During any staged decompression dive it is standard practice to switch from backgas to a more oxygen rich gas at least once during ascent. Because of the potential risks associated with breathing high partial pressures of oxygen, divers are strongly advised to adopt a set procedure for gas switching which includes standardized safety protocols. The following is a suggested procedure.
All scuba cylinders can be dedicated to high-oxygen content, cleaned and labeled as such. Many technical divers opt to use "standard" decompression gases for every dive. In either case, gas contents MUST be marked clearly according to Standards. In addition, decompression cylinders should be marked with actual Maximum Operating Depth (MOD) of contents with removable tape on two sides of cylinder valve. This MOD must be based on recent analysis and calculations for acceptable dose of partial pressure at that marked MOD and should show NOTHING but MOD in meters or feet clearly marked in large numbers. (See cylinder labeling procedures for full details.)
Decompression cylinders can be worn on diver’s left side or either side. ALl that is important is that the whole team adopts the same practice. The valve orifice should face the diver with valve on/off knobs pointing away from the diver (requires left and right valves if team wears cylinders on both sides). Divers enter water with regulator(s) on decompression cylinders charged and valve(s) closed.
During ascent, each diver will begin gas switch procedure prior to reaching switch depth (gas MOD). Deployment should follow the following steps.
Each team members “unstows” hose and second stage of selected decompression mix and pulls hose across her body with regulator second stage in right hand. Starting with dive leader, each members asks a buddy to “Look at my gas. Please confirm it is correct for next stop.” Buddy must follow hose to first stage, read actual MOD and confirm that the regulator will deliver the correct gas for the coming gas switch. This query / confirmation cycle will be done one diver at a time.
Divers will then follow schedule and proceed to MOD for gas switch. Once there, they will switch regulators and with left hand on cylinder valve will breathe hose dry while checking SPG on selected decompression gas. As reading drops, indicating once again that regulator is indeed connected to the correct cylinder, they will turn on the decompression valve allowing decompression gas to flow normally. Once they are sure regulator is breathing normally, they stow the backgas regulator they were formerly using. At the same time, each team member should indicate the status of their gas switch to dive leader. Once each team member has signaled “Switch went OK,” decompression at that depth will start.
This procedure is repeated for each gas switch made during the dive.
Some further thoughts and notes:
Do not breathe a gas which has not been analyzed by you or in your presence. There should be no exceptions to this rule.
It is imperative that all team members have similar decompression gases which can be switched within a depth of one meter or less.
Gas switching is perhaps the most stressful exercise performed during a normal ascent from a technical dive. It should never be executed in a cavalier or complacent way because the potential consequences of sloppy procedures are simply too severe. Second stage should be inspected for foreign matter before being breathed… muck, critters et al.
Whenever possible, use gases that all team members are familiar with such as 35/35 trimix, EAN50, pure Oxygen for decompression. However, when you are in the field and these “standard mixes” are NOT available, it is even more important (if that’s possible) that you follow the procedure outlined here!