Diving with altitude of more than 300M

[Keithliu]

I'm just a open water diver. I read on the text book that when diving in the ocean with altitude of more than 300M, it requires special procedure and special table for calculating the maximum time staying under water.
I understand why how depth you go would affect how long you can stay under water. But I get confused why the the depth of the water would also affect it. Isn't the pressure come from top to bottom ?

Regards,

Keith

 

[kylestugan]

I'm not sure if I understand you correctly, but I think you are confused because when the book uses the word altitude it refers to how high you are from sea level, not how deep you are in the water.

 

[Reef Man]

????? I'm with kylestugan....Altitude is diving above sea level... the ocean is at sea level every where!!!

 

[rocketry]

in the ocean with altitude of more than 300M

You are not making ANY sense.

I understand why how depth you go would affect how long you can stay under water.

First of all, you are referring to the NDL, not the 'time you can stay under water'.

But I get confused why the the depth of the water would also affect it.

We are dealing with the altitude of the body you are diving in ---- the higher the altitude of the water (NOT sea-level, a lake, for example) the less pressure your body is under and thus your tissues absorb nitrogen.

Why is your profile empty? Try to articulate better, and we can help you.

 

[rakkis]

Altitude diving refers to diving in a lake/quarry/other body of water that is on a mountain. The 300m/1000ft refers to the altitude the lake is at. Say.... a fictional lake on top of Mt Everest would be 29,000 ft above sea level.

Special procedures are required because your RDP's values are based on you exiting the water and off gasing nitrogen in a 1-atmosphere environment. Being at altitude means you are off gasing where the partial pressure of nitrogen outside your body is lower than your tables account for. Thereby putting you at risk of DCS at the surface since you would be off gasing too quickly.

 

[Rogue_Diver]

Diving at high altitudes does require a different set of tables or a computer that can adjust (manually or automatically) to the altitude. As the pressures are different, so are your NDL times at any given depth.

One example taken from the net:
Altitude-Adjusted Dive Depths

Left Column = Actual Depth of Dive in Feet -- Top Row = Altitude in Feet
Table Data = Effective Depth for Use in Standard Dive Table
Feet 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
10 10 11 11 12 12 12 13 13 14 15
20 21 21 22 23 24 25 26 27 28 29
30 31 32 33 35 36 37 39 40 42 44
40 41 43 45 46 48 50 52 54 56 58
50 52 54 56 58 60 62 65 67 70 73
60 62 64 67 69 72 75 78 81 84 87
70 72 75 78 81 84 87 91 94 98 102
80 83 86 89 92 96 100 103 108 112 116
90 93 97 100 104 108 112 116 121 126 131
100 103 107 111 116 120 124 129 134 140 145
110 114 118 122 127 132 137 142 148 153 160
120 124 129 134 139 144 149 155 161 167 174
130 135 140 145 150 156 162 168 175 181 189

Round upward to next 10 feet depth and next 1000 feet altitude.

Instructions: For the altitude of the dive, use your HIGHEST elevation for 24 hours after the dive. Round upward. (For example, if you dive at 6500 feet then drive over a mountain pass at 8900 feet, use 9000 feet as the dive altitude.) Find the appropriate altitude column in the top row. Now find the row with your actual dive depth in the left column. The number where the altitude column intersects the depth row is the adjusted depth. Use this depth in your standard dive table (instead of the actual depth), calculating bottom times, surface intervals, and residual nitrogen as usual.

 

[nadwidny]

Maybe some of the respondants here should check the location of the OP and realize that English is probably not his first language and cut him some slack before they start busting his grammar balls. I suspect his English is much better than their Chinese.

 

[Keithliu]

Thank you very much for the replies of you all! I'm impressed that the responses can be so quick and helpful. I understand why I get confused now. On the other hand, I have also updated my profile.

 

[namabiru]

Question. I know you have to have a different set of tables. Can you get them at the LDS, or order them easily?

 

[Charlie99]

One example taken from the net:
Altitude-Adjusted Dive Depths

Left Column = Actual Depth of Dive in Feet -- Top Row = Altitude in Feet
Table Data = Effective Depth for Use in Standard Dive Table
Feet 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
10 10 11 11 12 12 12 13 13 14 15
20 21 21 22 23 24 25 26 27 28 29
30 31 32 33 35 36 37 39 40 42 44
40 41 43 45 46 48 50 52 54 56 58
50 52 54 56 58 60 62 65 67 70 73
60 62 64 67 69 72 75 78 81 84 87
70 72 75 78 81 84 87 91 94 98 102
80 83 86 89 92 96 100 103 108 112 116
90 93 97 100 104 108 112 116 121 126 131
100 103 107 111 116 120 124 129 134 140 145
110 114 118 122 127 132 137 142 148 153 160
120 124 129 134 139 144 149 155 161 167 174
130 135 140 145 150 156 162 168 175 181 189

A reasonable approximation of the above table is to increase the equivalent depth by 4% for each 1,000 feet (or 300 meters). That's pretty accurate up through 6,000'/1800m, but underestimates the altitude correction a bit at higher elevations --- for 10,000' the correction is 45%, or 4.5%/1000'.

Charlie Allen