Fault Finding Guide
31
Problems relating to the Receiver.
Symptoms Cause Action
The detector output is at Fault (<1mA). The
HHT receives no data
The Receiver is not powered Check the power supply and cabling
The Receiver has an internal fault Remove the Receiver head and gimbal
assembly from its mounting, replace it with a
spare with the same configuration, and
return it to the factory. Realign and re-zero
the detector in the normal way
The detector output is at full scale (20mA) The self-zeroing sequence has not been
completed
Align and zero the detector in the normal
way
The detector fails to complete the self-zero-
ing sequence. The HHT shows the Optics
flag is set.
An internal check has detected abnormally
high signal strength. This is normally due to
the Attenuator Plate not being fitted
Measure the distance between the Transmit-
ter and the Receiver. The Attenuator Plate
must be fitted if the distance is below 16
metres. The central cut-out must be removed
if the distance is between 8 and 16 metres
The detector fails to complete the self-zero-
ing sequence. The HHT shows flags indicat-
ing Transmitter and/or Receiver
misalignment, even though no flags were set
in normal operation
The tests for correct alignment are made
more stringent during the self-zeroing proce-
dure. This is to give the detector the best
chance of working reliably, allowing for small
movements in the supporting structure over
a period of time
Check that both the Transmitter and
Receiver are rigidly mounted. Carefully
repeat the alignment procedure, ensuring
that both the Transmitter and Receiver are
aligned at the centre of the strong central
peak. Restart the self-zeroing procedure
The detector output is at Pre warning. The
HHT shows the Optics flag is set
The signal strength has remained for some
time significantly below the signal recorded
when the detector was last zeroed. The mar-
gin of signal loss, and the interval for which
it must be maintained, to generate the warn-
ing are both part of the Receiver configura-
tion. Typical values are 40% loss for more
than four days
Check the lenses of Transmitter and
Receiver are clean. If necessary, clean, rea-
lign and rezero the detector. In some loca-
tions fog may persist for longer than the
chosen interval, causing the warning to be
generated. It may then be ignored, or a
longer interval entered
The detector behaves erratically. It changes
unpredictably from normal operation with
the HHT showing a strong signal to a Beam-
block and little or no signal although the
path is not obstructed. The HHT continues
to receive data at the normal rate even when
power is removed from the Transmitter
The Receiver is seeing light from more than
one Transmitter on the same Channel
Ensure that Receivers of all detectors within
sight of each other are configured to sepa-
rate Channels, and that each Transmitter is
configured to the same Channel as its own
Receiver
The detector output is at Beam-block. The
HHT shows the 'RxAlign' flag is set, even
though it was not set before the Beam-block
occurred
The optical tests which detect misalignment
of the Receiver also serve to prevent its giv-
ing false gas readings due to a partial
obstruction of the beam. If a partial obstruc-
tion capable of causing a false alarm per-
sists for longer than the Beam-block delay it
will cause a Beam-block that is indicated like
this
Check for causes of partial obstruction such
as a vehicle, a crane or a build-up of snow or
ice intruding into the beam path
The Receiver has become sufficiently mis-
aligned to cause a Beam-block in a time too
short for the warning to be generated
Investigate the cause of the movement. The
Receiver may have been struck or the sup-
porting structure may not be sufficiently
rigid
We have installed several detectors with
similar configurations. One shows a lower
response to the plastic test sheets than the
others
The inbuilt calibration is for the specified
gas, not for solid plastic. The sheets are
intended to demonstrate that the detector is
working correctly, not to simulate any partic-
ular quantity of gas. It is normal for individual
units to show widely differing responses to
the sheets
If required, the gas response can be tested
directly and in situ using the gas cells sup-
plied in the GCK400 kit
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