Why 15 ft for a safety stop?

[spacemanspiff1974]

I find 20 feet much more manageable for deco stops/safety stops in the ocean. I suppose 10 feet would be fine in a lake.

 

[nereas]

I was just wondering - it seems a little arbitrary. Does that mean that a safety stop at 20 feet is less safe? Or one that begins at 20 ft and ends at 10 ft 3 minutes later won't allow proper off-gasing? Or, what if there is a very convenient place to stop at 25 or 10 feet, but not at 15? Should I try to hover at 15 anyway?

Good question, Murdrcycle! This shows that you are starting to think about what you have been taught so far.

Here are the givens, from the theoretical scientific laws of the physical universe, as least for here on this Earth (the planet Jupiter would have different laws of physics):

Ambient pressure for you while standing on a beach by the seashore is 1 ATA (atmospheres absolute). This equals 14.7 lbs per square inch, in terms of psi, if that helps you to understand what the unit means. Each square inch of your body is weighed down by 14.7 lbs of pressure from the sky above. (It's more on Jupiter!)

Ambient pressure for you while freediving or scuba diving at 33 ft in the ocean or seas is 2 ATAs.

Ambient pressure for you while diving in a fresh water lake near sea level is 2 ATAs at 34 ft depth. Note that these 2 are pretty close although still slightly different.

Thus, the ambient pressure upon you is double at these depths compared with the surface. One-half of these depths is the convention agreed upon by the diving intelligencia as a final safety stop, and this comes to 16.5 to 17 ft. It is published in the diving literature as 15 to 20 ft to make it easier to remember.

This is where the actual number comes from. Therefore you can see that it represents half of the additional pressure upon you from the 2 ATA point.

Haldane theory and Buhlman tables have been formulated from the observation that the 2 ATA point is a critical threshhold related to the development of DCS (decompression sickness/illness/injury). At less than 2 ATAs pressure, DCS does not occur. At pressures greater than 2 ATAs DCS begins to develop given sufficient time (called NDL time).

As a result, a safety stop at a depth of 1/2 this critical threshhold depth is agreed upon as a respite point to allow sufficient off-gassing (controlled decompression) before surfacing from a scuba dive.

I hope some of that made sense. I have attempted to omit the terminology and the discussions about the scientific models of atoms and molecules that most others refer to as if these truly existed and gave interviews/depositions and could be seen, when in reality they are really theoretical models only.

Our surrounding universe is merely a collection of observations and theories only. Science is only a theory, even as God is only a theory (for most people, except of course for the prophets themselves).

 

[PePaw]

Great read !
It's already been pointed out that many dive computers won't start counting down your safety stop time below 17-18 ft. You can really screw up your day if you pi** off the computer !

 

[PfcAJ]

Here are the givens, from the theoretical scientific laws of the physical universe, as least for here on this Earth (the planet Jupiter would have different laws of physics):

The "laws" of physics remain constant, however the conditions from Earth to Jupiter are different.

Interestingly, however, there seems to be some discourse within the scientific community about the true surface pressure of Jupiter, but you are correct that the pressure will be greater on the "surface", whatever exactly that may be.

 

[murdrcycle]

OK - so if DCI doesn't begin to occur until 33 ft or more, then that means that at 33 ft the human body can off-gas at the same rate at which the nitrogen would collect, meaning that no additional nitrogen will develop. Therefore, it can be assumed that at anything less than 33 ft the human body is able to off-gas faster than the nitrogen is building up.

Obviously every human body is different. However, assuming the above, 15 ft. is pretty arbitrary, and in fact any time that you spend at less than 33 ft. allows off-gas time. Therefore, it would seem to me that a stop at 30 ft followed by another at 20 or 15 would make more sense, or even better would be to slow your descent so gradually at the 30 ft mark that it takes 4-5 minutes to surface from 30 ft. That would allow for the most gradual return to normal as possible, and it is safe because as long as you are less than 30 feet you aren't taking on any more nitrogen.

Does that make sense?

