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A friend of mine (an old scuba instructor to be honest) told me that a too slow ascent is unsafe because it increases the time you stay at higher depths.

I think I have a good understanding of decompression algorithms (especially VPM and RGBM) and a good mathematical background, so I can roughly say that the risk of decompression sickness is a function of a derivative of pressure. In other words, the slower the better; if the ascent speed where near to zero, the risk would be near to zero too.

Am I wrong? Could my friend's theory be considered true at some point in the history of scuba diving?

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It does lower the risk of decompression sickness, but let's take an extreme example. Let's try for 1 foot every 2 minute. You are changing the dynamics of a typical ascent table, which are usually made for the 30ft/min (1 foot every 2 seconds) ascent rate. You are taking 60 times as long to ascend, so you are spending longer times at deeper depths, which does cause the build-up of more nitrogen, etc. However, as long as you're not going ridiculously slow, I think the difference is negligble in the grand scheme of things (remember, most numbers are worst-case scenarios anyways, as it takes you X time to actually get to your depth, you're not at 60 feet immediately, etc. etc.) Safety static stops (usually at 15 feet) are also recommended.

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    Can you give a mathematical proof (based on the currently accepted models) that the amount of absorbed gas causes more problems than the reduced bubble grown?
    – hey hey
    Jan 28, 2013 at 12:33
  • No, but think about extremes. Any realistic ascent rate would be ok. It's only when you get into the 10 or more times slower that you have to really ponder it
    – Aaron
    Jan 31, 2013 at 4:44
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    I've already thought about extremes: DCS is caused by bubbles; nuclei grow into bubbles when the pressure changes "too fast" (bubble grown is a function of the derivative of the pressure); if the ascent speed approaches 0 ft/s, the bubble grown should approach 0 bubble/s. Hence, the lower the speed, the better.
    – hey hey
    Feb 2, 2013 at 10:29
  • Where exactly is my fault in your opinion?
    – hey hey
    Feb 2, 2013 at 10:30
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    @heyhey you are correct that theoretically slower/less bubble growth as you ascent slower and slower. But the issue here is also about the rate your body absorbs and releases Nitrogen from various body tissues. Blood, Bone, Flesh, etc These all have different rates and which tissue compartments are off gassing and on gassing are influenced by depth, etc. So ascent rate may theoretically play a large role in this. And once at surface the difference between Nitrogen dissolved in various tissues and Nitrogen in surrounding air may cause DCS symptoms to occur.
    – AquaAlex
    Nov 7, 2013 at 11:00
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Lets be practical, for a too slow ascent rate to be dangerous it has to be so slow that you would basically not even be ascending.

A fast ascent rate creates way more problems than a slow ascent. If you ascent too fast you increase chance of DCS & AGE and the faster you ascent the harder it becomes to control your ascent rate.

Do people know where our ascent rates come from? 60feet/min or 18m/min ? It was becuase the US Navy wanted their divers to come up as fast as possible (and still be safe). The commercial divers used to do 10feet/min or 3m/min.

And rates where increased not necessarily because slower was unsafe, but because the sooner someone is out the water the better, so we keep trying to find the fastest way to get people out safely. Not for recreational use, but commercial and military.

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Historical ascent rates: Historical guidelines as to rates of ascent are pertinent. In the 19th century, for example, the French physiologist Paul Bert in 1878 quoted rates of 3 feet per minute and the English physiologist John Scott Haldane in 1907 recommended ascent rates between 5 and 30 feet (1.5 and 9 meters) per minute. From 1920-1957, rates of 25 feet (7.5 meters) per minute were recommended. Then in 1958, Cdr. Francis Douglas Fane of the U.S. Navy West Coast Underwater Demolition Team wanted rates for his frogmen of 100 feet (30 meters) per minute or faster.

Here is interesting article on safety stops and ascend rates. http://www.daneurope.org/c/document_library/get_file?folderId=13501&name=DLFE-133.pdf

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    Divers Alert Network is constantly doing studies on various aspects of diving and ascent/descent rates are one of the topics regularly studied. So search around and see what was found.
    – AquaAlex
    Nov 7, 2013 at 10:56
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Yes slow ascent can be more unsave.

It is proven to accent faster for a bit and make stops will me far more effective to decompress then to just assent slowly all the way.

This is because:

2) Building up less gas at depth.

1) Doing an more optimal decompression.

To put it in an example:

Say you make a deep dive. your tissues take up gas because of the pressure difference between your tissues and the surrounding pressure. ( I'm sure you already knew that ) There are fast and slow tissues. ( onces that take on gas quick and onces that take on gas slow ) Staying longer at depth you build up more gas in the tissues ( Saturate ) So as you ascent you actually still are building up more gas in your (slow) tissues.

While ascending your tissues become over saturated and you start to desaturate ( 'decompress' ) There is a point where the decompression is optimal ( larges pressure difference between your over saturated tissues and surroundings so gas exchange speeds up but not that large to cause dangerous bubbles to apear )

For this reason I do stops every dive ( even a shallow dive on nitrox I make stops of 1 minute ) The more effective you decompress the better you come out the water and the less change you'll have to a DCS. Also stopping every dive trains you to stay at one depth to do those stops more safely.

For more info also check out this wiki about decompression

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    It is proven to accent faster for a bit <--- This is potentially unsafe. What do you consider to be "faster"?
    – AquaAlex
    Dec 11, 2014 at 8:15
  • Depends on the depth you're coming from. But a for most sport divers 9meters per minute is considered the target speed.( And maximum also )
    – FLY
    Dec 15, 2014 at 13:25
  • actually 18m per minute is the accepted standard in recreational diving :) But most dive computers set default of around 12m.
    – AquaAlex
    Jan 9, 2015 at 9:35
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    I know :) in fact if you read the history the US navy went to 30 feet per minute and then to 60 feet per minute (they wanted to go faster but the hard hat divers could not practically go faster so 60 feet was a compromise between scuba and surface supply divers. Then much later US navy went back down to 30 feet per minute. Actually now a days 30 feet/10m is used as the guide line with deep stops and a safety stop at 6m highly recommended. It is even recommended to slow down the ascend rate from safety stop to surface as this is when largest percentage changes happen.
    – AquaAlex
    Jan 13, 2015 at 9:15
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    In fact if you read the research doc from DAN you will see a safety stop around 6m is more effective than slowing ascend rate from 18m to 10m. per minute daneurope.org/c/document_library/… (but research continues and we learn more and more about our bodies and on/off gassing) In technical diving we definitely aim more for 10m/min as we are already over limit but then we also do multiple decompression stops and deep stops as we ascend
    – AquaAlex
    Jan 13, 2015 at 9:15

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