CODEL Tunnel Tech 700 Series, Technical Manual

Page: 14 / 81

Technical Manual
CODEL
4
2. Principles of Operation
2.1. Air Quality Monitor (AQM)
2.1.1. Visibility Measurement
2.1.1.1.Transmissometry Principles
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
.
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.