Rounding altitude settings on Cressi Leonardo

[CycleCat]

Starting pressure, too: it needs to know if you're saturated at 13 or 10 PSI when you hit the water.

why do you need it in a "round" number of some dead king's shoe sizes?

Good point on the start pressure.

It is a lot easier for me to remember the steps are 2500, 5000, 8000 and 12,000 than it would be to remember the precise conversions 2296, 4921, 7874, and 12139!

The steps are already a kilometer wide and rounded to metric hundreds, and the actual ambient pressure will vary with weather. I can't imagine rounding a few feet in either direction will make too much of a difference.

That is kind of what I figured. The Loenardo is already known for being conservative. Rounding the first setting up to 2500' amounts to .1 PSI, rounding the second up to 5K amounts to .04 PSI, the third to 8K is .05 PSI, rounding the last setting DOWN to 12K adds a safety factor of .05 PSI.

 

[Esprise Me]

Another way of rounding that wouldn't sacrifice any conservatism is to just assume that a meter equals 3 feet. Thus the first setting, up to 700 meters, is up to 2100 feet. That's easy to do in your head.

 

[NAM001]

I was of the understanding that dive computers actually use the change in pressure for NDL, Deco, tissue, etc. calculations, rather than the depth and so from that perspective the altitude shouldn't matter that much.

Where it does matter is for the depth displayed by the computer. i.e. if the computer thought you were diving at sea level but you were diving at altitude, the depth displayed may be shallower than you actually are. It would only be an issue if you were using the computer in gauge mode and using tables.

I'm happy to be corrected.

True but what pressure equates to what depth display. just for grins if 14.7 is sea level and it takes a .3 to turn on the computer then it tuerns on at at 4 feet with the 15 psi. If you go to altitude and do not adjust for the surface pressure you will go to 15 feet or so before your computer turns on and it will read 4 ft. the It like you say uses pressure changes to calculate but the display needs the reference point to call zero. so if you need to do a stop at 50 ft( actually x psi) it will really be say 10 ft off when it translates it to depth in ft.

 

[CycleCat]

... if 14.7 is sea level and it takes a .3 to turn on the computer then it tuerns on at at 4 feet with the 15 psi. If you go to altitude and do not adjust for the surface pressure you will go to 15 feet or so before your computer turns on and it will read 4 ft...

I'm enjoying the nerdy science of this discussion!

I think the altitude settings have more to do with air pressure on your body during your time out of the water than anything to do with depth calculations. At .433 PSI per foot, water becomes the great equalizer regardless of altitude. At 5000 feet, surface air pressure is reduced from 14.7 PSI to 12.2. Those 5000 feet (or 2.5 pounds) are made up for by less than 6 feet of water.

 

[NAM001]

I'm enjoying the nerdy science of this discussion!

I think the altitude settings have more to do with air pressure on your body during your time out of the water than anything to do with depth calculations. At .433 PSI per foot, water becomes the great equalizer regardless of altitude. At 5000 feet, surface air pressure is reduced from 14.7 PSI to 12.2. Those 5000 feet (or 2.5 pounds) are made up for by less than 6 feet of water.

