Simplify the Surface Interval

This has been bugging me for a long time. - (UPDATE: We have been able to trace this back to 2011 at least.)

Decompression related injuries (DCS) have become a significant factor in advanced freediving and one way of reducing the risk of DCS is to work with well-defined surface intervals.

How to calculate this surface interval is the logical question. We do not have the tables established in the experimentation done for scuba divers and very little hard data to go by. So rules of thumb were created to at least have some idea.

For what feels like decades these two formulae are being quoted up and down the internet with little variation:

  1. For dives shallower than 30m: Surface Interval = dive time * 2
  2. For dives of 30m and deeper: Surface Interval = maximum depth (in metres) / 5

Now, why don't I like these? There are three main factors involved in estimating a risk of DCS:

  • time t
  • pressure p (in bar)
  • speed of ascent Va (in metres per second)

While the speed of ascent is completely ignored in the above formulae, Dive time and pressure (disguised as depth in metres) appear in them, but each formula uses one factor and completely ignores the other!

What makes these formulae more complex and less credible, is that arbitrary numbers are brought into the situation:

  • Why 30 metres and not 40 or 20? Or 23?
  • Why times two and not 3, 5 or 1.5?
  • Why divided by 5 and not 4 or 9?

A Simpler Formula

I suggest a single formula that removes the arbitrarily selected factors out of the above mentioned formulae, and makes use of all the proven important factors of pressure, time and speed of ascent. the suggested formula is

For fast divers: Surface Interval = t * p * Va

Speed of ascent varies somewhat by means of propulsion, Monofin freedivers have recently started a trend of fast dives with speeds over 2m/s, whereas the other disciplines leave us in the comfortable situation that the speed of ascent with bi-fins, no fins and in Free Immersion for most divers is just under 1 metre per second, so for freedivers swimming with bi-fins or without fins, we can work with a further simplified rule of

For most situations: Surface Interval = t * p

When we play with the numbers, we end up with numbers that stay relatively close to the established formulae, which appear to be reasonable, but we get more conservative numbers when a factor changes that had previously been ignored by the old formulae.

This result supports that my suggested one rule, which only works with relevant factors, makes calculations simple, reliable and - most importantly - stands up to reason.

My appeal to the educational agencies and freediving businesses is to replace current mention of the arbitrary and irrational old formulae with this until we have enough data to come up with an evidence-based solution.

A simplification of this formula will make it more likely for freedivers around the world actually use it as it is designed and may well reduce the incidence of DCS in freedivers significantly.

Of course I can by no means guarantee that this formula will keep anyone safe, but it certainly has a better chance than the old formulae have.