Safe travel altitude / lowest pressure allowed
Posted by: obetz ()
Date: July 18, 2019 09:05PM

Hi all,

is it correct that the OSTC currently doesn't indicate the current safe travel altitude (or lowest air pressure) in surface mode?

This would be very useful if I have to travel over a high pass after diving: I would like to know when it is safe to go up to the needed altitude, e.g 1500m somewhere in the Alps.

This could be seen as a natural extension of the "ceiling" value, just extended to lower pressures or "negative ceilings".

(kidding) Not suppressing negative ceilings would be sufficient for geeks but might confuse less technical users.

Oliver

Re: Safe travel altitude / lowest pressure allowed
Posted by: Ralph ()
Date: July 19, 2019 04:19PM

Hi Oliver,

it's not really about "currently". For quite a while, the tech version does calculate how long one has to wait before being allowed to ascent to 1.000, 2.000, 3.000 meter or board an aircraft. If this time calculates to zero, you are cleared for ascent. This function is likely to be introduced into the Sport version in near future, too.

Opposit to your proposal, the target height has to selected via the settings menu, then the "stop time" is calculated. Calculating a ceiling height would be very complicated, as with air there is no linear relation between depth/hight and pressure change due to the compressibility of air. Additionally, the air temperature will also effect the pressure-to-height relation. In the current approach, there are 4 distinct pressures stored for each hight selection, each pressure is pre-calculated using the barometric hight formular, with reasonable temperatures and safety margins plugged in.

best regards
Ralph

Re: Safe travel altitude / lowest pressure allowed
Posted by: dadefay ()
Date: July 19, 2019 04:59PM

Hi Ralph,

Thanks for this clear explanation.

Q : which altitude is taken for "flight" ? I could not find this information in the manual, maybe it's somewhere in this forum...

Kind regards.

Didier A. Defay
OC trimix instructor / ART CCR diver
France

OSTC Plus #16077
- OSTC 3+ #4806
-- OSTC 3 #3999

Re: Safe travel altitude / lowest pressure allowed
Posted by: Ralph ()
Date: July 19, 2019 05:06PM

Hi,

here's the original code with the definitions:

// ambient pressure at different mountain heights
#define P_ambient_1000m 0.880 // [bar] based on 990 hPa and 20°C at sea level, 15°C at altitude
#define P_ambient_2000m 0.782 // [bar]
#define P_ambient_3000m 0.695 // [bar]

// ambient pressure in aircraft cabin during flying - worst case according to Buhlmann
#define P_ambient_fly 0.600 // [bar], 0.600 bar is the value used by Buhlmann for his flying-after-diving calculations
// 0.735 bar is a typical cabin pressure for nowadays commercial jet aircrafts
// -----
// 0.135 bar safety margin

Re: Safe travel altitude / lowest pressure allowed
Posted by: dadefay ()
Date: July 19, 2019 05:44PM

Thanks Ralph !

Didier A. Defay
OC trimix instructor / ART CCR diver
France

OSTC Plus #16077
- OSTC 3+ #4806
-- OSTC 3 #3999

Re: Safe travel altitude / lowest pressure allowed
Posted by: obetz ()
Date: July 20, 2019 03:06PM

Thanks for the information, I'll wait this feature in the Sport version.

Regarding "typical cabin pressure": Using such a pressure may allow an earlier flight, but there is a certain (yet low) probability of a pressure loss resulting in much lower ambient pressure.

The oxygen masks will keep the passengers conscious but don't help against bubbles.

I don't know the rules how fast a descent to higher pressure will be in such a case.

Re: Safe travel altitude / lowest pressure allowed
Posted by: Ralph ()
Date: July 21, 2019 07:25AM

Perfectly right!

Standard procedure in case of cabin decompression is to decent to 10.000 feet (about 3.000 meters) if surronding terrain allows. In case of crossing mountains, there may be an intermediate flight path section where the aircraft stays (has to stay) higher. Emergency descent rate is about 9.000 feet/minute (2.750 meter/minute), so it takes approximately 3 minutes to come down from cruise level to 10.000 feet. A commercial jet engine aircraft won't go any deeper for aerodynamic reasons, it would have to slow down to not overspeed and would consume much more fuel in the thicker air lower down, potentially compromising ability to reach an appropriate emergency landing site.



Edited 1 time(s). Last edit at 07/21/2019 07:27AM by Ralph.

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