Trivia Question: Where Would You Be At 1/2 Ata?

[Walter]

"If your body reached an equilibrium with the pressure which it would and it would come close probably for how long it would take you to reach 18,000' the tables would still apply becuase they are generated from pressure, they have nothing to do with the atmosphere above you."

You are mistaken. That approach will get you bent.

First, the pressure is a result of "the atmosphere above you."

Next, the tables work because they are based on % of pressure change. When you reduce the pressure at the surface the % of pressure change increases. For this reason, you must adjust the tables to plan as if you were diving deeper than your actual dive depth. You can do this with tables designed for the altitude at which you are diving or you can use tables which tell you how much to adjust the sea level tables.

 

[Charlie99]

cnidae once bubbled...
Huh?

From a trip report posted elsewhere I got the impression that there were 4 students, but only 2 did the 120' 20 minute 30/30 trimix dive at Lake Tahoe, and only one, Kevin, did the 2nd trimix dive.

Anyway, what I'm curious about is the explanation behind the GUE teaching that you should just ignore altitude.

 

[cnidae]

Walter once bubbled...


First, the pressure is a result of "the atmosphere above you."

Next, the tables work because they are based on % of pressure change. When you reduce the pressure at the surface the % of pressure change increases. For this reason, you must adjust the tables to plan as if you were diving deeper than your actual dive depth. You can do this with tables designed for the altitude at which you are diving or you can use tables which tell you how much to adjust the sea level tables.

You are mistaken, The pressure is not a result of the atmosphere's above you. The pressure is determined only by the amount of pressure you apply from your zero point (surface). If you dive to 34 feet at 6500' you still ongass the same % of N2 as if you dove to 33 feet at sea level. If I'm wrong then show me an experiment or law for your reasoning.

Charlie99, yes I skipped the second dive but by my choice.

 

[cnidae]

http://scidiv.bcc.ctc.edu/wv/09/0009-006-henry.html

 

[cnidae]

Someone chim in here to support either theory's, this is bugging me.

 

[Doppler]

cnidae once bubbled...
Someone chim in here to support either theory's, this is bugging me.

Well, here is something to ponder... There are several variables not taken into account...

How will our depth gauges work in the mountains... let's say they are zeroed at 18,000 feet... they read 0.5 ATA as Sea Level effectively, right?

Now we go diving... does the pressure sensor read the increase as a percentage of Sea Level or does it read that an increase of one atmosphere equals 33 feet? I do not know the answer to this but I suspect that the depth gauge will give an inconsistant reading when it tries to express depth (an increase in pressure) as a lineal measurement (FSW).

OK. Let's put that aside. Let's say we have a shot line going down 100 feet and it has been previously measured and marked in feet or meters and now we dive to 100 feet.


The algorithm doesn't care what altitude we are at as long as we have been there long enough to offgass to a state of equilibrium.

HOWEVER, the partial pressures of oxygen and nitrogen and helium if we're using trimix, will differ because the AMBIENT PRESSURE AT THE SURFACE will differ and that IS factored into the gas loading at depth... Things like CNS and MODs

A EAN 36 at 100 measured feet delivers 1.42 ATA PO2 @ sea level and 1.24 ATA PO2 @ 18,000 feet. So there should be a difference in one's total runtime... albiet small (By the way, not a mix I'd use myself!)

 

[cnidae]

Doppler once bubbled...



Well, here is something to ponder... There are several variables not taken into account...

How will our depth gauges work in the mountains... let's say they are zeroed at 18,000 feet... they read 0.5 ATA as Sea Level effectively, right?

Now we go diving... does the pressure sensor read the increase as a percentage of Sea Level or does it read that an increase of one atmosphere equals 33 feet? I do not know the answer to this but I suspect that the depth gauge will give an inconsistant reading when it tries to express depth (an increase in pressure) as a lineal measurement (FSW).

