Gas switch using air

J

JukkaO

Member
How do you feel the idea of just adding some air to the loop to get the He portion lower? Let´s assume that I succeed to let out strong mix the same amount I add air and the result after one minute is mixed as planned.

The only problem I see is it does not mix evenly immediately. I suppose one breath of air at 60m is not that dangerous. Or is it?

Jukka
 
JukkaO said:
How do you feel the idea of just adding some air to the loop to get the He portion lower? Let´s assume that I succeed to let out strong mix the same amount I add air and the result after one minute is mixed as planned.

The only problem I see is it does not mix evenly immediately. I suppose one breath of air at 60m is not that dangerous. Or is it?

Jukka
Click to expand...

Once upon a time it was fashionable to do flushes so I experimented. I've tried Air and 20/30 at 42m and 60m. As a result the theory says you can knock some time from the schedule - about 20 mins or so.

However each time I have done it I'm extremely tired - definitely incomplete decompression.

I've also combined it with higher ppO2 of 1.5. This sheds another 20 mins.

The flush just doesn't work for me. I now take the high SP as soon as the ascent starts and stay on the gas all the way. This does work for me.

If you want to shave time then I'd say do it whilst on the gas, just shave the time.

Take care,
Matt.

I'm an idiot, so safer to ignore me.
 
Thanks for sharing your experience. I know the subject is not too clear. On a long dive in cold water you just need to choose between different risks.

Back to the question. Is it dangerous to have one breath of air at 60m after ascend with 70% He in diluent cylinder?

Jukka
 
JukkaO said:
Thanks for sharing your experience. I know the subject is not too clear. On a long dive in cold water you just need to choose between different risks.

Back to the question. Is it dangerous to have one breath of air at 60m after ascend with 70% He in diluent cylinder?

Jukka
Click to expand...

I'd say not dangerous, just not advantageous.

Matt.
 
Years ago Proplanner used to wipe out tons of deco if you put a couple of minutes in with a deep switch to air. I used to do it all the time, always felt absolutely shagged afterwards.

There have been a few deaths where deep air switches have been suggested as contributing to the accident. Rob Parker's accident is one that springs to mind, IIRC it was suggested that a deep air switch had caused some deco problems.
 
Did you check to see if IDC is a potential issue (although one breath wouldn't worry me - also not sure what dil you're on, xx\70??).

Matt.
 
There is a similar thread that I think Ccr Dave made a point that most of the helium is off gassed by the time u get shallow anyway... So technically I guess switching to air are 60m could increase deco? You are on gassing and off gassing in all stages of the dive... If u can't do the required deco due to cold conditions, perhaps shorten bottom time to allow a safe deco schedule...
 
From experience, after long, deep dives I have found the first half of the deco at the last stop (6m) I have been offgassing LOTS of inert gas into the loop. It slows down to almost nil toward the end of the last stop. It's evident by the inability to maintain the setpoint which is regained by conducting loop flushes to expel the inert gasses.

The only real advantage I have been able to see from doing an "Air Flush", is that a few mins on Air will cut your CNS loading by anything up to half what it would have been had you remained on the loop with the same high setpoint and is very handy when you are doing dives with a plan CNS loading of up past 150% !!!

As for my thoughts on doing it to reduce your deco commitment......Tread carefully and remember the old saying....."If it appears to be too good to be true then it probably is"

Regards,

Lance
 
I was not thinking to switch to air but to add some air to the mix to get He portion lower. I am interested to find new ways to change the diluent.

I guess ideal would be to get the He portion lower on ascend. Flushing is not very accurate methode and it is easy to understand that it is usually not recommended. I usually do not chage the mix on two hour dive but if the deco alone is three hours I do change the mix.

Jukka
 
Once you change proportions of inert gases in the loop you are just guessing your gas mix from then on. Soon after a flush it will be closest to your current mix with low helium, but the helium proportion will rise due to off-gassing from your body so you don't really know what proportion of helium you are breathing anymore.

Also as you ascend the impact becomes less and less as inerts make up a far decreasing portion of your gas mix anyway, by 6m you should be on 75-80% O2 depending on your setpoint.

If cold is a problem then buy a heated vest or add some layers under your drysuit.
 
JukkaO said:
How do you feel the idea of just adding some air to the loop to get the He portion lower? Let´s assume that I succeed to let out strong mix the same amount I add air and the result after one minute is mixed as planned.

The only problem I see is it does not mix evenly immediately. I suppose one breath of air at 60m is not that dangerous. Or is it?

Jukka
Click to expand...

