CODEL TunnelTech 100 Series, Technical Manual

Page: 10 / 34

Technical Manual
Page 2
CODEL
Issue : A Rev. : Date : 29/06/2015 Ref. : 100190
1.2. Measurement Elements
The visibility sensor produces a beam of light from a pulsed LED focused by a lens to a receiver unit mounted
6m away. An internal detector within the transmitter monitors the brightness of the emitted pulses of light. The
transmitted beam is gathered by the receiver unit lens and focussed onto a receiving detector. The ratio of
signals from the two detectors provides the measurement of transmissivity.
1.3. LED Control
Operation of the emitting LED is controlled by the on-board processor. A continuous series of pulses is applied
to the LED. Each pulse is less than 100µsec in duration. These very brief duration pulses enable the instrument
to operate without interference from other light sources within the tunnel.
1.4. Detector Element
Two silicon detectors are utilised one to measure the initial brightness of the emitted light (Vis Tx), the other to
measure the intensity of the received light (Vis Rx) after transmission to the receiver unit.
The processor takes a series of measurements immediately prior to 'pulsing' the emitter LED in order to monitor
the inherent background levels of light intensity. Then a series of measurements is made while the LED is
illuminated and again after
the LED is switched to check that the background levels haven’t changed. The high
frequency at which this occurs provides the device with extremely high immunity from the effects of background
lighting.
1.5. Diagnostic Data
Measurements of transmissivity and opacity are calculated from the two detector measurements. First the
detector measurements are smoothed to improve signal to noise and calculations made as follows:
Transmissivity = (Set Cal Vis) Vis Rx/Vis Tx
Set Cal Vis is the calibration constant to set the measurement of transmissivity to 100% in a clear environment.
opacity = 100
– transmissivity %
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.
In order to be able to resolve to a precision of 0.01%, opacity is redefined as:
opacity = 10000
– (Set Cal Vis) Vis Rx/Vis Tx
and to ensure a ‘live’ zero the opacity measurement is given an off-set of 2000. Thus:
opacity = 12000
– (Set Cal Vis) Vis Rx/Vis Tx
1.6. Calibration
It is normal for these instruments to be calibrated during a tunnel closure when it is expected that the opacity will
be zero. The instrument can be calibrated by selecting a calibrate mode where, instead of calculating opacity
using a fixed calibration factor, the instrument assumes an opacity value of zero and calculates the calibration
factor required.
Set Cal Vis = 10000 x Vis Tx/Vis Rx