Time to Desaturation?

[BoltSnap]

Diving with nitrox and setting to air is considered more cautious.

Who told you this?

 

[BoltSnap]

No way I would fly with a 13 hour SIT time only.

 

[tursiops]

No way I would fly with a 13 hour SIT time only.

It really depends on the dive history and the profiles.
The DAN Europe recommendation of 24h is all situations has no basis is fact.
The DAN USA recommendation of 18h after multiple days of diving is pretty conservative, and does not take into account the actual profiles. In fact, they recommend just 12h after a single dive. You know all that.
His profiles were pretty benign:

I have been diving over the last 3 days, with 3 dives the first day, 3 dives yesterday and 1 dive today early morning.

PS: Last days were on air, last dive yesterday was shallow (11m average) and today's dive was with 32% nitrox but computer on air profile.

Aside from the fact that he references depth in terms of the average (rather than maximum) for the dive it sounds pretty benign. If he provided actual profiles one could use the US Navy tables to estimate FAD time. For example, he probably was in Pressure Group I or less (Table 9-7). That would allow him to fly (8000 ft, table 9-6) without any issues after a 12h surface interval.

 

[Subcooled]

My new Geneses React Pro offers a 'Time to desaturation' count down on the 'Time to Fly' screen. Can anyone tell me about this number? For example-

1. Is this number total desaturation of Nitrogen from the body?

2. How is it best used after the dive?

3. Is it additional information for flight time or used for something else?

Thanks,

Stan

Decompression illness happens when the ambient pressure drops too fast and the nitrogen (etc) in your blood want to exit too fast. It can exit either to the exhaled air or, given some conditions (depth) are given, into the bubbles in your blood stream. Then latter, of course, is quite unhealthy.

Once on the surface, feeling good, everything is probably OK. But if you fly too soon, and the ambient pressure drops from 1 to 0.8, then that might be too much - especially if you have been diving extremely deep or for a whole week on a diveaboard.

The no fly counter is probaby only an overly conservative safety factor. You would not want to take chances though, as DCI is not fun. Some people use post dive surface oxygen, but this becomes an advanced skill.

 

[chris123]

Heya,
For anyone interested, just some additional information:
Dives on first days were all max. 28~35m deep and 45-55 minutes long (sometimes though only few minutes on deepest level and I believe that's what makes it difficult to use the US navy tables for reliably determining the repetitive group) while the last dive on the second last day was max. 14m. The one dive on the last day had max depth of 32m.

Suunto Ocean with factory default "medium" conservatism setting (40/85 GF) showed 18 hour no fly after the second day (after 2 days with 3 dives each), i.e. beginning of third day I was already good to fly. After the last (single) dive on 3rd day, computer showed a 12 hour no fly timer.

Flight had a cruising altitude of 4700m (determined via GPS) and the Suunto Ocean Altimeter showed an altitude of 950m (i.e. the "pressurised cabin environment equivalent altitude" assuming Suunto determines altitude via pressure).
Even group Z only has 9:13 required SIT before 3000ft ascent.

Long story short, all went fine.

 

[Blackcrusader]

Diving with nitrox and setting to air is considered more cautious.

In any case, second computer was set to nitrox but also shows desat time of few hours before flight time.

OK I understand your thinking. Use 21% setting and have less NDL and MOD is max 34m on 32%

However the reasoning omits the fact the DC now has incorrect information. If you had a DCS hit, go to a chamber and the chamber people look at your diving log they could make an incorrect assessment based on the gas mix in your DC.

If you want to use Nitrox and be more cautious then change the conservative settings in your DC. So if you have a DC with GF change the GF high to 50 but use the nitrox setup. That would be the correct way to be more cautious.

 

[Centrals]

24hrs NO Fly time since day one.
48hrs if I have been tec dive ie. deco dive.
Too conservative? My well being is my only consideration.

 

[lizardland]

However the reasoning omits the fact the DC now has incorrect information. If you had a DCS hit, go to a chamber and the chamber people look at your diving log they could make an incorrect assessment based on the gas mix in your DC.

No they won't. Treatment schedules are fairly standard unless either the dive profile or the symptoms are outliers. Plus hyperbaric medics are fairly switched on people who know how to interpret data.

 

[halocline]

24hrs NO Fly time since day one.
48hrs if I have been tec dive ie. deco dive.
Too conservative? My well being is my only consideration.

Where did you get the 48 hr no fly time after deco diving? Is that your idea?

For the sake of anyone reading, DAN has a recommendation of 18 hrs after multiple recreational dives. For decompression diving, they recommend ‘substantially longer’ than 18 hours. That’s it.

Dive computer time-to-fly counters are just that….counters. The DSAT feature on dive computers likely varies greatly with the computer. At the end of a dive trip, I usually have a look at the tissue compartment loading graph on my perdix, and I’ve never seen anything that causes me alarm after 18 hours.

 

[Belzelbub]

Dive computer will calculate with a higher N2 level of 21% instead of 32%, i.e. calculated N2 saturation is higher than actual saturation. This way is no problem, other way is more of an issue, based on my understanding.
From what I learned, O2 saturation shouldn't be an issue, rather N2 saturation I should be worried about when flying (13hr) after diving

On the surface, this is correct. O2 sat is mostly not an issue. At least for rec divers. However, you are now giving your computer inaccurate information, which is just not necessary and largely negates some of the benefits of using EAN.

When I took my EAN course it was explained via old school thinking. Mainly that EAN provides longer dive times/shorter surface intervals OR greater safety factor. That is essentially what you are doing by setting your DC to air when using EAN. You are ignoring the first part. In reality, it's not an OR question at all. It's a bit of a sliding scale. You can actually have both, provided you aren't bumping up against the NDL.

In full disclosure, I used to let my kids' DCs be set to air when they were using EAN. This was only because the computers they used would default to air after a while, and changing the setting was kind of a pain. I always set mine to what we were actually using. This is fixed now as they have Peregrines, which will hold the setting and are easier to change.

Dive computer time-to-fly counters are just that….counters. The DSAT feature on dive computers likely varies greatly with the computer. At the end of a dive trip, I usually have a look at the tissue compartment loading graph on my perdix, and I’ve never seen anything that causes me alarm after 18 hours.

Yep, exactly. Largely a dumb counter. Some DCs just default to 24 hours (or longer) after any dive. Others add a couple of levels based on the previous dives, but it's still a counter.

With my Garmin, I can choose between Standard or 24 hours. If I choose 24 hours, it will be a 24 hour counter for any dive longer than 3 minutes and deeper than 15', unless DC is in gauge mode or there was a deco violation. Then it is 48 hours. I use standard and the levels are below:

< 3 minutes duration, or < 15' max depth: 0 hours
Single NDL dive more than 48 hours from previous dive: 12 hours
Multiple NDL dives within 48 hours: 18 hours
Dive with a deco stop: 24 hours
Gauge dive or violation: 48 hours.