6
POISONING OF CATALYTIC SENSORS
Catalytic elements used in flammable gas sensors are liable to be rendered inactive due to ‘poisoning’ by certain
groups of compounds.
In general contact with any gaseous compound capable of producing an involatile residue upon heating is to be
avoided.
Examples of such substances are:
a. Silicon containing vapours, as emitted by silicone polishes, greases and oils.
b. Petroleum vapours containing tetra-ethyl lead or other organo-metallic compounds.
c. Phosphorus in the form of phosphate esters.
These compounds will permanently affect the detector and if their presence is suspected the response of the detector
should be determined by the calibration procedure.
It is also possible that the reaction of the detector to a flammable gas could be inhibited by halogen containing gases
such as chloroform, carbon tetra chloride and trichloro-ethylene. this effect is not permanent.
Commissioning
Before applying power to the instrument ensure that all detector heads are connected to the sensor terminals on the
printed circuit board (fig 2) and that each detector head is connected to its appropriate channel, identified by a small
circular, coloured label:
WARNING – DO NOT INSERT OR REMOVE ALARM CARDS FROM THE MOTHERBOARD WHILE THE POWER IS ON
Red = flammable
Yellow =
toxic
Blue =
oxygen
Green =
others
Switch on power to the instrument.
Check that the green ‘P’ power lamp is on.
Each channel alarm card has a green (ACTIVE) indicator located on the mother board (D102, D202, D302, D402).
On power up these will flash for 60 seconds indicating that the sensors are stabilising, during this period all alarms are
held in the off condition.
Where an internal standby battery has been supplied the connectors should be made on JP11 and JP12.
Re-set alarms by pressing the reset button located on the front panel.
Allow ten minutes for the sensors to stabilise.
Select channel 1 and for flammable or toxic sensors adjust meter to read zero by means of appropriate ZERO
POTENTIOMETER marked (Z) on the alarm module, or for oxygen sensor adjust the s-span potentiometer for a reading
of 20.8 repeat for Channel 2, 3 and 4.
CALIBRATION
Establish calibration figures with respect to the L.E.L. limit or the T.L.V. limit of the calibration gas being used.
See page 10.
The following calibration gases are recommended:
Flammable gases - 2.5% methane in air. Toxic gases - T.L.V. When using this gas ensure adequate ventilation.
If necessary zero each detector channel in clean air (for ambient oxygen monitoring the meter should be adjusted to
read 20.8% using the s-span potentiometer).
Apply the calibration gas to the appropriate head at a flow rate of approximately 1 litre per minute.
When the meter reads a steady value adjust the Span Potentiometer marked (S) to obtain the correct reading for the
calibration gas being used.
SERVICE ADJUSTMENTS
The following adjustments need only be made if the standard factory settings (see test certificate) are to be adjusted.
CALIBRATION WHEN USING CV TRANSMITTER (4~20MA DEVICE)
Where a sensor CV transmitter has been supplied the setting up procedure as described on page 18 or 19 should be
followed. The standard transmitter for toxic sensors is supplied as a two wire device set in a loop powered mode, and
the flammable sensor is supplied as a three wire device.
NOTE: Where a CV transmitter is used, adjustment of the alarm module calibration potentiometer is not required (factory
set for 4~20mA input signals), gas calibration need only be carried out at the detector head end.
