke production

According to tunnel specific conditions and difference between transported goods, monitoring could include also:

  • Nitrogen dioxide (NO2) (measurement range from 0 to 20 ppm NO2)
  • Oxygen (lack of O2) (measurement range from 0 to 25% O2)

Sensor system equipment

To ensure a reliable and stable control sensor of the best quality and reliability have been developed for each specific analysis.

Combustible gases

Alarm setting will adjust to the different concentrations and the air ventilation system will work in coordination evacuating non desired gases.

SENS BG-IR family sensor has been developed with the newest infrared technologies ensuring an advanced and long-standing electronic device.

Toxic gases and air quality

Electro-chemical cell sensors for toxic gas monitoring are made of the best quality ensuring detections for very low concentrations and low cross sensitivities.

Other toxic gases and air quality

The system is able to add O2 and NO2 sensors in accordance with the goods to be transported through the tunnel.

When O2 concentration drops below 19 % Oxygen sensor warns of possible problems due to lack of oxygen.

Carbon dioxide CO2

SENS-CO2-IR detects this gas through infrared absorption. The measuring range covers from 0 to 5000 ppm, giving evidence about the existence of a fire.


Two different kinds should be monitored, the opacity level as a result of fog presence or as a result of engine’s emissions and due to a fire.

Fire in an initial stage could cause high level of smoke emissions previous to reach high power capacity. We can compare between the engine’s emissions (5×10-3/m) to fire emissions (17×10-3 /m or higher)

Several studies confirmed the difference between situations, as shown in the table

Situation K
Normal Traffic 0,003/m
High Traffic 0,005/m
Traffic Jam 0,007/m
Tunnel Closure > 0,008/m

Against fire

Situation K
Initial Fire 0,018 to 0,023/m
Dense Smoke ~0,028/m

For railway tunnels there is no data available relating fire or engine emissions, but according to our broad experience in monitoring we know that normal values are always below 0,003 /m with temporary episodes resulting of humidity, dust, etc. With diesel engines values grow up to 0,004 or 0,006 /m.

Anyway the measuring device has to be efficient ensuring an effective margin form 0 to 0,015 /m to discern acceptable visibility from zero visibility under emergency conditions.

The VisGuard type opacimeter manufactured by Sigrist Photometer, a renown company with more than 50 years experience in the market, is a compact device working according to the principle of light dispersion offering a measuring range of 0 to 0,0015 /m and less.

These devices provide a highly stable measurement because of implemented self calibrating features which compensate aging of components.

Temperature and relative humidity

To complete the information reported a temperature sensor, range from -30º to 70ºC and precision about ±0,2 ºC, and a relative humidity sensor with 3% precision have been added.

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