Trimix advantage for recreational diving below 130ft. Reference to literature wanted.

[Scubadude_LA]

Good thread. Like the others said, if cost was no issue, why not. I have cave buddies that dive rebreathers that always use helium. Cost is not so much an issue since they use so little of it. Also, the GUE/DIR crowd have helium in their standard gases for less than 100'. My first read on all of this was IANTD Exploration and Mixed Gas Diving Encyclopedia. May be better materials out there now. But if you read it you may finish it at a 1st Dan level, or at least an honorary degree in meditation.

 

[OkByMe]

Originally Posted by walkonmars
Hi folks,


Can I assume if I mix a 20% to 30% helium blend with 30% O2 and use standard nitrox tables and computer algorithms I can be OK?

Thanks for your kind help.

T





***********************************************************************************

Unforutunately you can not. Why? The computer is tracking 2 gasses (assuming a nitrox computer), those being o2 ansd n2. for the n2 portion i would think it would be ok, because the n2 is behaving like like the computer expects it to.... IE 20/30 trimix the body and puter treats it ike the n2 is 50, and it is. The problem is on the o2 side of the house. Saying that you have 50%o2 because of using a 30/20 mix tells your puter youhave 50% o2. the o2bg wll go nuts if you can get 50% at all lin a nitrox puter. If you have a medical incident and they review the puter. You wil initially set into motion the wrong treatment. Plus from your nitrox course you know that 50% o2 has a mod of around 70-80 ft. So if youare going to dive trimix use the proper computer. ******************************************************************************

A NITROX Computer only has one user input for the gas and that is the oxygen percentage, the assumption is made that the remaining gas is Nitrogen. A suggested mix was 25% oxygen and 25% helium - the remaining gas would be 50% Nitrogen. Input the Oxygen content into the computer at 25% for a 1.4 PPO MOD of 151ft. The computer treats the remaining gas as Nitrogen and calculates NDL. So the issue is whether the NDL needs to be more or less using Helium to replace some of the Nitrogen. I say NDL is less, since you have an EAD for 50% Nitogen, but your computer does not know that and is goung to put you into a stop, if you have the gas to exceed the NDL. The 150ft. dive will be safer than on just 25% NITROX.

I have A Gallileo SOL with TriMix. When I turn off TriMIx and input 25% O2, The dive time is less (NDL calculation) because it assumes 75% N2. But with the TriMix turned on an input 25% O2/25%HE, the dive time is longer since the N2 content is adjusted to 50%. You can verify this using both NITOX and AIR tables... Or do it the easy way... use V-Planner.



---------- Post added January 28th, 2014 at 11:52 AM ----------

 

[barrmust]

Recreational trimix diving book

Amazon.com: Recreational Trimix Diving eBook: Kevin Evans, Richard Johnson: Books

Blending trimix book

The Art of Gas Blending: Anton Swanepoel: 9781463714185: Amazon.com: Books

 

[NAM001]

2 part answer First part is computer algorithm related and second part is outside the computer.

First part:::::

I would hagve to say you would be ok.. and this is why:

using the example of 20-30% heflium with 30 o2. (THIS IS A VERY BASIC ANSWER FOR WHAT I ASSUME IS A BASIC QUESTION) YOU WILL BE USING 30/25 30 o2 AND 25 He. When entering it into your comuter you enter as 30% O2 telling your puter that it has 30 O2 and 70 non O2. The puter will track the O2 correctly but not the N2. The Computer will give you something like this.


MOD 30% = 130
Ndl xxxx minutes

In reality the correct info will be

MOD 30% = 130
Ndl much much longer than xxx minutes above.

As Consequence your puter will have you surface (per ndl) much sooner that is needed, so no foul there.

So I will reluctantly say that there is no problem so long as you comply with ascent reates.

Second part:::::

In addition to this I would say to anyone not to fool with gasses that you are not trained to use. Also that way gasses react to the body at rec depths are certainly different at deep depth and time durations. Keep in mind that your computer also does not consider off gassing rates for other than N2. Some one correct me if i am wrong. He off gasses much faster than N2. If you are going to persue this kind of diving I emplore you to get the sutable training and the computer/equipment to match your diving.

Originally Posted by walkonmars
Hi folks,


Can I assume if I mix a 20% to 30% helium blend with 30% O2 and use standard nitrox tables and computer algorithms I can be OK?

Thanks for your kind help.

