Under pressure…

From the start of our training as Ocean Divers, the dangers of diving are drummed into us. The incident pit, getting narked, oxygen toxicity, the bends, embolisms; the list is endless and often startling for a new diver undergoing training. In reality, the chances of it all going wrong and getting whisked off to the chamber for recompression treatment is minimal if we stick to safe profiles and don’t take risks. However, Proper Preparation and Planning Prevents Poor Performance, and the first time you experience narcosis, or the other effects of pressure, it is best to be in a safe controlled environment. With it being winter and the weather not really suitable for diving, the club booked a training trip for the club to the London Diving Chamber at St John’s Wood.

The plan was for a 50m dry dive, using the US Navy tables to carry out a 170/25 dive, which translated means 170 ft for 25 mins. For conservatism, this converted to 51.8 metres, and bottom time would only be 15 minutes. After all, it gets embarrassing for the chamber if they end up bending you… The tables gave us stops at 12m, then 9 and 6 metres where we would switch to 100% O2.


Nitrogen Narcosis affects everyone, but the experience can be radically different depending on various factors. To complicate it, your perception is one of the first things to go, so identifying the effect it has is generally difficult. To give us a rough idea as to how it impaired each of us, we started off with a quick test that we would repeat at 50m and hopefully see the changes.


Then to the chamber. We were using a multi-place chamber for 10 people, although getting inside it was a bit cramped for the 5 of us and with many more people, it would have been a close session for any extended length of time. We were in scrubs, ear defenders on, and the doors were sealed closed. The roar of compressed air entering the chamber hit us and the pressure started creeping up. The first thing I noticed was the temperature rising as we went down. We dropped steadily down to 50m and bottomed out, the roar died away and we took the ear defenders off.

“Everyone ok?” asked the technician, his voice squeaking like he was on helium. Admittedly, he was Australian and already had a funny accent, but even so it sounded odd. We started giggling, then talking, and were swiftly reduced to uncontrollable sniggering. The increased pressure meant the speed of sound was faster, so the pitch of our voices was higher, as it was when you breathe helium. The laughter was the effect of narcosis kicking in as the nitrogen was forced into our cells in ever higher quantities as the partial pressure increased.


We spent what felt like a fraction of a second re-doing the tests, with the allotted 60 seconds flying by, then relaxed for a bit to play around. The thicker air made it impossible to whistle, and when waving your hands about you could feel the resistance in the air. Dropping light plastic water cups from the top of the chamber resulted in a gravity defying cup that fell strangely slowly to the floor. Time at 50m rushed past and we came back to the ascent phase.

As soon as the pressure dropped, the temperature cooled. From gently sweating, I went swiftly to goosebumps, and then as the pressure kept dropping, the water vapour in the air condensed, causing a cloud in the chamber. Slowly we came up, did the stops and then swapped to 100% O2 at 9 metres. I’d used 80% before as a decompression gas, and knew my breathing rate would drop off, but was surprised to see by how much. One breath would last me almost a minute if I was not thinking; there was no desire to breathe at all. I really had to force myself to exhale to help clear the nitrogen from my system.


Eventually, we surfaced and escaped from the claustrophobic chamber. The technician calculated our scores as we changed out of the scrubs and we came back for the results. All of us had performed far worse under water, and although the drop in performance varied from person to person, it wasn’t by that much. Sobering thought – if you are planning on doing dives deeper than 40m, start thinking about doing a Tri-Mix course!

London Diving Chamber