[Clare]

Find myself wondering how those of you without VR3s and suchlike manage with calculating CNS and UPTDs (OTUs) on dives - and whether you bother. Then go on to wonder what the VR3 (or any other computer which will work this out) calculates the percentage used on a particular dive and how much variation there is in the results, and finally whether people take any notice at all. So - for those who want to play.....

To get us all on the same page herewith a dive profile which has been posted before. Hope it is legible enough.

The reason I have posted a profile rather than a set of figures is that I caluclate using bottom time whereas some might calculate from surface. Gases were 18/45, 50 per cent from 21, 18/45 at 9 and O2 from 6. The exact time from surface to leaving the bottom was 27 minutes if you calculate on this.

If you dive in this depth range do you calculate your CNS at all?
If not - would you do so on a repetative series of dives?
If you do - how?
And what would you calculate it for on this dive?

 

[Colin Marriott]

If you dive in this depth range do you calculate your CNS at all?

Yusss

If not - would you do so on a repetative series of dives?
If you do - how?

N/A

And what would you calculate it for on this dive?

Dunno, what did the planning software calculate it as? (you did plan the dive, didn't you?)


ColinM

 

[1693]

When I did my TDI Adv. Nitrox/Deco Procedures course we spent a good couple of hours going through this. Calculating it all by hand (and calculator) from the NOAA O2 exposure table.

I went through it all again with Mark Powell on my Trimix course.

Since then I've let my VR3 and VPlanner take care of it.

 

[NotDeadYet]

I don't bother calculating. I'll have a look at the CNS on my deco software if I'm doing a long cave profile but generally for an openwater dive I wouldn't. I know from experience what profiles I need to at least be aware when/if I spike the pO2 on ascent, otherwise I never really worry about it.

As for UPTD/OTU's it's different, I've had post-dive symptoms on long dives/long decos so I'm a bit more aware if I'm doing multi-day dives. If it's a weekend then I don't calculate and certainly not for a one off (I'll run out of scrubber time before I hit the OTU limit).

Cheers,

Stuart

 

[dry suit diver]

When I did my TDI Adv. Nitrox/Deco Procedures course we spent a good couple of hours going through this. Calculating it all by hand (and calculator) from the NOAA O2 exposure table.

I went through it all again with Mark Powell on my Trimix course.

Since then I've let my VR3 and VPlanner take care of it.

same for the BSAC adv nitrox by hand/electronic adding machine from the NOAA tables and yes I do play with the numbers before and after a dive. for most weekends I dont get near the limits, i would have been closer to limits if I had dived all week at scapa.

 

[Colin Marriott]

If you do 45mins @ 1.6mtrs on O2, then you'll have 100% CNS, but only 87 OTUs, umm, I believe . . .


ColinM

 

[Gary K]

When I did my TDI Adv. Nitrox/Deco Procedures course we spent a good couple of hours going through this. Calculating it all by hand (and calculator) from the NOAA O2 exposure table.

I went through it all again with Mark Powell on my Trimix course.

Since then I've let my VR3 and VPlanner take care of it.

ditto

 

[Janos]

For CNS I plan on 1.2 on the bottom, 1.4 for intermediate deco, and 1.6 for the last 20 minutes or so of the 6m stop.

I can calculate all of this by hand (and it's part of the BSAC Advanced Nitrox course) but in practice I use planning software will calculate it all for me. The Shearwater will also monitor my CNS.

In practice I haven't found it to be an issue, except on Red Sea liveaboards, when I consciously run a lower ppO2 throughout the week to limit UPTD uptake.

Janos

 

[HP]

Also can someone provide any data / research as to how CNS / OTU came about and what data it was based upon? It just seems to be something that's been handed down through the mists of time?

 

[Scuby]

If its a single mix dive then I don't worry about it - done the calculations, and nothing I do gets anywhere near.

For multiple deco mixes / trimix, v-planner takes care of that in the pre-dive planning.

