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This is a classic problem for configuration management.
What is the purpose of the isolator?
- to isolate the tanks in the event of a failure where the offending side
valve cannot stop the loss of gas.
Requirements for an isolator valve:
Req. Precondition Requirements
==== ============ ============
1 Open It must flow enough gas in a variety of diving conditions.
1.1 Open The "enough" gas state is affected by environmental
conditions including, but not limited to:
- Increased flow due to depth.
- Increased flow due to diver exercise.
- Changes in valve geometry due to temperature.
2 Closed It must flow no gas when isolating.
4 Both There must be a means of toggling from one state to the
next.
4.1 Open The transition from open to closed must be done so as to
minimize gas loss in the event of an emergency.
4.2 Both There must be a means of determining the status of the
isolator valve.
4.2.1 Open Based on requirements 4.2, 4, and 1, there must be a
means of determining that it is flowing "enough" gas.
4.2.2 Open Given the position of the valve (behind the diver's head),
the valve status determination must either be tactile or
through visible remote indicator.
Assumption:
The valve manufacturer analyzed the gas flow requirements, and built the valve
to meet these requirements while in the fully open position (with a suitable
safety factor).
Convolution in the making:
By partially closing the valve, you are removing the safety factor, potentially
below requirements. Partially closing the valve makes it difficult to comply
with 4.2.2 under all conditions listed in 1.1. You could add a second pressure
gauge to monitor disparity between the tanks. You could listen carefully for
the sound of gas flow. Both of these solutions add complexity. The partial
closing is an attempt to address 4.1 (minimize gas loss in an emergency). The
loss of gas issue can be dealt with in a number of ways (large volume, low
pressure tanks come to mind).
Further analysis:
I have avoided the usability and reliability requirements thus far. There has
already been significant discussion around these topics.
Conclusion:
Leave it open, practise closing it.
Cheers
David Pearson
In message "seeking opinions on isolator valves", cobber@ci*.co* writes:
> Tony-
>
> IMHO I think that this isolator thing is something you can get fixated on
> while missing the big picture. I am trying to picture a scenario in ocean
> diving where you would need the isolator-
>
> - Your valve fell apart (avoidable by proper maintenance.)
> - You hit something so hard you snapped off your DIN and broke your valve
> wheel (avoidable by not diving off of waterfalls or improving your
> scooting technique.)
> - Your valve oring fails (this would be a slow leak, plenty of time.)
> - Your burst disk fails (double those bad-boys up.)
>
> Seems to me if you fuck with your valve so it can't open up all the way
> you might screw up PP fills as it would not let the gas moves as freely
> into both tanks.
>
> Jim
>
> On 3/29/98 3:19 PM Tony Phillips wrote:
>
> > Jim
> >Just a thought but it really wouldn't be difficult with many isolator
> >assemblies
> >to pack the valve stem so that it would only open say 2 or 3 (or whatever)
> >turns.
> >That way the valve could be "fully open" by feel but would be much faster
> >to shut
> >down. One would obviously need to make sure that in the full open position
> >the
> >gas flow would be adequate - say a little over half the requirement for a
> >diver
> >working hard since the diver.
> >Have been kicking this idea around for a while. Any comments?
> >Tony Phillips
>
>
> -------------------------------------------------------------------
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>
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