In all of the sidemount courses I teach we start off with the usual gear configuration session. I’m not one of those instructors who tells my students how to configure their gear. What I do is review all the different possible gear configurations and some of the reasons behind them. I’ve had the benefit of having discussed with the designers of some of the sidemount rigs available the reasons behind certain designs. While I may not agree with certain design characteristics for my own rigs, it doesn’t mean there is anything wrong with that configuration. It’s just not what works for me. I like to pass as much information on to my students as possible and have them make their own decisions.
Anyway, after spending a few hours getting gear configured we start discussing regulator configuration. Yes, we cover that as well. There are lots of ways to configure regulators. You can run the hoses around your neck, or straight up to your mouth, or one hose around the neck and the other straight up. The way you run your hoses will affect the way your rig is configured. But no matter how you configure your hoses and your rig, one thing remains the same – gas management.
My recommendation is to keep gas pressures between cylinders no more than 1/6 of total starting pressure from each other. If you start with 3300 psi, switch over 500-600 psi later, or at 2700 or 2800 psi. Then breathe the next cylinder down to your turn pressure (we’ll use 2200 psi here for simplicity). Switch back to the first cylinder and breathe that until turn pressure (2200 psi), turn the dive and keep breathing from that regulator until you get to 1600 or 1700 psi. Switch back and breathe this regulator until the dive is over. This means you will have a total of three regulator switches during the dive. The reason for switching is not to balance the cylinders, although some people will start to feel off balance if their cylinders are more than 600 psi difference in pressure. The reason for switching is in case a gas issue arises.
Some divers, however, don’t think it matters how sidemount cylinders are breathed from. They will breathe one cylinder down to turn pressure, switch to the other cylinder and breathe it down until they exit the cave. Sometimes they will breathe the second cylinder down enough to where they might need to switch back to the first cylinder before getting back to open water. While at first glance this may appear to work, and if nothing goes wrong it does, when things go pear shaped this practice may result in issues.
Let’s look at some examples using the above gas switches. Jim is diving HP100s that are filled to 3300 psi. He begins his dive in a low flow system breathing off of the right side cylinder. He breathes that cylinder to 2200 psi and switches to his left side cylinder. When he reaches 2200 psi on his left cylinder he turns the dive but continues to breathe off that cylinder. When Jim has breathed down the left cylinder to 1100 psi his dive buddy, Kim, who is diving backmount HP 100s has a first stage failure and loses all the air in her cylinders. Jim donates the regulator from his right cylinder to Kim and they continue to exit. However, during the incident, Jim breathed his left cylinder down to 900 psi. Even if Jim’s gas consumption remains the same he is still about 1100 psi from the exit. Remember, Jim and Kim swam 1100 psi in and Jim had breathed 1100 psi from one cylinder and had another 1100 psi to go. If Jim had managed his gas differently he would have enough gas to exit. As it is he is 200 psi short so Jim and Kim will have to buddy breathe the last part of the passage out.
Let’s look at this with Jim managing his gas as recommended above. Jim breathes his right cylinder down to 2700 psi and switches to his left. He then breathes his left cylinder to 2200 psi and switches back to his right. When he reaches 2200 psi on his right cylinder he turns the dive and continues to breathe that cylinder until it gets to 1600 psi. At that point he switches to his left cylinder and breathes that down to 1700 psi when Kim has her incident. Jim still has the same amount of gas. In the first example he had 1100 psi + 2200 psi = 3300 psi. In this example he has 1700 psi + 1600 psi = 3300 psi. The difference is when he donates the regulator from the cylinder containing 1600 psi, he still has 1500 psi left in the other cylinder (assuming he still breathes it down 200 psi during the incident). Now they both have enough to exit safely.
At this point some of you are saying to yourselves that you don’t ever dive mixed teams. Either everyone is in sidemount or everyone is in backmount. That’s fine. And to be honest, I can’t think of an incident in which a properly trained and experienced sidemount diver would need to share air when gas management is done properly so we’ll approach this in a different manner. Let’s look at another example. We’ll take this from the perspective of a self sufficient sidemount diver. But this example will work with two sidemount divers that may happen to share air at the time of the incident.
Joe is diving HP 130s filled to 3300 psi. As Jim did Joe breathes his right cylinder down to 2200 psi, switches to his left cylinder and breathes that down to 2200 psi and turns the dive. He continues to breathe his left cylinder down to 1100 psi. Let’s say Joe continues to breathe his left cylinder. About 200 feet from the exit he starts to feel his regulator breathing harder. He switches back to his right cylinder only to discover the second stage got jammed up with sand in a low sandy restriction he passed through earlier. He tries to purge the sand clear but it’s packed in there pretty tightly so Joe is left without a regulator to breathe from. Joe could go to his buddy and signal out of air, but if his buddy was managing his cylinders the same way he would also be switching to his right cylinder so he wouldn’t have a regulator to donate.
Had Joe and his buddy switched every 500-600 psi their cylinders would never get below 1100 psi so neither regulator would ever start to breathe hard. And it would be less likely that their second stage regulators would be unused long enough to become unworkable. Even with a second stage regulator out for 500-600 psi, it’s possible to get packed with sand a pretty good amount. I’ve had this happen to me to such a point I had to switch back to my other second stage while I worked on clearing the regulator and it had only been out for a couple hundred psi. Fortunately, I was able to switch back, get the regulator clear, and continue my exit uneventfully.
Yes, these examples are of incidents that are not likely to happen. But what incidents are likely to happen? How many cave divers do you know have had an out of air incident? How many do you know have ever been lost off the line? Yet, before every dive we do S-drills (or at least we should) and we all carry safety reels/spools. We prepare for unlikely events and this is just another way to prepare that doesn’t take more than one more regulator switch.
While in the above examples straight thirds was used for simplicity, I do not condone diving to thirds. Diving to thirds is not conservative enough. But that’s another article.