David

 

[dry suit diver]

Also can someone provide any data / research as to how CNS / OTU came about and what data it was based upon? It just seems to be something that's been handed down through the mists of time?

dunno and without wishing to offend anybody, the yanks probabnly threw a sailor in a water tank with pure O2 in his tins and timed how long it took him to dance like a dad at a wedding on acid at different depths.

seriously I would think the research/experimentationwould have started with Reddoes when he used Nitrox as a medical gas back in 1794. Henry Fluess of Siebe Gorman did a nitrox dive in 1879 so I would also imagine they did research into the effects on the human body. The NOAA (National Oceanic and Atmospheric Administration) did loads of research in the 70s (google is your friend)

 

[nigelH]

I confess I know that 1.3bar ppO2 (the setpoint of the Inspiration) is three hours in one hit and three and a half hours in one day so I don't bother. I'd love to do over three and a half hours diving in one day but I've never got close.

 

[PBrown]

dunno and without wishing to offend anybody, the yanks probabnly threw a sailor in a water tank with pure O2 in his tins and timed how long it took him to dance like a dad at a wedding on acid at different depths.

Nearly, but it was us Brits that did it. Summary of the research done is contained in this piece:
DAN Divers Alert Network : OXTOX: If You Dive Nitrox You Should Know About OXTOX


cheers,
Paul

 

[NotDeadYet]

Bill Hamilton came up with the UPTD formula which was renamed OTU as well as doing a lot of research into the decrease in vital capacity and recovery times.

Cheers,

Stuart

 

[Gareth J]

I believe the original reason for the research into O2 dose limits & pre-longed exposure was done for the NASA program.



Gareth

 

[Mark Chase]

There was also the unexplained high fatality rate on Navy divers on pure 02. This triggerd some reserch as well.


Personaly I pre plan dives on the PC and glance at the CNS numbers just to see if its dodgy. Thats about it. With the VR3 it prompts air breaks at a pre determind level. I had my one set for 90CNS.

As nigel said unless your doing sill dive times or silly deco using 100% for too long its not realy an issue.

One thing that reserch has found is adding a bit of Nitrogen to the mix reduces the risk of a tox.

ATB

Mark Chase

 

[Nick_lincoln]

My Vytec has as handy little bar on the left hand side which measures UPTD's and accumulates this for subsequent dives in nitrox mode (OLF I think its called) This doesnt give a CNS exposure though. But you would be going some to get near the limit of 80% CNS and 300 UPTD's.

In the Red sea in may, it looks like we will be on four dives a day on nitrox, certainly 3 a day, so I will be doing some calculations after each dive to see how I'm doing. Especially if I'm using EAN50 for deco.

Edit - actually - I seem to remember something in the vytec manual about the OLF being a combination of CNS and pulmonary exposure.............. either way if that bar on the vytec display is getting towards full I will be checking calculations carefully during the next surface interval.

 

[aquaholic]

I run 1.2 ppO2 at the bottom and as close to 1.6 as I can get for the 21m and 6m stops. On long deco dives I do breaks on backgas around 5 min for every 12-15 minutes of deco.

For Red Sea liveaboard type diving I run 1.2 ppO2 throughout the week if it is reef based diving. For deeper dives with air dil I run similar to above.

So far, so good.

 

[grazie42]

Learned to calculate during classes.
Watch whatever my software tells me it will be for a dive, but it´s never been even borderline so far...
I seem to remember my instructor saying something about OTU´s being "made up" and that it was an extrapolation of....something or other...it was a few years ago...

 

[P B]

My Vytec has as handy little bar on the left hand side which measures UPTD's and accumulates this for subsequent dives in nitrox mode (OLF I think its called) This doesnt give a CNS exposure though. But you would be going some to get near the limit of 80% CNS and 300 UPTD's.

In the Red sea in may, it looks like we will be on four dives a day on nitrox, certainly 3 a day, so I will be doing some calculations after each dive to see how I'm doing. Especially if I'm using EAN50 for deco.

Edit - actually - I seem to remember something in the vytec manual about the OLF being a combination of CNS and pulmonary exposure.............. either way if that bar on the vytec display is getting towards full I will be checking calculations carefully during the next surface interval.

If you then download it into Suunto Dive Manager 1.6, it will tell you which of the two is the higher between CNS and OTU and what % it was.

Paul