Altitude diving with junior OW diver

[scubajb]

Hey all,

have a question about diving at altitude with my son, junior OW. Understand the 40 foot rule and generally why. I have taken the AOW section on Altitude diving and when becoming certified up here, well, altitude is an added discussion during the course. Quite a few of our dives are at high altitude (8000 to 9000 feet).

OK, it's easy to adjust the actual depth and safety stop to the theoretical depths taking into altitude tables. However, what I do before the dive in the planning stage is to look at the altitude, the cold water, and the 40 foot rule. I always plan on making sure my son doesn't drop below the theoretical depth of 40 feet or 30 feet if in cold and difficult conditions.

i.e.
9000 foot elevation
40 foot dive
65 degree water

Adjust planned depth from 40 feet to 30 feet because of water temp, I conservatively adjust the 40 foot limit to a 30 foot limit due to the conditions. At 9000 feet elevation, table shows that an actual depth of 20 feet adjusted to altitude is theoretical depth of 28 feet. This is as close as I can get to the 30 foot limit (as set because of the cold water and the 40 foot rule).

So, in planning stage, after taking into account the altitude and cold, I won't let my son drop below an actual depth of 20 feet. He is involved with this planning process so he understands what we are planning for and why (doesn't like being limited to 20 feet though... ) Is this correct?

Thanks everyone!
Jim

 

[H2Andy]

i think it's great that you are putting this much thought into your son's diving, but realistically speaking, a dive to 40 feet and a dive to 20 feet are not that much different. if i was worried, i'd deal with this by reducing time at depth. that's going to make a larger difference than limiting him to 20 feet.

also, reducing depth by 10 feet for cold is not really going to make any difference at all. again, i would probably address that by reducing time of dive, not depth

my take on this would be to plan your dive as normal to 40 feet (considering altitude) then dial back the total dive time for safety. for example, if your dives are normally 30 minutes to 45 minutes long, i'd dial them back to no more than 30 minutes, and so on. or, if you know that your son starts getting cold after 20 minutes, then i'd dial back to 15 minutes in this case

again, i'd focus on time of dive, not depth, as the safety control here, since you are already limited to 40 feet anyway

(this is not to say that limiting depth is not a good safety option. it is, particularly at deeper depths. it's just that in this case, diving at 20 feet rather than at 40 feet isn't going to do much for you, safety-wise, so think time)

 

[shurite7]

As stated above I would reduce time at depth rather than reduce depth. Also control (acutally monitor it) your ascent and, add some time to the safety stop (5 min).

Based on the water temp you posted it appears you are diving in the summer.

Chris

 

[scubajb]

Yes, this is assuming diving during the summer. EVerything is locked up this time of year. Can't afford to keep him drysuits... wetsuits are expensive enough with how he's growing! As far as controlling time instead of depth, is there a standard that should be applied (i.e. depth standard of 10 feet)? We dive with tables although we use computers as well. Use the computers to monitor loading as well as a lot of other stuff (especially ascent rates). However, computer doesn't take into account Junior OW considerations. He would be really excited to adjust time versus depth. Just want to make sure I calculate it safely. My assumption here is that because it is still 40 feet adjusted to altitude (worst case at 9000 feet is theoretical depth of 56 feet), the main concern isn't pressure on the young diver but nitrogen uptake?

 

[H2Andy]

As far as controlling time instead of depth, is there a standard that should be applied (i.e. depth standard of 10 feet)? We dive with tables although we use computers as well. Use the computers to monitor loading as well as a lot of other stuff (especially ascent rates). However, computer doesn't take into account Junior OW considerations.

well, you know at 40 feet you have about 200 minutes dive time (depending on which tables you use), which is just not practicable, so pretty much anything under that will be "safe" NDL wise

the issue really is adding in additional "safety" time controls, which are mostly rule of thumb and since they are well below the max NDL, very conservative

basically keep dives short enough to keep cold from being an issue, get your ascent rates really slow (time again), do nice and log surface intervals (time again) and the rest should take care of itself at this stage

 

[scubajb]

Very good points and I agree with what you are saying. Diving conservatively and not pushing the limits with him is great advice and he is truly an exceptional diver - mentally and physically. Now I've been accused of being overly analytical, (no... seriously... ), but in here lies my real question I guess. Is it not necessary to adjust the dive plan at 40 feet for altitude considerations for a junior diver considering not pushing NDL, slow ascent, longer safety stop (basically diving conservatively)? If I just keep him underneath the NDL, really isn't that the same as applying the same rules as an adult diver to junior diver? I thought there were pressure concerns for a junior diver as well as nitrogen concerns. Is the pressure difference caused by diving at altitude at 40 feet not enough to worry about? I know, I'm probably being stupid here but I kind of need to be to make sure I understand with my kid and all...