Flushing the loop with a lower helium percentage diluent or even air was a 'popular' idea as deeper dive profiles were being explored back when CCR became available on the recreational market. In theory yes the deco can be reduced with diluent switching, however working in rebreather design and testing and seeing first hand what can actually go on in the loop in practice, I never bought into the concept, loop flushing can be very inconsistent and then determining what is in the loop upon which to base your deco calculation is imprecise. Of greater concern to me was the potential for a near immediate 'narcotic hit' that can potentially occur by flushing with air at depth. This has been implicated in one fatality.

On your back is a perfect gas blending machine automatically reducing the inert content in the loop as you ascend. I would suggest keep it simple and let it do its work; find alternative means of managing the thermal challenges. Rgds Paul
 
JukkaO said:
...to the mix to get He portion lower.

Jukka
Click to expand...

The He portion is dropping on ascent anyway. At 60m with 10/70 (inerts ratio 78:22) and SP=1.3 the fractions are 18\64\18 and at 6m 81\15\4. Hence the He has fallen from 64% to 15% during ascent. You can further reduce it by up-shifting the SP.

Would one breath make such a difference anyway as the majority of the off gassing will be He?

Matt.
 
Paul Haynes said:
In theory yes the deco can be reduced with diluent switching
Click to expand...

I agree with Paul here. Also, I dived recently with a guy who pulled the Air@60m flush stunt and he earned himself a rather nice chest-rash and swelling for it (80m dive, 45 mins bottom).

Matt.
 
matthewoutram said:
I agree with Paul here. Also, I dived recently with a guy who pulled the Air@60m flush stunt and he earned himself a rather nice chest-rash and swelling for it (80m dive, 45 mins bottom).

Matt.
Click to expand...

Did he switch to air or took just some air between using trimix.

Jukka
 
Lancer4545 said:
From experience, after long, deep dives I have found the first half of the deco at the last stop (6m) I have been offgassing LOTS of inert gas into the loop. It slows down to almost nil toward the end of the last stop. It's evident by the inability to maintain the setpoint which is regained by conducting loop flushes to expel the inert gasses.

The only real advantage I have been able to see from doing an "Air Flush", is that a few mins on Air will cut your CNS loading by anything up to half what it would have been had you remained on the loop with the same high setpoint and is very handy when you are doing dives with a plan CNS loading of up past 150% !!!

As for my thoughts on doing it to reduce your deco commitment......Tread carefully and remember the old saying....."If it appears to be too good to be true then it probably is"

Regards,

Lance
Click to expand...
I have observed this too with off gassing. Basically what i have observed is a lot of offgassing on the 15-10 m range and the first part of the 6m stop. after that very little difference.
I agree with Lance on reducing deco, i have actually come to padding deco over time case i didn't feel right after some dives.

Dimitris
 
Divers do still diluent switches with big steps and some have got symptoms. It would be easy to replace known portion (not all) of the loop volume with air many times during the ascend. I guess that might be something new. The problem is to know the mixture as you off gas etc. Effect of one air breath in deep is anotehr issue.

Long deco in cold water is risky with any equipment. Where to put the limit for a must of a habitat etc. Good loop mixture management during ascend means shorter deco obligation and less risk if you have to cut last hour of deco for some emergency.

I agree there is no way to do it with good controll today. We might still be far from optimal solution and that keeps me thinking.

Jukka
 
jptaylor9 said:
Do you risk/suffer from skin bends (not sure this term is used any more) due to cold & poor circulation thru outer skin layer?
Click to expand...

I have a very good suit etc. and I have not had any big problem ever.
When making longer dives the risk of a very serious problem increases. At some point a habitat is needed as a backup at least. As my ascend might be far from optimal I keep thinking how to make it better.

Jukka
 
If you were to conduct a deep and long dive on a high helium-content diluent and then on ascent switch to air and do a full loop flush, is it possible that the sudden lack of inspired helium coupled with a high saturation of helium in the tissues would cause the helium to offgas at a rate which might bring on a bend ?
 
steve6690 said:
If you were to conduct a deep and long dive on a high helium-content diluent and then on ascent switch to air and do a full loop flush, is it possible that the sudden lack of inspired helium coupled with a high saturation of helium in the tissues would cause the helium to offgas at a rate which might bring on a bend ?
Click to expand...

I am not writing about complete diluent or air flush.
It is possible to replace let´s say 1...2 liters of the loop volume with air. It will soon be mixed and the result includes less helium than earlier. It could be repeted using small steps avoiding big gradients.

Jukka
 
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