Failsafe
of oxygen addition valve (Solonoid)
This section provides a detailed description of the
ways in which the solenoid valve operation has been
considered in various failure modes.
The oxygen addition solenoid valve
allows the micro-controller system to automatically
add oxygen to the breathing loop when the detected
partial pressure of oxygen level falls below a configurable
target level.
The oxygen addition valve has two
main failure types:
1. Valve stays permanently shut
2. Valve stays permanently open
These scenarios are discussed below:
|
Above; ADV adjustment
control in the centre section of the unit.
|
Above; The oxygen
solonoid valve is external to the breathing loop for
obvious intrinsic safety and is sealed within its
own case. |
1. Valve stays permanently shut
From a diver safety point of view, a permanently shut valve
causes a gradual decrease in oxygen level that is warned
of by the display systems. This failure scenario can happen
due to an electronics failure, or a mechanical failure of
the valve. This failure mode is gradual, and a diver has
time to sort out this by using the manual addition valve
on the rebreather. The manual oxygen bypass valve has no
connection to the electronics micro-controller control system,
so is isolated from any effects of an electronics failure.
2. Valve stays permanently open
This failure scenario can cause an increase in the partial
pressure of oxygen level. This failure type could be caused
by a mechanical or electronics failure. Due to the possibly
rapid nature of the onset of diver problems from this failure
type, special attention has been given to reduce the risk
of this type of failure from an electronics and mechanical
standpoint. The following list gives the potential scenarios
and methods for reducing the probability of failure:
a. Valve stuck open due to debris under the valve mechanism
This has been guarded against by the installation of a filter
in the O2 line. However, should this still happen, the diver
can shut off gas using the main cylinder shut off and or
the oxygen solenoid shut-off valve or both– see below.
b. Software and Electronics drive circuit failure.
This has been guarded against by a failsafe electronic circuit
that ensures no single component failure can force the valve
to stay on.
In the event of either of the above
failures happening, despite the system precautions, the
diver can shut down the oxygen supply by turning off the
oxygen supply at the main cylinder. Alarms on the HUD, Rear
facing O2 display and Main display will show high Partial
pressure O2 warnings, and the display backlights will flash.
The gas piping system has been designed with a minimum of
reservoir, so that turning off of the cylinder O2 supply
will quickly stabilise the O2 level. The diver can then
add diluent to bring the O2 back down to safe limits.
ADV
| The ADV designed into
the Ouroboros rebreather is located inside the unit
(not externally where it could be damaged) and is built
from a high performance regulator. The ADV can be isoltated
during a dive by the simply sliding shut a valve located
under the case. |
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