 

[Charlie99]

OK - so if DCI doesn't begin to occur until 33 ft or more, then that means that at 33 ft the human body can off-gas at the same rate at which the nitrogen would collect, meaning that no additional nitrogen will develop.

Does that make sense?

It makes sense, but is not at all correct.

A good, non-technical, very readable article that explains basics of decompression is Diving Physics and "Fizzyology"

.

 

[DeepSeaExplorer]

It makes very little difference to your deco. Do what's convenient, what's pretty, whatever. quote]

Absolutely... unless it's mandatory deco, you can do your safety stop where ever you want, within reason of course.

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Other thoughts...

Remember, the primary reason for a safety stop is to compenstate because most divers ascend too quickly. If you don't ascend too quickly then you don't need to insert an artificial shallow stop to compensate. However, most divers now do both ascend slowly and do a safety stop, which adds extra conservatism. It's good practice, but it also makes the mechanics of the SS less important, allowing more freedom and flexibility. As long as you're not in deco, your computer shouldn't care either.

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Other than creating difficulty holding your position, wave action doesn't affect deco. No one ever got bent because a wave passed over while they were doing deco.

 

[Desert Pirate]

Thanks DSE, thats the point in a sence that I was going to make, other than I am a firm believer in a 3-5 min saftey stop anyways. I also believe anywere from 20-10' is fine, but equaly as important is once the saftey stop has been completed.. say at 20'....

we know that the greatest pressure change is from 33' to the surface.

so that bein said if you take the bottom 13' off of that, the last 20' from your saftey stop to the surface is extremely crutial, ascend extremely slowly, this is the highest rate of pressure change you will experience on the entire dive.

of all the saftey stop agruments, theories, and different ways it is taught, I believe this last leg.. is the most overlooked factor that we all experience every time we go on a dive.

DP

 

[scubajb]

What a great post!

 

[nereas]

OK - so if DCI doesn't begin to occur until 33 ft or more, then that means that at 33 ft the human body can off-gas at the same rate at which the nitrogen would collect, meaning that no additional nitrogen will develop. Therefore, it can be assumed that at anything less than 33 ft the human body is able to off-gas faster than the nitrogen is building up.

Obviously every human body is different. However, assuming the above, 15 ft. is pretty arbitrary, and in fact any time that you spend at less than 33 ft. allows off-gas time. Therefore, it would seem to me that a stop at 30 ft followed by another at 20 or 15 would make more sense, or even better would be to slow your descent so gradually at the 30 ft mark that it takes 4-5 minutes to surface from 30 ft. That would allow for the most gradual return to normal as possible, and it is safe because as long as you are less than 30 feet you aren't taking on any more nitrogen.

Does that make sense?

Be careful with your logical analysis. The ancient Greeks taught the world how to do logic, but when push came to shove (battles between heavily armed infantry with shields and spears) the ancient Greeks relied on discernment of animal entrails rather than strictly logic, which is often flimsy at best.

Since DCI doesn't begin to develop until 33 ft or more, we don't know why. And now you are guessing why. Tissue and blood tolerances to elevated levels of nitrogen are my guess as to why, rather than perhaps on-gassing (compression) or off-gassing (decompression) rates for various gasses (nitrogen or helium) in various tissues (muscle, fat, organs, etc) and blood. But your guess is still as good as anybody else's.

Observations are facts, but only to the extent they are correctly percieved. The ideas that follow from them, called variously hypotheses, theories, laws, etc. are merely some person's conclusions. Conclusions are constantly being revised and updated as new facts are observed. History tells us that everyone on the Earth once thought that it was flat and you could sail off the edge and fall into oblivion.

I strongly suggest that you hold your 15 ft safety stop for 3 to 5 minutes. But that is just my opinion. That's what I do, and it has worked really well for me. This is in addition to a slow ascent rate of 1 ft every 2 seconds when you are above 70 ft (which comes from 3 ATAs, or 66 ft rounded), and not in lieu thereof. Both.