Ascending faster than 60ft/minute

[uglyredshoes]

Thanks Charlie99 for the insightful read.

Thalassamania, yup i should clarify myself. I dive my computer because i plan all my dives on a basis of gradual multi level diving. Thus i usually dont see the need to make a conscious effort to do that mid level stop unless the comp informs me too. It has happened only twice when i reached depts of 130-143ft.

But either way, it is still a very safe suggestion!

 

[TheRedHead]

Slowing ascent rate, the closer to the surface you get, is because of Boyle's Law and its affect on the nitrogen bubbles, correct?

Yeah, if you have bubble it will expand upon ascent and you want to keep it small until it is expired. There are several deco models which involve tissue compartments, perfusion, diffusion and a bunch of other factors which have gone into determining the rate of ascent.

Deep stops are the way to go. I do multi-level diving mostly so those stops are already incoporated into the plan. You'll notice in Cozumel that you are doing multi-level and that the DM winds his reel up very slowly so you spend a lot of time offgassing at various depths. Those guys understand deeps stops and are very careful not to get bent.

I just ascend slowly and make little stops along the way. You can just hover a minute or two at 15 feet, 10 feet, 5 feet and that way you will slow down your ascent and get rid of the dissolved nitrogen.

The guy who got bent made is 3 minute stop at 15 feet and just rushed to the surface after every dive. He was also diving air on all of his dives.

 

[Fish_Whisperer]

Yeah, if you have bubble it will expand upon ascent and you want to keep it small until it is expired. There are several deco models which involve tissue compartments, perfusion, diffusion and a bunch of other factors which have gone into determining the rate of ascent.

Deep stops are the way to go. I do multi-level diving mostly so those stops are already incoporated into the plan. You'll notice in Cozumel that you are doing multi-level and that the DM winds his reel up very slowly so you spend a lot of time offgassing at various depths. Those guys understand deeps stops and are very careful not to get bent.

Yep. It was a real pleasure diving with those guys.

I just ascend slowly and make little stops along the way. You can just hover a minute or two at 15 feet, 10 feet, 5 feet and that way you will slow down your ascent and get rid of the dissolved nitrogen.

The guy who got bent made is 3 minute stop at 15 feet and just rushed to the surface after every dive. He was also diving air on all of his dives.

Oh boy... We had a couple of new divers with us who kept bopping up to the surface. The DM told them a number of times, both on the boat and in the water, to get down to depth and stay there. They weren't listening. Poor DM was sooo frustrated.

 

[Thalassamania]

My pleasure, it's a very complex topic and sometime counterintuitive, especially as concerns the behavior of gas once it changes phase. Bottom line: 3 minutes at 1/1 you max depth. To paraphrase that old SNL line, "my spinal cord has been berry, berry good to me!"

 

[FyshEye]

Gas embolism in diving
Gas embolism is one of the diving disorders SCUBA divers sometimes suffer when they receive pressure damage to their lungs following a rapid ascent. It is named "gas" because the diver may be using a diving breathing gas other than air. The gas bubbles can impede the flow of oxygen-rich blood to the brain and vital organs. They can also cause clots to form in blood vessels.

I know that when I taught my OW students that the rule was Don't Ascend Faster than 60ft/18m per minute. Slower is always better. I used to tell all my students that the last 33ft. is where the Dysbarism changes the greatest and will do the most damage.


~Cheers~

 

[Charlie99]

Slowing ascent rate, the closer to the surface you get, is because of Boyle's Law and its affect on the nitrogen bubbles, correct?

Another way to look at is that in the classic multicompartment dissolved gas model, you have both fast, medium and slow compartments. The purpose of the deeper stops is to offgas the fastest compartments, while not further loading up the slow compartments more than needed. Since you are offgassing the fastest compartments at the deepest stops, they don't have to be long. As you ascend and either do stops or a slower and slower ascent, you progressively are working on offgassing slower and slower compartments/tissues. So you need to get slower as you ascend.

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The faster compartments have significantly higher overpressure limits allowed. The slowest compartments have the lowest allowable dissolved gas supersaturation limits.

At the end of a typical recreational single tank dive, the fastest compartments may be saturated, but still within the limit for surfacing. The medum halftime compartments are nearing their limits. The slow compartments have much lower gas loads, but since their limits are much lower, the are loaded to a significant fraction of their limit.

Popping directly with a fast ascent to the surface will drive the medium compartments right up to their limits. (And keep in mind that it is not a hard black and white limit, but instead a fuzzy "how much are we going to bubble today" sort of limit).

Doing an excessively slow ascent is just the same as doing a long multilevel dive, and the medium to slower tissues will continue to ongass to beyond their limits.

An important concept is that the allowable limit for each compartment is a function of depth. The deeper you are, the higher the limit at that depth.

Heading up towards 40' lets you get to a point where the fastest compartments will start offgassing, but are still well away from their limits for that depth. Meanwhile, you have signficantly slowed the ongassing in the slowest compartments. Depending upon the dive, the medium speed compartments could be either ongassing or offgassing.

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After just a short while at 40', the fastest compartments will have significantly offgassed, and you can now go shallower. Going to some intermediate depth, such as as 20' repeats the situation as above, but you are now offgassing the medium compartments, while the pressure from being at 20' increases the compartment limit higher than what it would be at the surface. Staying well away from the compartment limit minimizes bubbling. Since at this shallower depth, you are now trying to offgas the medium speed compartments, you need to spend longer time than you did for the deepest stops, where you are offgassing the fastest compartments.

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I'll repeat this post with some real numbers when I get a chance.

 

[Thalassamania]

I know that when I taught my OW students that the rule was Don't Ascend Faster than 60ft/18m per minute. Slower is always better. I used to tell all my students that the last 33ft. is where the Dysbarism changes the greatest and will do the most damage. ~Cheers~

No! Ascent rate has nothing to do with Arterial Gas Embolism (AGE) if you hold your breath and ascend, regardless of speed, you will pop a lung eventualy. Rocketing to the surface makes an AGE more likely due to the surrounding situation, but is NOT THE CAUSE.

 

[Fish_Whisperer]

Nice explanation, Charlie. The whole behavior of nitrogen, air, DCS, nitrox, and trimix, and how they behave at depth is just fascinating to me.

 

[engdiver]

I was always taught to never ascend faster then your slowest bubbles and the max was 60 ft/min. Not using a computer I have no idea what this ascent rate is but I know it is less then 60 ft/min and seems to change with depth. Has anyone else followed this or tried to measure this ascent rate?

 

[FyshEye]

Bubbles move faster as they get closer to the surface and expand.