Partial Pressure of 2.0 ?!

[sammyseadog]

Hi,

I'm going to start off in these forums with a big ask that may scare a lot of you!

What I need to know is if there is a dive computer that allows you to conduct accelerated decompression on 100% Oxygen at 9 metres? That's right 9 metres!

Most of you will say that's a partial pressure of 1.9, which is way over 1.6 (the recreational max...I know!!), but I have heard that CMAS and some Navy tables allow a partial pressure of 2.2. ALSO, if you have ever been in a chamber ( I have as a tech!), you'll know that they take people down to 18 metres and give them 100% oxygen, that's 2.8 partial right?!

I know the computer might not like it and it may flash a lot, but I need one to track the Oxygen exposure on dives where accelerated deco is happening at 9 metres, so that I can get a graph of the Oxy exposure and correct Nitrogen off-gassing.

Please don't reply unless you have technical knowledge in relation to partial pressures exceeding 1.6. I actually know what I'm doing, I just need a computer that will monitor it without shutting down on me.

Thanks

 

[Tortuga68]

Most of you will say that's a partial pressure of 1.9, which is way over 1.6 (the recreational max...I know!!), but I have heard that CMAS and some Navy tables allow a partial pressure of 2.2. ALSO, if you have ever been in a chamber ( I have as a tech!), you'll know that they take people down to 18 metres and give them 100% oxygen, that's 2.8 partial right?!

1.6 is generally considered the max PO2 for the non-working (i.e decompression) part of a dive. Hence MOD for O2 is 6m, MOD for 50% is 21m etc. It's not a recreational limit.

Navy Tables have an 'contingency' schedule which is designed for getting fit, young Navy divers out of the water ASAP, and onto a boat where they have a recompression chamber immediately available. You shouldn't compare that, or exposures in a chamber for treatment purposes, with 'recreational' technical diving (i.e non-commercial).

I fail to see a benefit to doing what you're proposing, compared to the addditional risks - and I say that as someone who has dived air to 100m which is a PP of 2.31. In other words, I wouldn't do what you are suggesting. If you compare run times to conventional MODs I don't think you'll see much difference.

I don't think basing your diving plan on what you heard might be in some table is a good idea, without fully understanding the application of those tables (not to mention their actual content).

Having said that, any 'technical' computer (Liquivision, Shearwater etc) should produce the iformation you're looking for.

It might interest you to do some research on the early WW2 rebreathers and the changing MOD that was adopted, and the reasons why - but don't forget this was soldiers at war so the 'acceptable risk' was much higher than it should be for something we are all supposed to be doing for fun.

 

[HowardE]

The Table also calls for max of 30 minutes in a 24 hour period for a PO2 of 2.0 (that equates to 2% of the allowable CNS O2 per minute). If you're doing deco on a dive and running a PO2 of 2.0, then you'll most likely exceed the CNS O2 allowable for a 24 hour period, because of the CNS (and OTU's from the dive itself). This includes the US Navy limits.

Conducting a high PO2 in the dry (a chamber), if you tox, you just pass out. Keep that in mind.

 

[Luis H]

Back about 40 years ago a PP of 2.0 was considered very acceptable (at the time the limit for using air was 298 ft or 10 atm).

Just keep in mind that the Navy does that type of decompression using full face mask or (more often) ultra light Kirby Morgan helmets. Going into convulsions in a chamber or using a helmet/ FF mask is considered a very acceptable risk since there is basically no real risk of drowning.

The Navy procedures are normally for surface supply and it include gas switching including air breaks (similar to the air breaks in a in a chamber).

Note: I am not (or have ever been ) a Navy diver, but I have done some the classroom portion of the Navy mix-gas, decompression, and saturation diving training.

 

[Tortuga68]

There is some great, relevant information in this thread that I would encourage the OP to read: http://www.scubaboard.com/forums/diving-medicine/440726-oxygen-toxicity-limits-symptoms.html

 

[sammyseadog]

Navy Tables have an 'contingency' schedule which is designed for getting fit, young Navy divers out of the water ASAP, and onto a boat where they have a recompression chamber immediately available. You shouldn't compare that, or exposures in a chamber for treatment purposes, with 'recreational' technical diving (i.e non-commercial).

I fail to see a benefit to doing what you're proposing, compared to the addditional risks - and I say that as someone who has dived air to 100m which is a PP of 2.31. In other words, I wouldn't do what you are suggesting. If you compare run times to conventional MODs I don't think you'll see much difference.

I don't think basing your diving plan on what you heard might be in some table is a good idea, without fully understanding the application of those tables (not to mention their actual content).

Having said that, any 'technical' computer (Liquivision, Shearwater etc) should produce the iformation you're looking for.


Thanks for all info guys!

I'm aware of dangers and full face masks and navy fitness over recreational demographics and the difference between wet and dry Oxy hits. The tables I am using are formulated for a very fit group of male workers. They were not created by me, but by a leading Hyperbaric doctor. I am just trying to monitor the oxygen consumption and off-gassing in a real world environment and then look at potential changes to the tables, using nitrox and shallower O2 stops.

I'll look into the Liquivision and Shearwater and see what the manuals and companies say.

Cheers

 

[victorzamora]

I'm pretty sure Petrels will do what you're asking as they don't lock you out for anything. BUT, I don't know how valid the algorithms are (under ANY scuba model) at predicting such high PPO2s.

 

[sammyseadog]

Thanks guys, researching the petrels and the liquivision and have also stumbled upon the VRX computers, anyone know anything about them? Seems to be that I have heard of then before and there's some clearance one's out there that are a bit more affordable.

Cheers

 

[Tortuga68]

Anecdotally at least, most tech divers today seem to favour SW & LV products over VRx, which are infamous for their uh... somewhat erratic deco schedules, not to mention being the size of half a housebrick. Just my two cents.

I'm pretty sure Petrels will do what you're asking as they don't lock you out for anything. BUT, I don't know how valid the algorithms are (under ANY scuba model) at predicting such high PPO2s.

This is a valid point which bears repeating; it's one thing for the computer to operate under these circumstances; whether the data provided is accurate is another question altogether.

It's also worth mentioning that people have been bent in the past, sometimes badly, following experimental tables designed by "leading hyperbaric doctors" who some might say (some did say) were using the divers as guinea pigs to validate their software (as recounted in Mark Powell's 'Ocean Gladiator' book, for example)

 

[victorzamora]

VRx are much older technology. They've been rebranded as the Hollis DG05. However, you haven't really given us much information to go off of as far as what you want out of the computer.

I will tell you, Hollis has been suffering a rash of computer-related issues with the DG03 and the DG05. I've heard of 6 DG03s failing and a couple DG05s failing....and that's in a small community based around a Hollis shop. The owner of the Hollis shop is now getting numbers together to become a Petrel dealer. Also, I believe the VRx computers lock you out if you violate their protocols....similar to the liquivision computers. You might find yourself locked out of the data you want.

I'd stick with a Petrel.