GDS Defender – Sensor
FLAMMABLE / TOXIC / OXYGEN / REFRIGERANT GAS ALARM
Not for use in potentially explosive atmospheres (zoned areas)
Technical Sheet
ref C1714A v.2
Operation
The gas sensor continuously monitors the atmosphere for any hazardous gas conditions, on detection of gas the sensor will indicate
the level status by red LED indication and sounder.
LOW alarm (A1), the green indicator will change to slow flashing red and repeat ON/OFF sounder.
An increasing level of gas will cause the HIGH alarm (A2) to activate which is indicated by rapid flashing of the red alarm
indicator, and corresponding increase in sounder rate. Should the over range DANGER alarm (A3) be reached, the red alarm
indicator and sounder will go constant.
Pressing the reset will mute the sounder and only reset the alarm condition once the gas has cleared. Unit Fault is indicated by the
indication LED turning AMBER with a very slow repeat ON/OFF sounder. Alarm Relay Operation will activate end user selected
controls or alarms, details of which should be recorded and kept adjacent to the alarm unit.
All sensors are supplied gas tested with preset alarm
levels, ready to install and operate.The alarm unit should be mounted
in a position which is accessible, away from direct heat, in the field of
vision and in accordance with the units IP Rating. Mains supply should
be from a 1A fused spur.
SITING THE SENSOR (DETECTOR)
A key feature of the installation is the correct siting of detectors. Several
considerations must be taken into account, the most important being the
density of the gas with respect to air.
HEAVIER THAN AIR GASES
(LPG, Propane, Butane, Refrigerant
Gases) – locate at 15 to 20cm above floor level.
LIGHTER THAN AIR GASES
(Methane, Natural Gas, Town Gas) –
locate at 5 to 10cm from the ceiling.
EQUAL TO AIR GASES
(Carbon Monoxide, Oxygen) – locate at 1.5
metres above floor level.
Under still air conditions, a ‘lighter than air’ gas such as methane leaking
from a small orifice at ground level, will rise in a plume the shape of
which approximates an inverted cone. As the gas rises, it draws air
from the surroundings and creates a turbulence. Resulting from this there
occurs rapid dilution and, unless a sensor is positioned within the plume,
there will be no initial indication of a leak.
As gas continues to escape, the diluted concentration rises to ceiling
level and begins to layer. In time the concentration at ceiling level will
increase and this, in turn, will displace air downwards.
Dangerous levels will, therefore, tend to occur at ceiling level and the
thickness of this layer will increase with the passage of time.
Ventilation of the room will of course alter the situation significantly but
it should be remembered that if the ventilator is not at ceiling level, a
dangerous concentration can still occur between the top of the ventilator
and the ceiling.
For heavier than air gases such as propane or butane, the formation of
dangerous layers occurs at ground level. These gases tend to behave
like water and will run down gradients and pool at the lowest point.
The number of sensors required in individual rooms is determined by the
number of possible hazards in the vicinity.
Gas leakage may occur around valves, flanges and anywhere where
gas pipes are jointed. It may be possible to cover several probable
gas leaks in one room by the careful siting of a single detector. Ducts,
trenches and manholes are also likely places where a build up of heavy
gases may collect.
When siting a detector in such places it is most important to ensure that
there is no likelihood of flooding by water, or excessive dust which may
block the sensor mesh element and prevent gas reaching the sensor.
When monitoring gases outside, those lighter than air will be quickly
dispersed, but gases heavier than air will tend to form in layers and
again causing a dangerous hazard. When siting detectors outside
prevailing winds must be taken into consideration and adequate
protection given against wind and rain.
POISONING OF CATALYTIC SENSORS
Catalytic elements used in most 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.
Switching ON
Having fully installed the unit switch ON the power from the fused
spur. During the one minute stabilising period the green LED indicator
will flash going constant on warm up completion. Control or warning
devices which have been connected to the alarm relays should be tested
for correct operation.
Unit Test
The unit may be electrically tested by pressing the reset button for 15
seconds. The LED indicator will flash RED and the sounder will toggle
ON/OFF, alarm relays will activate at which time attached warning
devices or valves should be checked for correct operation.
Gas Test:
To ensure that the sensor responds correctly to the presence
of gas it should be exposed to test gas at six–monthly intervals. For
further details please contact your product supplier.
Installation
– By approved electrical installer
