My First Stupid Question...

[Ant Slegg]

Looking through the on-line version of the AP Evolution manual I came across the following (p87)
"When deeper than 20m too much O2 may be added using this method..."

The paragraph is part of a section dealing with actions to be taken with a high Oxygen (ppO2) warning is displayed.

The problem I am having (bear in mind I don't [yet] own an AP machine) is that I have always considered cylinder valves to be pretty much constant flow rate devices (for "constant" pressure) when fully opened. If I am right then opening a cylinder valve for a fixed (short) time will add a fixed quantity of O2 (i.e. a fixed number of molecules). At a certain ambient pressure this would equate to a fixed increase in ppO2.

I would therefore have thought that as the depth increases that the increase in ppO2 would be less.

It would appear from the statement in the manual that the cylinder adds more O2 at greater depth.

Someone please put me out of my misery and explain where I'm going wrong!

Ant S

[jradomski]

Looking through the on-line version of the AP Evolution manual I came across the following (p87)
"When deeper than 20m too much O2 may be added using this method..."

The paragraph is part of a section dealing with actions to be taken with a high Oxygen (ppO2) warning is displayed.

The problem I am having (bear in mind I don't [yet] own an AP machine) is that I have always considered cylinder valves to be pretty much constant flow rate devices (for "constant" pressure) when fully opened. If I am right then opening a cylinder valve for a fixed (short) time will add a fixed quantity of O2 (i.e. a fixed number of molecules). At a certain ambient pressure this would equate to a fixed increase in ppO2.

I would therefore have thought that as the depth increases that the increase in ppO2 would be less.

It would appear from the statement in the manual that the cylinder adds more O2 at greater depth.

Someone please put me out of my misery and explain where I'm going wrong!

Ant S

The deeper you get the more critical your mastery of the skill has to be.. Small oxygen additions at depth can make a big impact on po2. what worked ok at 10m might be deadly at 20m or 30m..

Whn flying an rb with a stuck open solenoid you need to learn to contol the valve so that the SPG doesnt move when opening.. you will bleed some o2 into the loop, if it shoots up, the o2 will continue to bleed until its exhausted.. There is alot of available gas in the lines that feed the system when you take into account the length of the hoses for the SPG and LP oxygen manual add..

If you really want to prove it to yourself.. Shutoff your oxygen and time how long it takes the system to dump 50bar and how long it can maintain your po2.. no think about how much an effect this would have if the solenoid was stuck open anad allowed to dump all this gas at once



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[Ant Slegg]

Thanks Joe
This leaves me with 2 questions:
1) What does "bleen" mean?
2) If I am adding a "small" amount of oxygen (at whatever depth) then it's the technique and not the physics that drives the ppO2 changes?

Ant S

[Simon A]

Thanks Joe
This leaves me with 2 questions:
1) What does "bleen" mean?
2) If I am adding a "small" amount of oxygen (at whatever depth) then it's the technique and not the physics that drives the ppO2 changes?

Ant S

Ant
1) i guess it's a typo for "Bleed"
2) It's the physics, but the result is you need better technique to compensate at depth.
You are adding O2 through a depth compensated reg which means that due to the physics you are adding differing "Mass" or "Surface equivalent volume" the deeper you go.

For example say you add 1 litre of gas to the loop, at 10m this is at 2ATA so the surface equivalent is 2 Litres. at 20m it's at 3ATA so your adding 3 litres surface equivlent, and so on.

With a bit of luck someone a bit more eloquent will be along shortly to explain it better.

HTH
Simon A

[jradomski]

Ant
1) i guess it's a typo for "Bleed"
2) It's the physics, but the result is you need better technique to compensate at depth.
You are adding O2 through a depth compensated reg which means that due to the physics you are adding differing "Mass" or "Surface equivalent volume" the deeper you go.

For example say you add 1 litre of gas to the loop, at 10m this is at 2ATA so the surface equivalent is 2 Litres. at 20m it's at 3ATA so your adding 3 litres surface equivlent, and so on.

With a bit of luck someone a bit more eloquent will be along shortly to explain it better.

HTH
Simon A

I'm tired.. It was a typo...

2 things change with depth.. the IP rises which will push more gas through the orifice (solenoid) faster, but this really isnt the issue since it will continue until the pressure is zero which is quite fast anyway..

The big difference small differences in o2 make biger differences in po2 as the pressure rises, some of this is partially offset in the fact that the loop actually has more gas molecules in it than shallower..



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[Ant Slegg]

I think I get this now...

A fixed volume of pure O2 (more mass/molecules as depth increases) added at ambient pressure to the loop with 1.3bar ppO2 in it will have a greater influence on the ppO2 in the loop as the depth (ambient pressure) increases.

I did a little bit of fiddling with numbers in a spreadsheet and (with some assumptions for loop volume etc. i.e. I assumed 6 litre loop volume and adding 1 litre pure O2) the effect of adding pure O2 (at ambient) on the ppO2 in the loop is 6 or 7 times greater at 50m than at 10m. The bigger the loop volume the less the impact so I guess that makes taller divers (or just those with bigger lungs) have very slightly less of a problem.

Like a said - my first stupid question - I'm sure there'll be others!

Ant S

[Simon A]

Like a said - my first stupid question - I'm sure there'll be others!

Ant S

Ant
this question was only stupid if a) you didn't understand originaly, b) didn't ask to discover more, and c) then went and dived despite not understanding.

The only stupid question is the one you didnt ask!

Simon A

[Ant Slegg]

Thanks to you both gentlemen!

The good news is:
a) I did learn something that seems at first counter-intuitive to an OC diver
b) My spreadsheet confirms Joe's figures so I can play with the numbers without having to get wet or risk my life
c) I can even see how the loop venting (which with a substantial quantity of new gas in it is likely) would make things even worse.

Now where's that AP Order Form?

Ant S

Ant
this question was only stupid if a) you didn't understand originaly, b) didn't ask to discover more, and c) then went and dived despite not understanding.

The only stupid question is the one you didnt ask!

Simon A

[Ben Field]

Now where's that AP Order Form?

Try sales@apdiving.com

Come on- you know you want to... :D

BEN

[Sutty]

I don't dive an inspo so I may have this all wrong so feel free to correct me - surely the cylinder size is irrelevant, if you start with cylinder off, crack it open (SPG moves up to 10bar) and turn it off again the volume of O2 will be 10 x volume pressurised, the volume pressurised will be the volume of the hoses from cyl to solenoid including SPG etc. Plus the volume that feeds during the time taken for the on-off manoevre.

Or have I misunderstood

Neil