Dil switch off helium during deco

[Wibble]

For clarity -- and please don't shoot me for asking dumb questions:

Are we talking about deep dives that rack up long deco times in the range of hours? Switching from the very helium rich bottom diluent gas to an "air" type gas when shallower and flushing the loop would mean no helium in the loop along with further periodic loop flushing to expel exhaled helium build up from the loop.

Questions:
* At which depth would one consider doing this switch?
* Which gasses would be used? (The 'weakest' bailout deco gas would likely be 50% but that's 1.6 at 21m/70ft)

 

[wedivebc]

For clarity -- and please don't shoot me for asking dumb questions:

Are we talking about deep dives that rack up long deco times in the range of hours? Switching from the very helium rich bottom diluent gas to an "air" type gas when shallower and flushing the loop would mean no helium in the loop along with further periodic loop flushing to expel exhaled helium build up from the loop.

Questions:
* At which depth would one consider doing this switch?
* Which gasses would be used? (The 'weakest' bailout deco gas would likely be 50% but that's 1.6 at 21m/70ft)

I can't speak for others but for sub 300ft dives I carry a 32% bailout that I use to flush my loop during the ascent phase of the dive in order to reduce helium.
Don't forget just because your lloop has been flushed with a non helium mix does not mean there is no helium in your loop. You are still off-gassing helium as you decompress and that means you are not creating a total helium void unless you periodically flush during the ascent. I do not consider nitrogen flushes in my deco planning but I believe it provides me with a more efficient washout.

 

[stuartv]

I keep going around in circles in my head on this. Maybe somebody can help me understand the thought process better.

- Flush with something with less helium to help off-gas helium more quickly. But, that is at the expense of off-gassing nitrogen more slowly, right? You lower ppHe by increasing ppN2.

- The Shearwater post about the Helium Penalty boils down to telling us that we were not doing enough deco before we started using helium (and factoring it into deco calculations).

So, I keep thinking that:

- Taking more time to decompress because "I'm using helium" is really more time that is needed - to off-gas the nitrogen.

- Getting rid of He faster by getting rid of N2 slower seems like a tradeoff in the wrong direction.

- Lowering ppHe and also changing your computer to give you less deco time seems like traveling even further in the wrong direction.

Like I said, I feel like the more I think about this, the more I feel like I'm just going in circles and confusing myself. If somebody can explain the support for flushes to reduce or eliminate helium in some way that helps me understand this better, I would appreciate it.

 

[GF99/99]

For clarity -- and please don't shoot me for asking dumb questions:

Are we talking about deep dives that rack up long deco times in the range of hours? Switching from the very helium rich bottom diluent gas to an "air" type gas when shallower and flushing the loop would mean no helium in the loop along with further periodic loop flushing to expel exhaled helium build up from the loop.

Questions:
* At which depth would one consider doing this switch?
* Which gasses would be used? (The 'weakest' bailout deco gas would likely be 50% but that's 1.6 at 21m/70ft)

I keep going around in circles in my head on this. Maybe somebody can help me understand the thought process better.

- Flush with something with less helium to help off-gas helium more quickly. But, that is at the expense of off-gassing nitrogen more slowly, right? You lower ppHe by increasing ppN2.

- The Shearwater post about the Helium Penalty boils down to telling us that we were not doing enough deco before we started using helium (and factoring it into deco calculations).

So, I keep thinking that:

- Taking more time to decompress because "I'm using helium" is really more time that is needed - to off-gas the nitrogen.

- Getting rid of He faster by getting rid of N2 slower seems like a tradeoff in the wrong direction.

- Lowering ppHe and also changing your computer to give you less deco time seems like traveling even further in the wrong direction.

Like I said, I feel like the more I think about this, the more I feel like I'm just going in circles and confusing myself. If somebody can explain the support for flushes to reduce or eliminate helium in some way that helps me understand this better, I would appreciate it.

Just like @wedivebc has put it I can't speak for others and as I have said be for sub 100m diving is very very personalized and everyone has there own style.

@Wibble yes I am talking about dives that rack up multiple hours of deco (think over 3hr you really see the gain with dives over 5hr). I gave an example of a typical dil switch I would do but the gas and depth varies greatly on your dive plan. But generally I have an intermediate trimix gas I will switch to first then a deco gas ill switch to later. I have never done air dil switches. To answer you question about the Po2, I just plan my gases and switches at the depths and setpoints I want to keep things simple. If you look at the previous example I gave at 60m switching to a 17/40 you will see that the dil has a Po2 of 1.2 and that is what my setpoint is and at 18m switching to a 50% you will see that the dil has a Po2 of 1.4 and that what my setpoint. Additionally this makes for a good cell check, and flush check.

@stuartv Yes you are correct in the fact that doing a dil flush is reducing the He and adding N however the N is the lesser of the two evils. So yes you are actually reducing your decompression obligation.

 

[Wibble]

If you look at the previous example I gave at 60m switching to a 17/40 you will see that the dil has a Po2 of 1.2 and that is what my setpoint is and at 18m switching to a 50% you will see that the dil has a Po2 of 1.4 and that what my setpoint. Additionally this makes for a good cell check, and flush check.

Thanks for clarifying. Makes complete sense with those numbers.

 

[stuartv]

Just like @wedivebc has put it I can't speak for others and as I have said be for sub 100m diving is very very personalized and everyone has there own style.

@Wibble yes I am talking about dives that rack up multiple hours of deco (think over 3hr you really see the gain with dives over 5hr). I gave an example of a typical dil switch I would do but the gas and depth varies greatly on your dive plan. But generally I have an intermediate trimix gas I will switch to first then a deco gas ill switch to later. I have never done air dil switches. To answer you question about the Po2, I just plan my gases and switches at the depths and setpoints I want to keep things simple. If you look at the previous example I gave at 60m switching to a 17/40 you will see that the dil has a Po2 of 1.2 and that is what my setpoint is and at 18m switching to a 50% you will see that the dil has a Po2 of 1.4 and that what my setpoint. Additionally this makes for a good cell check, and flush check.

@stuartv Yes you are correct in the fact that doing a dil flush is reducing the He and adding N however the N is the lesser of the two evils. So yes you are actually reducing your decompression obligation.

But, isn't your presumption that you are "actually reducing your decompression obligation" based on a model that the Shearwater article on "The Helium Penalty" explains to be faulty?

 

[J-Vo]

But, isn't your presumption that you are "actually reducing your decompression obligation" based on a model that the Shearwater article on "The Helium Penalty" explains to be faulty?

N2 starts @ pp0.79 while He starts @ pp0.00, so right of the bat He is going to absorb faster due to the differential pressure. The Helium Penalty article only says the compartment half times don't need to be different. Add to that that your breathing 10/70 and now you only have 20% N2 being pushed into your tissues, so on the way up the He is certainly the biggest concern.

My problem with the switch is you PDC now assumes no He in the loop even though I'm off gassing. How much He is in there at 20' when my PPO2 drops below 1.6? Seems prudent from that perspective to just leave the PDC set to bottom gas.

 

[stuartv]

N2 starts @ pp0.79 while He starts @ pp0.00, so right of the bat He is going to absorb faster due to the differential pressure. The Helium Penalty article only says the compartment half times don't need to be different.

Okay, but the algorithm being used does still have them as different....