The pressure gauge if digital works on pressure alone, resetting it's self once activated or reset at altitude. If the gauge was calibrated for sea water it would read 33' at 34'. In reality your gauge is'nt exactly accurate in the ocean. Depends on what the manufacture calibrated it to as far as the salinity.

Doppler once bubbled...

OK. Let's put that aside. Let's say we have a shot line going down 100 feet and it has been previously measured and marked in feet or meters and now we dive to 100 feet.


The algorithm doesn't care what altitude we are at as long as we have been there long enough to offgass to a state of equilibrium.

HOWEVER, the partial pressures of oxygen and nitrogen and helium if we're using trimix, will differ because the AMBIENT PRESSURE AT THE SURFACE will differ and that IS factored into the gas loading at depth... Things like CNS and MODs

A EAN 36 at 100 measured feet delivers 1.42 ATA PO2 @ sea level and 1.24 ATA PO2 @ 18,000 feet. So there should be a difference in one's total runtime... albiet small (By the way, not a mix I'd use myself!)

Yup you are correct. We are now going to have fewer molecules saturated into our tissues. Seems it would be safer to dive at altitude than sea level using the standard tables. Anyone else have any thoughts?

 

[Charlie99]

cnidae once bubbled...
Someone chim in here to support either theory's, this is bugging me.

Here's some simple history and theory.

Haldane, by bending a sufficient number of goats, determined that one could stay on air indefinitely at a given pressure, then directly ascent to 1/2 of that pressure. This 2::1 ratio (1.58 to 1 if you just consider N2) was reasonably constant independent of the two depths chosen.

For example, one can stay an extended period at 33 fsw (2ata), then ascend directly sea level (1ata). More conservative models have reduced this number so that today we consider about 22-25' the limit for no deco sat dive, but that is more a reflection of less tolerance of bending divers and caisson workers than a change in theory.

Lets look at what happens when you repeat that 33 fsw dive at say 6,225' (Lake Tahoe altitude). The lake level pressure is about 0.8ata. So at 33 fsw below the lake (34 ffw if you electronic depth gauge automatically switches over) you will be at 1 + 0.8 = 1.8ata. Surfacing to 0.8ata subjects you to a 1.8/0.8 = 2.25 pressure ratio ---- significantly more than the 2::1 of sea level. (Using N2 only numbers it is 1.78 to 1, compared to sea level 1.58 to 1 ---- the same 25% increase).

Traditional adjustments of tables is to use equivalent depths, scaled by this 25% fudge factor so that surfacing ratios stay constant.

That is the basic theory behind Walter's earlier comments about treating an 80' dive at 18,000 feet as a 120' dive.

While the number crunching gets more complicated once you start having multiple compartments of differing halftimes, the results remain pretty much the same.

When you go to the dual phase models, such as RGBM and VPM, the altitude still has pretty much the same effect.

============

You might run the 120', 20 minute, 30/30 trimix dive through GUE's own DecoPlanner for sea level and for 6200' and look at the results. Although I only have VPM-B and GAP (ZHL16B algorithm) and ZPLAN (also Buehlmann), I'll wager that GUE's own deco software shows a significant effect.

------------------

In Kevin's trip report, he described the dive planning as 120' x 0.8 to get 96' EAD for 30/30 trimix. Then applied 120 rule to get 20 minutes (why not 24??) of NDL. What surprised me is that he didn't apply a 25% fudge factor to turn the 96' EAD back into a 120' sea level equivalent. His statement was that he was explicitly told to ignore altitude.

So, back to my original question, what sort of theory or supporting material did GUE provide to justify ignoring altitude?

Thanks,

Charlie Allen

 

[Walter]

That will get you bent.

"The pressure is not a result of the atmosphere's above you."

Pressure is the weight of the atmosphere. When we dive, it is also the weight of the water.

"The pressure is determined only by the amount of pressure you apply from your zero point (surface)."

