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Off-gassing and the role of safety stops

Published on: 12th August, 2015 | Dive Safety and Medicine
Photo credit: Peter Southwood
Is it really necessary to complete a safety stop? It’s chilly and the surface is almost within reach – is it really necessary to complete a safety stop before leaving the water? Even recreational dives are decompression dives, a fact so many of us forget.

It’s chilly and the surface is almost within reach – is it really necessary to complete a safety stop before leang the water? Even recreational dives are decompression dives, a fact so many of us forget.             

It’s often the last thing you hear yelled by the Dive Master before the water fills your ears – “Don’t forget your safety stop, 3 minutes at 5 metres!”. You might be thinking ‘I could always skip it, it’s not like I’m doing a technical dive’.

 Wrong - every dive is a decompression dive. Regardless of the type of dive, your tissues are under pressure and they are on-gassing (absorbing Nitrogen). So even when you ascend slowly and steadily – your tissues are still off-gassing (releasing Nitrogen) each time you exhale.

 The first studies into safety stops were undertaken in the 1970s, when a lot of our current knowledge was being developed by military divers, particularly in the US Navy. Your body experiences an additional one atmosphere of pressure for every ten metres depth, meaning the Nitrogen in the air you are breathing is absorbed more quickly at depth. So longer, deeper dives require more time to off-gas than shallow dives.

 Becoming certified to use Nitrox mixes is one way of reducing the amount of Nitrogen you absorb during long dives. In Nitrox mixes, the level of Nitrogen in your gas mix is lower than the background level in air (roughly 79% Nitrogen, 21% Oxygen) due to the addition of oxygen. Common Nitrox range between 29-32% Oxygen (often written as EANx32 or similar).

 It isn’t advisable for recreational divers to exceed their no decompression limits and go into ‘deco’ during a dive, because they don't have the necessary training and equipment to manage the added risk. However, technical diving training, offered by TDI accredited instructors provides the know-how to dive deeper and longer safely. This is often associated with an even longer time on safety stops, at several points during the ascent (and you thought three minutes was boring)!

 With a sport you love and want to return to time and time again, those three to five minutes are worth it, and you never know what marine life you might encounter suspended just below the surface…

 Read more about how recreational divers are not immune from the laws of physics at SDI Diver News here: https://www.tdisdi.com/i-dont-decompression-dive/ 

AUTHOR

Emma McIntosh

TDI Adv. Nitrox & Deco. Procedures. IANTD Cavern Certified. PhD Candidate in Ecology