Veo 100 and ascent to altitude question

[raftingtigger]

I'm hoping an Oceanic rep can answer this question. First let me set the scenario.

1. Computers (a Veo 100 and a DataMax Sport) are with the diver at all times.
2. Day starts at sea level
3. Drive to the dive site takes 1 hour and peaks at 6000'.
4. Dive site is at 6000'

My question is does either of the listed computers take into consideration the ascent from SL to 6000'?

Using the information taught to PADI altitude divers and the RDP: using 2 pressure groups per 1000', the diver arrives at the dive site in pressure group L. 5 hours and 10 minutes later the diver exits the table as now in equilibrium. Does either of the above computers do the same?

Sample problems:

#1: The diver is all dressed and ready to hit the water on arrival and executes a square profile dive to a theoretical depth of 50' (actual depth of ~41 ffw). According to the RDP they have residual nitrogen time (RNT) of 38 minutes with an allowable NDL of 42 minutes

#2 The diver takes one hour to get ready and executes the same square profile dive. According to the RDP their pressure group is now C, and they have a RNT of 17 minutes and an allowable NDL of 63 minutes

#3 The diver is in no hurry and waits 6 hours from arrival at altitude to begin diving. At this point they have equilibrated. Their RNT is zero, and their allowable NDL for 50' theoretical depth is back to the full 80 minutes.

There is a significant difference in each of these scenarios. So which does the Veo 100 and/or the DataMax Sport follow? Does it matter if the unit is turned on at SL, on arrival at altitude, or just prior to water entry?

I have done an informal experiment where I activated the Veo 100 at SL and watched it through the drive. First hour was from SL to 1000', next 45 minutes was from 1000' to 6000', the last 15 minutes from 6000' to approx 5200'. The NDLs displayed on the Veo changed as the elevation increased but once I passed 6000' they did not change over the next 24 hours at altitude. So it seems the Veo is assuming the diver will do #3 above. Granted that there is off gassing during the drive up that is unaccounted for in any of my scenarios.

Thanks

 

[scubamiller]

Raftingtigger,

It doesn't look like you ever got an answer (on here); but I know the answer.

I too own a Veo 100 (and a Veo 1.0) that I use for diving everywhere from sea level to about 8,000'.

The situation you described has nothing to do with off-gassing, the NDL for the dive you are "planning" decreases due to the mean atmospheric pressure at altitude, not nitrogen loading while you were at a lower altitude (higher pressure).

This phenomenon occurs since ~34 ffw still equals 1 atmosphere (gauge) regardless of altitude; at sea level that is 2 atmospheres Absolute. The problem arises when you surface since you surface into an environment with less that 1 atmosphere absolute (mean sea level is ~1 atmosphere Absolute). So th CHANGE in pressure is greater at altitude as soon as you break the surface of the water in a "lower pressure" environment.

Example: at 18,000 feet above sea level (yeah, not much diving up there and most computers shut down at around 14,000') the atmospheric pressure is about 1/2 that at sea level or 1/2 Atmosphere Absolute, but 34 ffw still equals ~1 atmosphere (gauge), when you break the surface after being underwater you are instantly in a lower pressure and more dangerous environment for microbubbles and DCS/DCI.

I hope this makes sense the way I typed it.
Michael

 

[raftingtigger]

Scubamiller, what you say is true to a point, but it doesn't address my original question (of which I have experimentally answered). The drive from sea level to altitude IS offgassing. It is in effect a prior dive and therefore should affect the NDL at altitude until your body has again equillibrated. This is accounted for by adding 2 pressure groups for every 1000' of altitude (PADI altitude diving). Arrival at altitude starts the surface interval.

If the computer took this into account I would expect the NDLs shown on the computer to decrease as altitude is gained (which it does), and then increase slightly (but not to SL values) as nitrogen is off-gassed during the pre-dive surface interval (it does not).

This phenomenon occurs since ~34 ffw still equals 1 atmosphere (gauge) regardless of altitude; at sea level that is 2 atmospheres Absolute. The problem arises when you surface since you surface into an environment with less that 1 atmosphere absolute (mean sea level is ~1 atmosphere Absolute). So th CHANGE in pressure is greater at altitude as soon as you break the surface of the water in a "lower pressure" environment.

Example: at 18,000 feet above sea level (yeah, not much diving up there and most computers shut down at around 14,000') the atmospheric pressure is about 1/2 that at sea level or 1/2 Atmosphere Absolute, but 34 ffw still equals ~1 atmosphere (gauge), when you break the surface after being underwater you are instantly in a lower pressure and more dangerous environment for microbubbles and DCS/DCI.

