301UM0031_01
TD136 Enhanced Vehicle Detector User Manual
Page: 23 of 29
If the Frequency reading from the DU100 is close to the Minimum Frequency the
inductance of the LOOP is too high and you need to remove turns from the loop
If the detector is operating close to either limit it is possible that either the frequency drift caused
by environmental changes or the shift in frequency caused by a large
∆
L/L detect will cause the
frequency to go outside the limits and cause a retune.
8.2.1.2 Frequency drift
The TD136 Enhanced Vehicle Detector can handle environmental conditions that cause the
frequency to drift up to at a rate of approximating 1%
∆
L/L per minute.
If the Drift reading approaches this value the detector will have problems tracking the
environmental change
If the drift is higher than say 0.5 %
∆
L/L per minute this will indicate a possible fault with the loop
or feeder cable. Possibly the wire insulation has deteriorated and moisture is causing a short to
earth or that wires of the loop are no longer encapsulated and are moving.
For more information about Frequency drift refer to the “Theory of Application” section in
Diagnostic Unit DU100 User Manual Document No. 895UM0001
8.2.1.3 Sensitivity
For a standard loop of 1.0 metres by 2.0 metres with 2 turns (circumference less than 10 m) and
a ten meter feeder cable the following table shows typical sensitivity values for different vehicle
types
VEHICLE TYPE
%
∆∆∆∆
L/L
Metal Supermarket Trolley
0.3 to 2
Bicycle
0.04
Motorbike
0.12
Articulated Truck
0.38
Four Wheel Drive
0.40
5 Ton Tip Truck
0.45
Motor Car
> 1.00
Forklift
> 1.00
For more information about Sensitivity refer to the “Theory of Application” section in Diagnostic
Unit DU100 User Manual Document No. 895UM0001
8.2.1.4 Time
This is a powerful tool in identifying problems with an installation. The time since the last retune of
the detector will let you know when the event occurred and the reason will inform you of what
caused the event