Faking Altitude for NDLs, etc. for DRIVING to altitude

[cheeseandjamsandwich]

OK, we off diving at sea-level, but then have a slow drive up and over a mountain pass at about 1000m altitude, a couple or three hours after diving...

How can i set up my OSTC to think it's at 1000m altitude such that it calculates the No Deco Limits, etc as if i was altitude divng... to give me some conservatism...

And there doesn't seem to be a lot of info out there about driving to altitude after diving.

[scubatinoo]

cheeseandjamsandwich Wrote:
-------------------------------------------------------
> And there doesn't seem to be a lot of info out
> there about driving to altitude after diving.

Driving to altitude is the same like flying after diving. Don't do it.

And for the planning: Grab an "Adventures in Diving" book from PADI and read the section "Altitude Diving" - there you'll learn how to plan a dive on certain altitude...

[swissdiving]

You cannot set up your OSTC to make it think it's at altitude. It sets up its altitued according to air pressure.

Whether you drive to altitude or fly is not relevant. The OSTC tells you the No Fly time after a dive. In that time you should not go to altitude.
(in reality its not quite that clear cut, I live in Switzerland and most of the diving we do is diving at altitude at least in theory)

Diving at altitude

[cheeseandjamsandwich]

But is there a way of changing the conservatism such that the model calcs the shorter NDLs for a given altitude?
like dividing actual depth by 0.8657 to workout theoretical depth at say 1200m

My crappy Suunto D4 has an altitude option
[pre]
Table 5.5. Altitude Adjustment settings
Alt. adjustment value Altitude range
A0 0 - 300 m / 0 - 1000 ft
A1 300 - 1500 m / 1000 - 5000 ft
A2 1500 - 3000 m / 5000 - 10 000 ft
[/pre]


Cheers for the link btw

[Bardass]

sorry but,
did you understand what is Bühlmann's algorithm ?
did you read what Swissdiving wrote ?
"It sets up its altitued according to air pressure. "

Suunto computers need to make an altitude adjustment
OSTC don't need.

[swissdiving]

You can make your OSTC more conservative (or less for that matter) by changing to Gradient Factors.
http://forum.heinrichsweikamp.com/read.php?2,3344,3590

But then that does not have anything to do with diving at altitude.
Don't fiddle with GF's unless you know what you're doing.

Also here another forum discussion on the altitude implementation on the OSTC:
http://forum.heinrichsweikamp.com/read.php?2,7399,7412#msg-7412

Also note that at altitude, the OSTC will change to dive mode when reaching a depth of 3m. This was implemented to prevent the OSTC from switching into dive when flying and the pressure in the cabine is increased again when landing.

When I am talking about altitude I am speaking of somewhere between 1600m and up. I had this behaviour when diving at 2500m but not at 1600m. But then that depends also on the air pressure.

[cheeseandjamsandwich]

What we're trying to do is dive to the 1200m tables... but at sea level.

Then the trip back on the boat, packing up the gear is like a very shallow last 'safety stop' before then driving up the hill, which isn't a super fast ascent (like it can be in the Swiss Alps), this is like a super, super slow ascent to the 'surface' i.e. the 1200m altitude (actually just under 1000m).

[swissdiving]

Mate,

There really isn't anything you can do with the OSTC to replicate a dive table.

I suppose the question is, how long are you going to be on 1000m?
If you just go across a mountain pass and then back down...

On the dive, prior to the trip to altitude, what dive profile have you planned? How many dives? Air or NX?

[cheeseandjamsandwich]

What i mean is replicating the method of increasing the depth by the atmospheric pressure difference, as opposed to an actual table... Using the theoretical depth...

When diving in Switzerland, does the OSTC give you the shorter NDLs that are like those that an altitude table would?

If it did, then couldn't there be an 'Altitude fudge-factor' CF that altered the NDLs presented to us?

[swissdiving]

You really are flogging a dead horse.

In your case the discussion is not in relation to diving at altitude but going to altitude after diving.

If you really want to be on the save side, don't "fly" after diving, meaning don't go to altitude.

The decompression calculation model is Bühlmann ZH-L16. This algorithm and its calculations also include considerations for altitude diving.
So there is no need for a "fudge" factor.

[cheeseandjamsandwich]

As i understand it, the real problems with Flying after diving is:
1) The dives would have followed the standard tables (un-altered DC)
2) The RATE of change of altitude is huge in a plane... Zero to 8,000ft (cabin pressure) in only a couple of minutes. (plus a very small risk of 'loss of cabin pressure' that'd probably fizz you to death)

But the Driving after Diving situation IS different.
We're going to be ascending really quite slowly to only 1,000m (3,000ft)
AND we're going to dive following the 1200m atitude adjustments to the tables...

So the analogy of getting back on the boat/packing up being a 'stop' and the drive up the mountain being a really slow ascent to the surface, does seem to be correct.


The standard arse-covering answer is probably going to stay as 'Follow No-Fly time' for a while, as there's not a lot of data to work with... And that'll keep us all very, very safe... but the reasoning above seems correct to suggest it can be done with the correct planning (and diving the plan).

The 1000m pass is just a pass... we then go straight back down to about 300-400m.

I would like to know how the OSTC does behave doing you altitude diving.

[heinrichsweikamp]

cheeseandjamsandwich Wrote:
> I would like to know how the OSTC does behave
> doing you altitude diving.

All decompression calculations are made based on the surface pressure. Your deco times will be longer compared to sea level diving.

To compensate the return over a high pass use "classic" methods: extend your deco stops, especially the shallow ones, generously. Use a table for high altitude diving for comparison.

Regards,
Matthias

[cheeseandjamsandwich]

heinrichsweikamp Wrote:
-------------------------------------------------------
> cheeseandjamsandwich Wrote:
> > I would like to know how the OSTC does behave
> > doing you altitude diving.
>
> All decompression calculations are made based on
> the surface pressure. Your deco times will be
> longer compared to sea level diving.
>
> To compensate the return over a high pass use
> "classic" methods: extend your deco stops,
> especially the shallow ones, generously. Use a
> table for high altitude diving for comparison.
>
> Regards,
> Matthias

Cheers Matthias.

Rich.