Beyond 130 feet: always a deco dive?

[boulderjohn]

And I wonder what it meant that a dive was "very unsuitable". how does that differ from an "unsuitable" dive?

Suitable was for scuba (self-contained breathing apparatus carried on the back) as opposed to surface supplied air. Unsuitable ones were dives in which it would be better to have surface supplied air, but if circumstances demanded it, scuba was acceptable. Very unsuitable meant you really needed surface supplied air.

 

[windapp]

I have actually been doing a lot of thinking about the reason for some of the depth limits, and have come up with the following:

12m (40 ft) ... Limit for children(10+) and first time divers. This limit is established because the risks of DCS due to rapid accent, or misapplication of tables in the case of children is fairly minimal.
18m (60ft) ... Narcosis is non-existant, and a skilled diver could make it to the surface in the event of a freeflow. Also, the really fast tissue groups saturate before reaching the NDL at this depth, so emergency decompression stops are minimal. This is my limit for diving with strangers.
30m (100 ft) ...Narcosis may be present. A reliable redundant air source is required. In cold water, a redundant first state regulator is a must. Gas planning needs to be more conservative. A reasonable length dive may be done at this depth.
40m (130 ft) ... Narcosis may be pronounced. Serious consderation must be given to having an independant redundant air source (pony bottle). Buddies at this depth provide a redundant brain in case of narcosis, and must therefore be extremely reliable. Dives at this depth are very short. Any OOA emergency ascent will probably not end well.
Beyond 40m ... Narcosis can seriously impair safe diving. A miniscule amount of time is available for no decompression diving. Fast tissue groups continue to accumulate nitrogen even through the beging of the ascent. An OOA emeregency ascent is not an option from this depth.

 

[Gilldiver]

I have actually been doing a lot of thinking about the reason for some of the depth limits, and have come up with the following:

12m (40 ft) ... Limit for children(10+) and first time divers. This limit is established because the risks of DCS due to rapid accent, or misapplication of tables in the case of children is fairly minimal.
18m (60ft) ... Narcosis is non-existant, and a skilled diver could make it to the surface in the event of a freeflow. Also, the really fast tissue groups saturate before reaching the NDL at this depth, so emergency decompression stops are minimal. This is my limit for diving with strangers.
30m (100 ft) ...Narcosis may be present. A reliable redundant air source is required. In cold water, a redundant first state regulator is a must. Gas planning needs to be more conservative. A reasonable length dive may be done at this depth.
40m (130 ft) ... Narcosis may be pronounced. Serious consderation must be given to having an independant redundant air source (pony bottle). Buddies at this depth provide a redundant brain in case of narcosis, and must therefore be extremely reliable. Dives at this depth are very short. Any OOA emergency ascent will probably not end well.
Beyond 40m ... Narcosis can seriously impair safe diving. A miniscule amount of time is available for no decompression diving. Fast tissue groups continue to accumulate nitrogen even through the beging of the ascent. An OOA emeregency ascent is not an option from this depth.

You are thinking too much, by the way saying that "12m (40 ft) ... Limit for children(10+) and first time divers. This limit is established because the risks of DCS due to rapid accent, or misapplication of tables in the case of children is fairly minimal." is dead wrong as the last 30 feet has a 100% gas expansion (30 feet vs. surface) and the risk of a diving related over expansion and embolism is the greatest. But I will grant you that it would take a lot of tanks to get into DCS problems at 30 feet.0

 

[Moonglow]

The answer to the thread title is, no.

 

[windapp]

You are thinking too much, by the way saying that "12m (40 ft) ... Limit for children(10+) and first time divers. This limit is established because the risks of DCS due to rapid accent, or misapplication of tables in the case of children is fairly minimal." is dead wrong as the last 30 feet has a 100% gas expansion (30 feet vs. surface) and the risk of a diving related over expansion and embolism is the greatest. But I will grant you that it would take a lot of tanks to get into DCS problems at 30 feet.0

I don't know where this idea of percentage change in pressure has anything to do with anything comes from. If you hold your breath, and ascend from 60 ft to 30 ft or 30 ft to 0 feet, the air in your lungs still is 15 psi greater than on the ambient. It's not like your lungs are going to double in volume. You do have a rib cage, and when your lungs squish into it, it's the pressure difference that is going to cause the damage.

 

[ianr33]

It's not like your lungs are going to double in volume.

But the alveoli will,and then they rupture, that gives you an Arterial Gas Embolism which is quite likely to kill you.

 

[windapp]

But the alveoli will,and then they rupture, that gives you an Arterial Gas Embolism which is quite likely to kill you.

So, if I hold my breath from 80 ft to 40 ft, I won't die?

I am saying that the shallower depth minimizes the risk of DCS, not lung overexpansion. You can get a lung overexpansion injury in a swimming pool. Also, I used the word MINIMIZE, and not ELIMINATE. I still stand by my thought that 12m is a good depth limit for minimizing the effects diver error, and minimizing the possiblity of DCS.

 

[Dive Bug Bit Me]

So, if I hold my breath from 80 ft to 40 ft, I won't die?

Probably still dead. Holding your breath from 330 ft to 297 ft you maybe have a better shot at not rupturing your lungs. Still a real bad idea for other reasons though.

The ambient pressure at the surface is 1ATA. The ambient pressure at 33ft is 2ATA. The ambient pressure at 330 ft is 11 ATA while at 297t it is 10ATA.

Moving from 33ft to the surface reduces your ambient pressure from 2ATA to 1 ATA. Basically a 50% reduction. If you do the math the volume of gas in your lungs doubles.

Moving from 330ft to 297ft (the same distance) reduces pressure from 11ATA to 10ATA. Basically a 9% reduction. If you do the math the volume of gas in your lungs increases by 10%

I am saying that the shallower depth minimizes the risk of DCS, not lung overexpansion. You can get a lung overexpansion injury in a swimming pool. Also, I used the word MINIMIZE, and not ELIMINATE. I still stand by my thought that 12m is a good depth limit for minimizing the effects diver error, and minimizing the possiblity of DCS.

It takes a serious amount of diving to get DCS at 12m. I have seen some US Navy tables that saturate at slightly shallower depths before you are able to accumulate enough nitrogen to get DCS. Put another way, if you dive shallow enough, you can't get DCS. Please don't try this at home.

Oh, and for the OP I have done 145 ft bounces without putting myself in deco. I chose to do some serious safety stops of my own accord. Please don't try this at home.

 

[windapp]

Probably still dead. Holding your breath from 330 ft to 297 ft you maybe have a better shot at not rupturing your lungs. Still a real bad idea for other reasons though.

The ambient pressure at the surface is 1ATA. The ambient pressure at 33ft is 2ATA. The ambient pressure at 330 ft is 11 ATA while at 297t it is 10ATA.

Moving from 33ft to the surface reduces your ambient pressure from 2ATA to 1 ATA. Basically a 50% reduction. If you do the math the volume of gas in your lungs doubles.

Moving from 330ft to 297ft (the same distance) reduces pressure from 11ATA to 10ATA. Basically a 9% reduction. If you do the math the volume of gas in your lungs increases by 10%

This all assumes that your lungs have enough room to actually double in volume, which they don't. It becomes a matter of pressure difference, and not how much expansion.

 

[Dive Bug Bit Me]

This all assumes that your lungs have enough room to actually double in volume, which they don't. It becomes a matter of pressure difference, and not how much expansion.

Technically if your lungs are less than half full when you ascend you might be ok. If they are more than half full you pop like an over inflated balloon.