We recently did a chamber dive to 50 metres. A hyperbaric chamber is a sealable chamber, or pressure vessel, somewhat like your dive cylinder (just larger), and has hatches large enough for you to climb in. It is connected to an air compressor or a bank of compressed air, and once you’re in and it is sealed the pressure is increased just as the pressure around you increases as you descend. You need to equalise as you do under water, the only real difference being is that you are dry. A fast descent means constant equalizing and ensuring deep breaths are taken. You will experience nitrogen narcosis, the extent will vary from person to person and you voice will change.
The video below shows us counting backwards from five, showing the correct number of fingers and turning our hands round after each number. It’s hard when you’re narced!
[youtube=http://www.youtube.com/watch?v=zWX-X5RngUs&w=540]
As a recreational diver you will find that your PADI eRDP will not allow you to enter a depth greater than 40 metres when you plan a dive, and the recreational dive planner (RDP or dive tables) will not allow planning deeper than 42 metres. The chamber operator or anyone that does deep technical diving will have a program to enable proper dive planning and ensuring the correct dive profile is maintained, by means of decompression stops.
On our dive we descended to 9 metres, and paused whilst two way communication was tested and the operator checked everyone was okay. Thereafter we dropped like a stone down to 50 metres in two minutes. Our bottom time was nine minutes. Nine minutes at this depth gives you quite a decompression commitment and we ascended slowly doing several deco stops on the way up with a total dive time of 39 minutes.
Our dive computers all joined us for this dive and were placed in a bucket of water (some dive computers will not go into dive mode unless the water contacts are activated). I had a Suunto Mosquito, a Mares Nemo Wide, an Uwatec Aladin Prime and a Citizen dive computer as well as a wrist mount depth gauge.
The computers were all similar in readings and were all between 50.1 metres and 50.4 metres whilst the depth gauge showed 59 metres! It’s safer for your instruments to err on the side of conservatism (i.e. tell you you’re deeper than you are, rather than the other way around). This depth gauge probably didn’t know what hit it!
A very important deep skill on a PADI Advanced course is to compare your depth gauge with your buddy and your instructor. There can easily be huge variations in depth gauges.
