OFF-GASING & MINIMUM SURFACE INTERVAL

As a consequence of Henry’s Law, while diving our body starts to accumulate nitrogen in the tissues due to the higher environmental pressure of the water.

While ascending, the body begins the desaturation process, releasing excess nitrogen so far dissolved in the tissues (so called “off-gasing”).

This process is not immediately completed when we reach the surface: according to DAN (Divers Alert Network) the off-gasing can be considered complete after a period of 12 to 24+ hours, in conformity with the type of diving activity (single recreational dive, multi-day / repetitive dives or demanding deco dives).

Nevertheless, in the diving industry it is a common and popular use to opt for repetitive dives: all of us have been diving at least two times per day during holiday, if not three or more, without particular problems. In fact, this is a safe practice if we respect the recommended surface intervals between the dives.

We can determine the minimum surface interval between two repetitive dives by calculating the Pressure Group and Residual Nitrogen Time with the help of more or less conservative dive tables (e.g. US NAVY) that generally consider surface intervals from 10 minutes to 12 hours.

But what is the recommendable minimum duration of a surface interval, and why?

Among other topics, a DAN research from the year 2017* (see fig. 1) has investigated, in a significant amount of volunteers, the trend of bubbles formation after surfacing, localizing a peak between 30 and 45 minutes after the dive. In other words, when surfacing even from a very simple recreational dive, we reach the maximum amount of bubbles in our body tissues between 30 and 45 minutes after the dive.

Fig. 1

 Thus, diving in this time interval is highly discouraged at least for two reasons:

• Synthoms of potential DCS usually are visible more likely 40 minutes after the dive, and we do not want to be in the water when they appear.

• Potential bubbles in our body tissues, during the second dive, are subject to uncontrolled recompression and this could lead to higher uncontrolled mobility within the body, thus resulting in serious pathologies. The more bubbles we have in the body, the higher is the risk of incurring in serious pathologies!

Considering the above, it is highly recommendable to wait at least 1 hour before a consecutive dive: in fact, only after one hour the amount of bubbles starts decreasing regularly. In any case only after 90 mins bubbles significantly reduce!

Note that this post just represents my personal opinion and does not have scientific value.

*Source: Cialoni D., Pieri M., Balestra C., Marroni A., Dive Risk Factors, Gas Bubble Formation, and Decompression Illness in Recreational SCUBA Diving: Analysis of DAN Europe DSL Data Base, Frontiers in Psychology.