 

[H2Andy]

yup, you're right. there are pressure concerns for young divers, which is why the 40 foot floor (or any other shallow requirement, depending on who you listen to) exists for them. the shallow depth limit addresses those concerns

it's just that the 40-foot depth requirement isn't affected all that much from diving at altitude, so i would keep the 40 foot depth but dial back the time spent at depth. or, if you must, bring the depth up to 30 feet for peace of mind, but don't worry about taking extra depth off for cold ... 10 feet less won't make any difference in that regard. dialing back on time would work better

you're not being stupid at all; you're actually doing this thing called ... dang .. what is it called... crap ... er.... oh, thinking!

 

[scubajb]

Thanks a for the answers and advice... I appreciate it!

 

[MauiScubaSteve]

I see this from a different perspective. At 9,000 ft elevation the atmospheric pressure is ~73% of the atmospheric pressure at sea level (~10.7 vs 14.7 psi). Fresh water is not quite as heavy as sea water (1.000 vs ~1.025 g/ml). Therefore, the pressure of a 40 ft deep dive in fresh water at 9,000 ft elevation would be less water pressure and less air pressure compared to ocean diving. If pressure on the developemental body is the only reason we limit jr divers from going below 40fsw, that same pressure would be found at a depth deeper than 40 ft in fresh water at 9,000 ft elevation (or any elevation above sea level).

The special procedures for altitude diving are taken due to the fact that returning to the surface is not returning to sea level atmospheric pressure, but to a lesser atmospheric pressure. The Nitrogen bubbles can/do expand to a larger size much quicker in the final feet of ascent, so we advise ascent no quicker than 30 ft/sec and 5 min SS at 10 ft deep(?).

According to this page from a Utah dive shop, they would work the tables as a 60 ft dive. If the temp is what you would consider cold the general rule is to add 10 ft to the depth, so work it as a 70 ft dive, giving 40 minutes max bottom time on a square profile dive (vs 140 min at sea level). Working it this way also puts you in pressure group T at the start of your SI, vs M at sea level, so you have the added conservatism of a longer required SI. 40 minutes in cold water without a DS is a long enough dive for me and a nice long SI sounds good too!

If I have messed this up, I'll blame my dad for taking me to ~100 ft at age 8

 

[Charlie99]

I see this from a different perspective. At 9,000 ft elevation the atmospheric pressure is ~73% of the atmospheric pressure at sea level (~10.7 vs 14.7 psi). Fresh water is not quite as heavy as sea water (1.000 vs ~1.025 g/ml). Therefore, the pressure of a 40 ft deep dive in fresh water at 9,000 ft elevation would be less water pressure and less air pressure compared to ocean diving. If pressure on the developemental body is the only reason we limit jr divers from going below 40fsw, that same pressure would be found at a depth deeper than 40 ft in fresh water at 9,000 ft elevation (or any elevation above sea level).

The problem in diving, either for younsters or adults isn't the absolute pressure. The problem is more that of bubbles than the absolute pressure. Going to depth isn't a problem, coming back up is.

To the first approximation, both dissolved gas models (Haldane, et. al) and dual phase or bubble models (RGBM, VPM, etc.) respond to the RATIO of bottom pressure to surfacing pressure. All the altitude adjustments do is to recalculate the tables, using multiples of the local atmospheric pressure rather than sea level pressure. In other words, the table limits for 99' or 3atm or 4ata at sea level will be the same for whatever depth at altitude corresponds to 3 local atmospheres of depth or 4 local atmospheres absolute pressure.

In other words, if the air pressure at 9,000' is 0.73ata, then the equivalent depth correction is to multiple the actual measured depth (in reality, pressure if you are using an electronic depth gauge or computer) by the factor 1.00/0.73 or a factor of about 1.4. Diving to 40' depth at 9,000' altitude is roughly equivalent to diving to 1.4 x 40' = 55' at sea level.

Charlie Allen