3.3 Gas Supply Safety Devices
3.3.1 Gas Supply Cut-off Device
A gas cut-off device, triggered by low oxygen
supply pressure, cuts the supply of nitrous
oxide, and carbon dioxide or other fourth gas
(if fitted).
The cut-off operates when the oxygen
pressure falls to 200 ± 14 kPa (29 ± 2 psig).
Gas supplies are reinstated only when the
oxygen supply pressure rises above 227
kPa (33 psig).
3.3.2 Oxygen Supply Failure Warning
Whistle
WARNING: Refer to WARNING 11 (section 1).
A whistle gives an audible warning when
there is a reduction of oxygen supply
pressure.
Operated solely by the remaining oxygen in
the machine system, the warning whistle is
prolonged by an oxygen reservoir built into
the gas circuit, allowing a minimum warning
whistle of 7 seconds duration.
The whistle will start to sound when the
pressure falls to 200 ± 21 kPa (29 ± 3 psig),
and will continue to sound until the pressure
falls to approximately 70 kPa (10 psig).
Oxygen consumption of the whistle is
approximately 2 L/min when sounding and
nil at other times.
3.3.3 Fresh Gas Pressure Relief Valve
A pressure relief valve is mounted between
the vaporizer back bar and the common gas
outlet (CGO) on the inside face of the
machine right hand upright.
It is designed to prevent fresh gas being
delivered to the breathing system at
pressures exceeding 41 kPa (6 psi).
This valve also protects machine
components against excessive pressure in
the event of a total blockage of the CGO.
3.3.4 Oxygen Supply Visual Indicator
WARNING: Refer to WARNING 11 (section 1).
The indicator (1) is mounted on the front of
the machine and is operated from the
oxygen supply and shows GREEN when the
supply is at working pressure, and RED if the
pressure falls.
3.3.5 Mechanical AHD
A mechanical link between the oxygen
control valve and a needle valve in the
nitrous oxide flow ensures that the machine
delivers a fresh gas mixture with a minimum
30% (±3%) oxygen, irrespective of the flow
of nitrous oxide set by the anaesthetist.
With the nitrous oxide control valve fully
open, the oxygen and nitrous oxide flows are
then both controlled by the oxygen control
valve.
See section 3.4 for a full description
3.3.6 Low Pressure Gas Tubing
Diameter-indexed tubing is used for the low
pressure gas system - see section 4.
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DESCRIPTION
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