T





***********************************************************************************

Unforutunately you can not. Why? The computer is tracking 2 gasses (assuming a nitrox computer), those being o2 ansd n2. for the n2 portion i would think it would be ok, because the n2 is behaving like like the computer expects it to.... IE 20/30 trimix the body and puter treats it ike the n2 is 50, and it is. The problem is on the o2 side of the house. Saying that you have 50%o2 because of using a 30/20 mix tells your puter youhave 50% o2. the o2bg wll go nuts if you can get 50% at all lin a nitrox puter. If you have a medical incident and they review the puter. You wil initially set into motion the wrong treatment. Plus from your nitrox course you know that 50% o2 has a mod of around 70-80 ft. So if youare going to dive trimix use the proper computer. ******************************************************************************

A NITROX Computer only has one user input for the gas and that is the oxygen percentage, the assumption is made that the remaining gas is Nitrogen. A suggested mix was 25% oxygen and 25% helium - the remaining gas would be 50% Nitrogen. Input the Oxygen content into the computer at 25% for a 1.4 PPO MOD of 151ft. The computer treats the remaining gas as Nitrogen and calculates NDL. So the issue is whether the NDL needs to be more or less using Helium to replace some of the Nitrogen. I say NDL is less, since you have an EAD for 50% Nitogen, but your computer does not know that and is goung to put you into a stop, if you have the gas to exceed the NDL. The 150ft. dive will be safer than on just 25% NITROX.

I have A Gallileo SOL with TriMix. When I turn off TriMIx and input 25% O2, The dive time is less (NDL calculation) because it assumes 75% N2. But with the TriMix turned on an input 25% O2/25%HE, the dive time is longer since the N2 content is adjusted to 50%. You can verify this using both NITOX and AIR tables... Or do it the easy way... use V-Planner.



---------- Post added January 28th, 2014 at 11:52 AM ----------

 

[Dr. Lecter]

2using the example of 20-30% heflium with 30 o2. (THIS IS A VERY BASIC ANSWER FOR WHAT I ASSUME IS A BASIC QUESTION) YOU WILL BE USING 30/25 30 o2 AND 25 He. When entering it into your comuter you enter as 30% O2 telling your puter that it has 30 O2 and 70 non O2. The puter will track the O2 correctly but not the N2. The Computer will give you something like this.


MOD 30% = 130
Ndl xxxx minutes

In reality the correct info will be

MOD 30% = 130
Ndl much much longer than xxx minutes above.

As Consequence your puter will have you surface (per ndl) much sooner that is needed, so no foul there.

So I will reluctantly say that there is no problem so long as you comply with ascent reates.

No! Replacing a portion of the nitrogen in 30% EAN with helium will decrease the NDLs, because you will saturate with the faster on-gassing helium sooner than you would with the nitrogen. Not to mention that "complying with ascent rates" tends to matter a good deal more with helium than with nitrogen.

Just for an example: VPM-B at nominal conservatism will give no stops for an 11 minutes dive to 130' on 30% EAN, but will give a brief stop at 10' with 30/25 trimix. The richer in helium your trimix, the shorter your NDLs.

 

[NAM001]

i was trying to keep the answer simple. The ndl as I stated for the N2 portion of the gas will be longer than what the nitrox computer will calculate. i will agree that the He component throws curves into the problem that the computer is not taking into consideration. Thanks for that clarification. I had to write the other post twice. I did have something about "as long as you comply with ascent rates in it" for the helium off gas rates..



30% EAN with helium will decrease the NDLs, because you will saturate with the faster on-gassing helium sooner than you would with the nitrogen. Not to mention that "complying with ascent rates" tends to matter a good deal more with helium than with nitrogen.



Just for an example: VPM-B at nominal conservatism will give no stops for an 11 minutes dive to 130' on 30% EAN, but will give a brief stop at 10' with 30/25 trimix. The richer in helium your trimix, the shorter your NDLs.[/QUOTE]

 

[TSandM]

V-planner definitely doesn't like helium. I'm not sure if all the technical algorithms are equally stern about it.

 

[Dr. Lecter]

V-planner definitely doesn't like helium. I'm not sure if all the technical algorithms are equally stern about it.

Depending on how you play with them, gradient factors can be much easier on helium mixes than VPM, but no matter how you tweak 'em they'll never give longer NDLs for nitrox than trimix. Then again, there's not much doubt that helium is treated with excessive conservatism by all major tech models: I'm finding it's common for people doing some truly serious CCR dives to just lie to their computers about how much helium is in their mix. It's a jungle out there...