Ok I agree I think, what about a dive at 10000 ft. when you make a computer you have to make it to work at any environment. I agree there is a certain amount one thing creating its own correction. Much like frequency changing speed form 4800 to 5000 fps. you do not get a higher frequency at the receiver because that increase on speed also changes the wave length to counter the the increase in volosity. or a better example being flying at 400 kts and going to 500 kts. it does not make the trip distance shorter because you advance more miles per unit of time. the time times speed product has not changed although the individule elements have. Something in my gut says you are right but only in the sense of deco. the error in the measurements may also counter them in the result. Its the trranslation to the visual interface. so when you do a plan and it says for instance MOD is 100 ft. the compurter may read 100 ft when you are actually at 120 ft. Yes exagerated values but the effect is still there. As long as the diver knows about it and can adapt to it or know how to reconfigure for it it is not a problem. I dont know about the specifics opf any one computer. The reasultand total pressure would be the same because of hte lower barometric pressure. I feel comfortable with mine ( shearwater) so I will speek regarding it. my preditor at an old firmware level had a setting for barometric pressure of 0 ft. so if I went to 5000 ft i would set it to 12.2 pounds (your measurement terms) it changed later to someting else and now I dont think it is even there because it uses the depth sensor to make the measurement when first turned on at the dive location. The gotha is you have to to rn int on before you enter teh water to read that atmosphere pressure. that has its own problems. once you hit the water it locks it in as a reference to do calculations on. Now since calculations are done in ATA's then the surface pressure diefines what an ATA is. if it is say 7.2 psi then 100 real feet is now 7 atmospheres instead of 4. that may or may not be a problem because it could use the same reference to print the depth with and say that 7 ata is now 100 ft. those fine details i dont know. I do know that once you hit the water the ATA pressure is locked and I am guessing it will stay locked for 24 hours or until you fully deco from your SI's. If I go to say sea level again in that periiod of locked value i hit the beach and the computer thinks I am still at high altitude. Such is the price you pay for auto sensing. When it was a configuration setting I could set it for what ever I wanted and also could (acccidentally) totally forget it and be in the same situation. Its a what is bigger 12 oir a dozen. No matter what people shold be aware of things like this especially when they travel. Imagine some one with a new computer playing with it on a plane at 40 thousand feet shutting it off and then diving with it on auto turn on that happens at 4 feet. It may automatically turn on because of the higher pressure at sea level compared to the pressureized cabin pressure. I dont know. I have seem [posts where people are saying that 2 identical computers are reading different depths because of a setting like 100 ft vs 94 ft. to go way out on alimb here what happens when you are in a diving bell and the bell has say 10 atas of pressure in it??? That situation would make setting a real issue unless surface is now going to be 300 ft and being saturated may end the issue all together since it will be a chamber deco when you surface. They may have their own dspecial firmware for bell diving if needed at all.

 

[boulderjohn]

The decompression algorithm used for your dive changes with altitude. It is important to understand why.

Altitude training in traditional OW classes was based on tables, which cannot adjust in themselves. That means that the diver had to make adjustments for altitude, and there were two such adjustments.

1. Adjust for tissue loading prior to the dive because of increasing altitude by pretending you have already done a dive. This was usually a pretty negligible adjustment, frankly, because the diver had usually been at the new altitude long enough to make it a non-factor.
2. Adjust depth using a theoretical depth table. If you were diving to 80 feet at the Blue Hole in New Mexico, the theoretical depth is 96 feet. You would round that to 100 feet and use 100 feet for your table calculations. That would impact your NDL dive time. At sea level at that depth, your maximum time would be 30 minutes; at that altitude it would be 20 minutes.

Computers set to altitude, whether automatically or manually as in this case, will make the same sort of adjustment described in #2 above for you. They can't do #1 above, but, as I said, that is usually not a significant factor at reasonable altitudes. (It is a huge factor in much higher altitudes.)

The reason for the change is the lesser pressure on the surface. As a diver ascends, the diver must make sure the pressure of gases in the body is not too much greater than the surrounding ambient pressure. Decompression algorithms are designed to make sure the ascent allows enough off-gassing for this to be safe. When you are diving, the overwhelming factor in ambient pressure and thus the degree to which you on-gas is the water pressure. If you are diving at at 102 feet of fresh water at sea level, you are under 4 atmospheres of pressure; at the Lake Tahoe, you would be under 3.8 atmospheres of pressure. That is not a lot of difference (95%), so your ongassing would be nearly the same as at sea level. As you ascend, there is still not a whole lot of difference for the first 2/3 of the ascent, but then the air pressure starts to become a bigger portion of ambient pressure. At 34 feet, the sea level ambient pressure is 2 atmospheres; at Lake Tahoe, it is 1.8 atmospheres (90%). At the surface, sea level ambient pressure is 1 atmosphere; at lake Tahoe it is 0.8 Atmospheres (80%).

In summary, when diving at altitude, your body on-gases pretty much the way it does at sea level, and it off-gasses about the same for most of the dive. When you get shallow, things change rapidly, and if you follow a sea level ascent profile, you will be in danger of DCS.

If I were diving at the Blue Hole in New Mexico (about 4,600 feet) with a computer that adjusted for altitude manually, I would set it for the altitude it calls for.

 

[NAM001]

this would be a great time for the manufacturers to chime in on how their products handle this along wht the protocols for when you change altitudes.

 

[NAM001]

The decompression algorithm used for your dive changes with altitude. It is important to understand why.

Altitude training in traditional OW classes was based on tables, which cannot adjust in themselves. That means that the diver had to make adjustments for altitude, and there were two such adjustments.