There is no "zero point." At sea level, we are under 14.7 PSI pressure. Our bodies (assuming we've been at that pressure a while) are saturated with N2 at 14.7 PSI. If we descent into the water, the pressure increases. The deeper we go, the more the water above us weighs and the more pressure we are under. As we increase the pressure by diving deeper, more N2 can dissolve into our bodies. If, for example, we were to dive from the surface at sea level to 68 ffw (66 fsw), we would triple the pressure from 14.7 PSI to 44.1 PSI. The longer we stay at that depth, the closer we approach nitrogen saturation at 44.1 PSI. The tables are designed to limit our time at depth to allow us to safely return to 14.7 PSI. They are not designed to allow us to safely return to the surface unless the surface is 14.7 PSI.

If, for example, we were to dive from the surface at an altitude of 8000 ft to 68 ffw (66 fsw), we would almost quadruple the pressure from 10.9 PSI to 40.3 PSI. A similar change occurs at sea level when we dive to 100 ft. We will need to return safely, not to 14.7 PSI, but to 10.9 PSI. Logic would indicate when diving at 68 ft, we should use 100 ft on the Sea Level dive tables. When I check the Sea Level Equivalent Depth table, I find the sea level equivalent depth for 70 ft at an altitude of 8,000 ft is 100 ft. Amazing!

 

[cnidae]

Charlie99 once bubbled...
Here's some simple history and theory.

Haldane, by bending a sufficient number of goats, determined that one could stay on air indefinitely at a given pressure, then directly ascent to 1/2 of that pressure. This 2::1 ratio (1.58 to 1 if you just consider N2) was reasonably constant independent of the two depths chosen.

For example, one can stay an extended period at 33 fsw (2ata), then ascend directly sea level (1ata). More conservative models have reduced this number so that today we consider about 22-25' the limit for no deco sat dive, but that is more a reflection of less tolerance of bending divers and caisson workers than a change in theory.

Lets look at what happens when you repeat that 33 fsw dive at say 6,225' (Lake Tahoe altitude). The lake level pressure is about 0.8ata. So at 33 fsw below the lake (34 ffw if you electronic depth gauge automatically switches over) you will be at 1 + 0.8 = 1.8ata. Surfacing to 0.8ata subjects you to a 1.8/0.8 = 2.25 pressure ratio ---- significantly more than the 2::1 of sea level. (Using N2 only numbers it is 1.78 to 1, compared to sea level 1.58 to 1 ---- the same 25% increase).

Traditional adjustments of tables is to use equivalent depths, scaled by this 25% fudge factor so that surfacing ratios stay constant.

That is the basic theory behind Walter's earlier comments about treating an 80' dive at 18,000 feet as a 120' dive.

While the number crunching gets more complicated once you start having multiple compartments of differing halftimes, the results remain pretty much the same.

When you go to the dual phase models, such as RGBM and VPM, the altitude still has pretty much the same effect.

============

You might run the 120', 20 minute, 30/30 trimix dive through GUE's own DecoPlanner for sea level and for 6200' and look at the results. Although I only have VPM-B and GAP (ZHL16B algorithm) and ZPLAN (also Buehlmann), I'll wager that GUE's own deco software shows a significant effect.

------------------

In Kevin's trip report, he described the dive planning as 120' x 0.8 to get 96' EAD for 30/30 trimix. Then applied 120 rule to get 20 minutes (why not 24??) of NDL. What surprised me is that he didn't apply a 25% fudge factor to turn the 96' EAD back into a 120' sea level equivalent. His statement was that he was explicitly told to ignore altitude.

So, back to my original question, what sort of theory or supporting material did GUE provide to justify ignoring altitude?

Thanks,

Charlie Allen

Now lets look at it this way. I believe the 2:1 ration was determined somewhere near sea level. What it's missing is the amount of molecules absorbed into the tissues is the real reason for getting bent it’s not just the pressure decrease. Just as an example, if I was on the moon with an ATA of 0.1 and I submerged myself to 0.3 of an ATA and saturated, your telling me I would have a good chance of being bent? Think about how few molecules your actually absorbing with the pressure change because you started out with far fewer than if on earth. Do you have a link to Kevins post? Tell me how my logic is flawed.