I disagree with the statement above. At sea level with a depth of 34 ffw you are at 2 ATA absolute. If you make this dive at 18,000 above sea level and decend to 34 ffw you are at 1.5 ATA absolute. The pressure change from 1" below the surface to 1" above the surface, while rapid, is the same regardless of the altitude. It is still equal to 1" of water pressure. So from on off-gassing standpoint it shouldn't make a difference (pressure gradient is the same). However, from a physiological point of view surfacing at 18,000' takes you from a near normal partial pressure of oxygen to a half-normal partial pressure of oxygen. An experience that is likely to take your breath away.​

 

[scubamiller]

I am specifically referring to alveolar partial pressure when I say that the pressure gradient is much more extreme when you surface at altitude and take the regulator out of your mouth. when you do this at sea level you experience almost no pressure gradient inside your lungs and at the cellular (red blood cell/hemoglobin) level. So when you say that it "takes your breathe away"...this is the WHY behind that...and I totally agree. However, you will still NOT return all the way to the same NDL time while diving at altitude as diving at sea level due to the alveolar pressure gradient (once you take your regulator out of your mouth).
I fly in un-pressurized aircraft (plus hypobaric chamber, and hyperbaric chamber) all the time, and the effect is the same...we are just fine as long as we are breathing through a air tight, sealed, and pressurized oxygen mask; when you remove your mask at 24,000', the alveolar pressure gradient causes all sorts of problems (not to mention the partial pressure is ~1/3 that at sea level). This decompression is the same decompression that we all experience diving, the regulator in your mouth maintains alveolar pressure based on the pressure of the water at depth (one of the reasons that you blow bubbles when you take your reg out to buddy breath, this creates back pressure and reduces the chance of glottic spasms and alveolar collapse).

 

[Agility]

I own a Veo 2.0 and it does not calculate the ascent to the dive site.
As far as I know no diving computer does this. This seems to be a limitation of the used alorithms, here brain 2.0 has to kick in.
My Veo does not reduce the recommended ascent rate either and it does not to my knowledge move the deep stop deeper down.
In Austria at least one altitude dive site requires you stay overnight before diving. It might be a business model but the lake is private property so they make the rules and in this case it seems sensible.

 

[scubamiller]

That is just consevative business practice....and probably a good idea; there are dive site all around the gulf of mexico/bahamas that restrict you from leaving the islands via airplane for the full 24 hours (they hold your passport to be sure) after diving too...good business practice is not getting a bad reputation about an unusually high occurence of decompression sickness at your dive location....scares people from coming/coming back!

 

[raftingtigger]

Agility got my question quite well, and has come to the same conclusion I came to nearly a year ago.

Scubamiller, I'm not sure you fully understand the physics and physiology involved. There are several statements that are highly questionable. Run some of your thoughts by TSandM and Dr. Deco here on the board. They can probably explain things more precisely than I can.

I am specifically referring to alveolar partial pressure when I say that the pressure gradient is much more extreme when you surface at altitude and take the regulator out of your mouth.​

​

The partial pressure (pp) of oxygen (o2) at altitude is lower than at sea level, but that doesn't mean the pressure gradient across the water/air interface is any different. Also a scuba regulator is not a positive pressure delivery system, so whether or not it is in your mouth makes no difference.

One foot below the water surface at 6000' MSL (mean sea level) is less absolute pressure (and therefore has a lower pp of o2) than being one foot above the water at 0 MSL (ATA is 1). Yes you surface at 6000' MSL to a lower atmospheric pressure (than sea level) but the change isn't as abrupt as you suggest. Also the body compensates quite well to lowered pp o2 levels until about 8000'. http://www.flightstat.nonin.com/documents/Hypoxia, Oxygen and Pulse Oximetry.pdf

To use numbers (18,000 MSL is convenient since it is about 0.5 ATA and air is compressible so the change is not linear like it is in water):

	Sea Level dive	18,000 MSL dive
absolute ATA at surface	1.0	0.5
absolute ATA at depth of 1'	1/34 + 1.0 = 1.03	1/34 + 0.5 = .53
change in ATA from 1' deep to surface	0.03	0.03

Alveolar pp mirrors the atmospheric pp, but may be slightly different due to dead spaces. Alveolar pp also changes during the respiratory cycle as oxygen and carbon dioxide are exchanged. The amount of o2 carried in the blood isn't so simple. This depends on the effectiveness of the alveolar gas exchange and the hemoglobin content of the blood among other things.

http://www.flightstat.nonin.com/documents/Hypoxia, Oxygen and Pulse Oximetry.pdf

you will still NOT return all the way to the same NDL time while diving at altitude as diving at sea level

Agreed, read my original post, I say that the NDLs should lengthen with a period at altitude but will stabilize at less than sea level NDLs. This is a flaw in the computer algorithms, and one that is making the dive less conservative.

...we are just fine as long as we are breathing through a air tight, sealed, and pressurized oxygen mask; when you remove your mask at 24,000', the alveolar pressure gradient causes all sorts of problems (not to mention the partial pressure is ~1/3 that at sea level). This decompression is the same decompression that we all experience diving, the regulator in your mouth maintains alveolar pressure based on the pressure of the water at depth (one of the reasons that you blow bubbles when you take your reg out to buddy breath, this creates back pressure and reduces the chance of glottic spasms and alveolar collapse).

Not sure where to start with this one. First, the tight fitting mask you are using at altitude IS NOT pressurizing your lungs. It IS increasing the pp of o2 available. Taking the mask off is not like diving decompression. Second, the reason why you are taught to blow bubbles when your regulator is out of your mouth is to train you to never hold your breath underwater. If you hold your breath and ascend you then pressurize your lungs (something they don't tolerate well) and risk serious injury or death. Positive pressure breathing is used in medicine, but at very low pressures, with lots of monitoring, and carries significant risk.