max off-gas time?

[jon m]

hey all-
noticing that after multiple dives (3) my computer gives me about the same time (18hrs+) to off-gas completly (no fly) as after 1 dive... would this ever be more than that? would there ever be a situation (more dives per day or more days diving) were you would get a no-fly time longer than 18 hours or so?
thanks!

 

[Rick Murchison]

hey all-
noticing that after multiple dives (3) my computer gives me about the same time (18hrs+) to off-gas completly (no fly) as after 1 dive... would this ever be more than that? would there ever be a situation (more dives per day or more days diving) were you would get a no-fly time longer than 18 hours or so?
thanks!

Depends on the algorithm used. In Haldanean theory, for example, tissue ongassing and offgassing theoretically occur in "halftimes" - if a tissue gets half saturated in 10 minutes, then half of what's left gets filled in the next 10 minutes, so the tissue is 3/4 saturated and so forth. In these algorithms the full saturation/desaturation time is generally considered to be six half times - a 10 minute tissue will be fully saturated in 60 minutes, or, going the other way, fully desaturated 60 minutes after arriving at the surface.
So... if the algorithm uses 120 minutes as its "longest" tissue, then the longest it'll take to desaturate once you reach the surface is six times that, or 12 hours. That's why no matter what group you surface in, it takes 12 hours to clear the Navy tables.
If your computer uses a Haldanean algorithm and six halftimes for its sat/desat rules, and the longest tissue compartment used is 180 minutes, then no matter what kind of diving you do, once you're out of the water it'll take 18 hours to clear.
Or.... some computers simply have a count-down timer for "time to fly" and yours may be set at 18 hours.
Many computers today use bubble models that can clear at a variable time, and those can give you longer times to fly after lots of hard diving... and some... some combine a minimum count-down timer with a bubble model, so you'll get the minimum time-to-fly until you cross a theoretical threshold, then you'll get more than the minimum.
Which computer do you have? Someone here may know the algorithm and the rules it uses.
Rick

 

[sharkattack]

I give it 24 hours to fly

 

[Meng_Tze]

Depends on the algorithm used....
Many computers today use bubble models that can clear at a variable time, and those can give you longer times to fly after lots of hard diving... and some... some combine a minimum count-down timer with a bubble model, so you'll get the minimum time-to-fly until you cross a theoretical threshold, then you'll get more than the minimum.
Which computer do you have? Someone here may know the algorithm and the rules it uses.
Rick

What he said. Depends how things are calculated and padded by the manufacturer. I personally have seen 12hrs to 24 hrs, but one time didn't feel right... so added another 24 hrs.

 

[jon m]

thanks , i have an uwatec smart com.

 

[fairybasslet]

thanks , i have an uwatec smart com.

Uwatec has a reputation for being conservative but I've noticed that their no fly times are ridiculously short. One day after 3 dives, pretty deep ones of a max of 100-90fsw on 30% EAN it gave me a no fly of 6 hours. My Aeris back up gave me 20+. I wasn't flying that day anyway, but still, my buddy thought it was weird.

 

[jon m]

hmmm
so here is the obvious follow up question-
do you (or do you THINK) it is o-k to fly after multiple dive days and an 18hr interval, or do you (or think) you need to wait 24 hrs?

 

[bajashack]

hmmm
so here is the obvious follow up question-
do you (or do you THINK) it is o-k to fly after multiple dive days and an 18hr interval, or do you (or think) you need to wait 24 hrs?

That is a difficult question to answer based on your body weight, fitness level, how fat or not you are. For multiple dives shallow or deep go the 24 hours. If diving shallow 12 hours. As far as the 18 hours, its better then 12.

 

[Charlie99]

hmmm
so here is the obvious follow up question-
do you (or do you THINK) it is o-k to fly after multiple dive days and an 18hr interval, or do you (or think) you need to wait 24 hrs?

Usually 18 hours is enough, which is why that is the DAN recommendation for multiple NDL dives.

Normally the desat time on my computer is less than the 24 hour dumb countdown timer. Unless I have been doing some pretty agressive diving, then I'll fly at 18 hours. OTOH, I have driven the Desat time on my Oceanic Data Plus 2 up above 30 hours. That was the result of several days of 4+ very long dives in the 60 to 80' range on air --- repeatedly going right up to, or a bit beyond the NDL on the rather liberal Oceanic computer. Probably 6+ hours of bottom time each day. In that case, I'm not willing to fly even at the 24 hour point.
My solution has been to cut back to two dives on the last day, and to go on the "shallow boat" on my last day of diving in a place like Cozumel.

Your time to fly is limited by the slower compartments after the first few hours. For those very slow compartments, a reasonable approximation of the loading is your average depth over the last day or two, INCLUDING the SI times.

 

[Charlie99]

Uwatec has a reputation for being conservative but I've noticed that their no fly times are ridiculously short. One day after 3 dives, pretty deep ones of a max of 100-90fsw on 30% EAN it gave me a no fly of 6 hours.

I'm pretty sure that the Uwatec just runs the Buhlmann algorithm with a surfacing pressure of about 0.75ata to find out when you are cleared to ascend to that point. To put it another way, Uwatec calculates the needed stay at 0' depth before an ascent to 8,000' altitude exactly the same way that they calculate the time you need to stay at 10' depth before ascending to 0' sea level. The time to fly has no safety margin added.

A short, deep dive will not load the slow compartments very much, so the time to fly will be much shorter than a dive to NDL in the 60' range. A dive to 40' NDL would make time to fly even longer, but recreational divers never spend that length of time in the water.

When doing a square profile deep dive, you are limited by NDL to such a short dive that the medium to slow compartments aren't loaded very much. The heavily loaded fast compartments offgass fast, so time-to-fly is short. When diving to 40' range, you are limited by air consumption to much less than NDL.

For time-to-fly problems, the worst sort of dive is a deep multilevel dive where one rides the NDL back up. The loading at the end of the dive in the slower compartments is much greater than you can achieve on either shallow dives or deep dives to NDL.