CODEL TunnelTech 100 Series, Technical Manual

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Technical Manual
Page 4
CODEL
Issue : A Rev. : Date : 29/06/2015 Ref. : 100190
2. Principles of Operation
Visibility and Cold Smoke Detection both rely on the amount of light obscuration within the tunnel atmosphere
being determined. This obscuration is quantified by determining the visibility coefficient. A visibility sensor is
programmed to measure low levels of visibility coefficient and thus light obscuration experienced during normal
tunnel operation. A Cold Smoke Detector is programmed to detect the high levels of visibility and light
obscuration that would be produced by smoke emissions from a tunnel fire.
Both sensors use the same technology and principles at operation.
2.1. Visibility Coefficient
Visibility Dimming Coefficient (abbreviated to Dim K or Vis K)
Fine particles suspended in the atmosphere will scatter a beam of light so that the intensity of the beam reduces
as it passes through the air. This reduction in visibility is directly proportional to the concentration of suspended
dust particles.
The intensity of a beam of light follows the Lambert Beer law:
I
=
I
o
e
-KL
where K is a parameter known as the visibility coefficient and is proportional to the concentration of the
suspended particles and L is the path length of the beam.
I
/
I
o
is the ratio of the measured beam intensity and that of the initial intensity
I
o
and is known as the
transmissivity (T) of the system:
T = e
-KL
Thus visibility coefficient :
K = 1 . log
e
1
L T
where L =
6m & T is the measured transmissivity.
A visibility sensor measures the transmissivity of a light beam from a source of known brightness over a fixed
path length to enable a value of the visibility coefficient to be deduced. The units of measurement for this
coefficient are m
-1
and the typical span range would be 0 - 0.015m
-1
or 0 -15(km)
-1
.
2.2 . Meteorological Visibility
An alternative method of presenting this data is in the form of meteorological visibility. This is defined as the
distance over which the intensity of transmitted light falls to 5% of its initial value. It represents the distance over
which a person can see in a hazy or dusty environment.
In this case,
I
= 0.05 x
I
o
thus T = 0.05
and since K = 1 . log
e
1
L T
visibility L = 1 log
e
20 = 2.99
K K
Thus, for a K value of 0.003, the visibility in metres is 2.99/0.003 = 1000m. Both K factor and visibility are
calculated by the sensor and are available for output.