What is the advantage of BD Loops loop wire having no air pocket in its design?
Why is it a poor design to have loop wire encased in a PVC flex conduit or air hose?
The best place to start is in answering this question would be to understand
how an inductance detector and loop works. the detector will energize the
loop wire with an oscillating signal current. this current will cause EMF
fields around each wire and a level of inductance will result. The inductance
level will change if any conductive material enters the detection filed.
this change in induction will cause a change in current flow. The detector
has a circuit that looks for a change in current and will trip a switch
when a change is detected. A change in inductance can be caused by slightly
moving the loop wire windings closer or further apart from each other. this
moving of the loop winding can trigger the detection circuit. this type
of detection is usually referred to as phantom or fault detection. When
loop windings are lying loose on a conduit or air hose, simple ground vibrations
from a vehicle could cause loop winding to move ever so slightly that end
up setting off the detector. To see this effect for yourself, hook up a
coiled up induction loop to a detector and slightly disturb the coil. You
will experience a detector trip with the slightest movement. This happens
because each loop wire has its own EMF field that will interact with the
adjustment wire's EMF field. Depending on the field relationship, either
the fields will double in value or cancel each other out. this change will
either increase or decrease the inductance that will result in a change
in loop current. This change is detected by the detector and sets off the
detection circuit. Another problem that is associated with a PVC conduit
or air hose design is the fact that water can enter the chamber at a later
time causing a ground short. In the case of DoorKing loop the "T" is highly
subject to water leakage. The possibility of a short is caused by the fact
that when the wire is pulled through the loop housing, the wire insulation
is subject to be stripped away. This causes the bare wire to be exposed
often without the knowledge od the manufacturer. BD Loop design is not subjected
to this type of problem. In fact each BD Loop is submerged in water and
must pass over 500-mega ohm reading. The only way to do this type of test
with a air chamber design would be to fill the air chamber with water and
do a mega ohm check. Still another problem with an air pocket design in
concrete encasement is the fact that the air pocket will weaken the concrete
structure. In some cases this could be as much as 40%. In the case of a
parking garage where the concrete is a vital part of the structural integrity,
air pockets such as this are not allowed at all.
Conclusion: BD Loop design is far superior design as compared to
an air pocket loop design. Using BD Loops will result in fewer service call
over the life of the loop. |