1. Adjust for tissue loading prior to the dive because of increasing altitude by pretending you have already done a dive. This was usually a pretty negligible adjustment, frankly, because the diver had usually been at the new altitude long enough to make it a non-factor.
2. Adjust depth using a theoretical depth table. If you were diving to 80 feet at the Blue Hole in New Mexico, the theoretical depth is 96 feet. You would round that to 100 feet and use 100 feet for your table calculations. That would impact your NDL dive time. At sea level at that depth, your maximum time would be 30 minutes; at that altitude it would be 20 minutes.

Computers set to altitude, whether automatically or manually as in this case, will make the same sort of adjustment described in #2 above for you. They can't do #1 above, but, as I said, that is usually not a significant factor at reasonable altitudes. (It is a huge factor in much higher altitudes.)

The reason for the change is the lesser pressure on the surface. As a diver ascends, the diver must make sure the pressure of gases in the body is not too much greater than the surrounding ambient pressure. Decompression algorithms are designed to make sure the ascent allows enough off-gassing for this to be safe. When you are diving, the overwhelming factor in ambient pressure than thus the degree to which you on-gas is the water pressure. If you are diving at at 102 feet of fresh water at sea level, you are under 4 atmospheres of pressure; at the Lake Tahoe, you would be under 3.8 atmospheres of pressure. That is not a lot of difference (95%), so your ongassing would be nearly the same as at sea level. As you ascend, there is still not a whole lot of difference for the first 2/3 of the ascent, but then the air pressure starts to become a bigger portion of ambient pressure. At 34 feet, the sea level ambient pressure is 2 atmospheres; at Lake Tahoe, it is 1.8 atmospheres (90%). At the surface, sea level ambient pressure is 1 atmosphere; at lake Tahoe it is 0.8 Atmospheres (80%).

In summary, when diving at altitude, your body on-gases pretty much the way it does at sea level, and it off-gasses about the same for most of the dive. When you get shallow, things change rapidly, and if you follow a sea level ascent profile, you will be in danger of DCS.

If I were diving at the Blue Hole in New Mexico (about 4,600 feet) with a computer that adjusted for altitude manually, I would set it for the altitude it calls for.

wonderful explanation.

 

[dmaziuk]

this would be a great time for the manufacturers to chime in on how their products handle this along wht the protocols for when you change altitudes.

Workman's model was tied to sea-level altitude and atmospheric pressure relative to 1 atm. It couldn't be easily adjusted for altitude: atmospheric pressure changes non-linearly with altitude. One of the key points of Buhlmann's work was diving in Swiss mountain lakes. So his formula works in absolute pressure and is suitable for altitude diving with nothing special to "handle". You just need to feed it the correct atmospheric pressure.

 

[boulderjohn]

Workman's model was tied to sea-level altitude and atmospheric pressure relative to 1 atm. It couldn't be easily adjusted for altitude: atmospheric pressure changes non-linearly with altitude. One of the key points of Buhlmann's work was diving in Swiss mountain lakes. So his formula works in absolute pressure and is suitable for altitude diving with nothing special to "handle". You just need to feed it the correct atmospheric pressure.

Buhlmann adjusted the algorithm after Swiss divers had problems at altitude. His adjustments were used successfully by Swiss divers at Lake Titicaca. I would not say, though, that there has been a whole lot of true research on those adjustments at very high altitude, and I would hesitate to trust them (or any other algorithm) at a very high altitude.

If you plug a 200 foot decompression dive into a Buhlmann software program (like Multi-deco) for an altitude of 16,000 feet, you will get a recommended profile, one that is not a whole lot different from the same dive at, say, 6,000 feet. To my knowledge, there has never been a successful 200 foot dive at 16,000 feet, so how can they be sure? I do know of one attempt (probably not using Buhlmann), but it resulted in one of the divers paralyzed and the other deceased. (They never actually completed the dive as planned.) I discussed that dive with a high altitude decompression expert who consults for the Pentagon (U2 flights) and NASA, and he said he believed there were only 6 people in the world capable of planning such a dive (he being one of them). He said it would take him a long time to calculate it, but his off-the-cuff guess was not remotely close to the Buhlmann profile--it was many times longer.

The US Navy manual's section on diving above 10,000 feet essentially says don't do it unless properly approved, and that section is in bright red, bold print. I have dived above that altitude, but never deep and never long.