CODEL Tunnel Tech 700 Series, Техническое руководство

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Technical Manual
CODEL
6
In order to calculate the opacity from the data collected by the detectors, a Y value has to be
established. This is done by using the Vis TX and Vis RX data, along with the Set Cal as shown in
section 2.1.1.5;
Y Value = 1000000- (Vis Rx/Vis Tx x Set Cal)
This Y Value is then used to establish the Opacity as a percentage as shown below;
Opacity = Y/10000
Opacity is a direct reading of the attenuation of light. Zero opacity equates to a totally clean light path
and 100% to total light attenuation.
This can then be used to calculate the transmissivity as a percentage and vis K as shown below;
Transmissivity = 100
– Opacity %
K = 1 . log
e
1
L T
2.1.1.7. Auto Zero
The measurement of opacity is dependent upon the optical surfaces of the instrument remaining
clean. If the surfaces of the instrument lenses or the retro-reflector become contaminated it will
reduce the intensity of the received light and increase the opacity measured value. Over a period of
time, it is normal to observe a slow build-up of optical contamination resulting in a steady increase in
the measured opacity value. This appears as a persistent positive output drift.
There are two cures for this problem. One is to clean the optical surfaces on a regular basis.
However, in a road tunnel regular access is not usually possible without a tunnel closure.
The second method is to automatically compensate for such a build-up of contamination. The
technique used by Tunnel Tech
relies on the assumption that there will always be periods of low real
opacity during the course of the day. These periods usually occur at night when traffic loading is low.
These periods provide a reference condition for the measurement.
During normal operation, the effect of contamination will be a slow increase the measured value of
opacity. To compensate for this the instrument is programmed to slowly reduce its measured value of
opacity at a rate faster than that at which contamination would increase.
In this way periods of zero opacity, whenever they occur, are utilised to correct the calibration of the
measurement allowing long periods of operation without maintenance and the cleaning of optical
lenses.
2.1.2. Electro Chemical Cell
– CO/NO/NO²
In order to measure the level of gas (CO/NO/NO²), the Tunnel Tech 700 uses an electro chemical cell
to convert the gas in to readable data. The gas diffuses in to the sensor and interacts with the
electrode within the cell. This causes a chemical reaction in which the gas is reduced or oxidised. The
electrochemical reaction results in an electric current which can then be read, with the generated
voltage being linear to the gas response, and the detector level is derived from the incoming voltage
and subsequently a gas measurement.