I ask this merely out of curiosity as I am just a very green OW diver. I am interested in the principles underlying various equipment setups and I have come across a bit of a question. I may not have all my facts straight but I will explain my question as I ask it. I am probably missing something fundamental to the principles of diving with two cylinders.
When diving a back mounted twinset, the gas supply is connected with a manifold. While the two cylinders can be isolated with the isolation valve, a nice option to have when dealing with problems such as a free flow, my understanding is that the tanks are not normally isolated from one another. Therefore under normal conditions, both regulators essentially provide gas from the same supply so the ability to rapidly donate a regulator is obviously the primary consideration when choosing whether to breathe from the long hose or the short hose by default. Obviously you would use the long hose as the primary regulator, because if it is in your mouth then it is also the fastest choice should you need to donate it to a team member in an out-of-air emergency.
Now, the answer to this may be clear cut to anyone with more experience than me, but his choice of which regulator to use as a primary regulator seems to be complicated a bit when the cylinders are used in a sidemount configuration. Since the cylinders are no longer connected with a manifold, they no longer act as a single gas supply but rather two independent gas supplies under normal conditions. As such, it seems that utilizing the long-hose as one's primary regulator is not such a clear choice, and factors other than the speed at which you can donate the regulator come into play. Here is why:
Imagine two divers in a shallow wreck that for some reason mandates that the divers proceed single file. The long hose allows for air sharing while proceeding in this manner, but the divers are using sidemount 80s filled with air. Both divers are using their long-hose as their primary regulator (which I believe is the proper thing to do even with sidemount?) and breathing it dry before switching to the secondary regulator on the other tank (this seems like it is probably not what you do). The divers hit the turnaround point in their gas supply while penetrating the wreck, so the tank with the long hose is down to one-third capacity because they have both been breathing off of that tank on their way in (one third of total gas supply subtracted from half of the total gas supply leaves one sixth of the total gas supply in the long-hose-tank, or a third of that tank.) Right at the turnaround point, a vicious moray eel, ala "The Deep," that also happens to be a Jurrasic-Park-velociraptor-intelligent-mutant due to exposure to radioactive waste, lunges out and bites clean through one of the diver's regulator hoses and then bites clean through the other. All of the sudden, diver A is in an OOA emergency, but the eel backs off because he wants to watch the chaos that he is certain will ensue. (The basic idea behind this scenario could play out in any manner where a diver has an OOA emergency at some point at or after the one-third-of-gas-used turnaround point, so long as the emergency happens after a good amount of gas has been consumed, obviously the dive would be aborted so this situation is avoided if the emergency occurs before significant gas consumption has taken place).
Diver B offers his primary regulator, the long hose. Both divers must now proceed single file back out of the wreck and to the tie in-point where their deco cylinders are secured, air sharing the whole way. But the tank attached to the long hose that has been donated only has a third of it's total capacity, and diver A is breathing fast because he was freaked out by that eel. The divers also kicked up some sediment on their way in that hasn't settled, so going back out is going slower. Diver B is breathing off his fresh tank hooked up to his secondary, short hose regulator, but diver A can't access that air supply because it is on a short hose and the divers are going single file.
Even worse, what if this happened on the way out when diver B just about almost finished off his long hose tank and his long hose was a few drags away from being a dead regulator?
This may be an overly contrived situation, but say something happens like a diver becomes entangled on the way out. The rule of thirds basically gives you a third of your air supply should something like that happen, but if you already breathed through the tank at the end of the long hose, you can't really air share at all.
Do sidemount divers breath on the short hose as their primary instead of the long hose? Do they manage their gas differently, such as switching to another regulator when a tank is at X% capacity or something?
When diving a back mounted twinset, the gas supply is connected with a manifold. While the two cylinders can be isolated with the isolation valve, a nice option to have when dealing with problems such as a free flow, my understanding is that the tanks are not normally isolated from one another. Therefore under normal conditions, both regulators essentially provide gas from the same supply so the ability to rapidly donate a regulator is obviously the primary consideration when choosing whether to breathe from the long hose or the short hose by default. Obviously you would use the long hose as the primary regulator, because if it is in your mouth then it is also the fastest choice should you need to donate it to a team member in an out-of-air emergency.
Now, the answer to this may be clear cut to anyone with more experience than me, but his choice of which regulator to use as a primary regulator seems to be complicated a bit when the cylinders are used in a sidemount configuration. Since the cylinders are no longer connected with a manifold, they no longer act as a single gas supply but rather two independent gas supplies under normal conditions. As such, it seems that utilizing the long-hose as one's primary regulator is not such a clear choice, and factors other than the speed at which you can donate the regulator come into play. Here is why:
Imagine two divers in a shallow wreck that for some reason mandates that the divers proceed single file. The long hose allows for air sharing while proceeding in this manner, but the divers are using sidemount 80s filled with air. Both divers are using their long-hose as their primary regulator (which I believe is the proper thing to do even with sidemount?) and breathing it dry before switching to the secondary regulator on the other tank (this seems like it is probably not what you do). The divers hit the turnaround point in their gas supply while penetrating the wreck, so the tank with the long hose is down to one-third capacity because they have both been breathing off of that tank on their way in (one third of total gas supply subtracted from half of the total gas supply leaves one sixth of the total gas supply in the long-hose-tank, or a third of that tank.) Right at the turnaround point, a vicious moray eel, ala "The Deep," that also happens to be a Jurrasic-Park-velociraptor-intelligent-mutant due to exposure to radioactive waste, lunges out and bites clean through one of the diver's regulator hoses and then bites clean through the other. All of the sudden, diver A is in an OOA emergency, but the eel backs off because he wants to watch the chaos that he is certain will ensue. (The basic idea behind this scenario could play out in any manner where a diver has an OOA emergency at some point at or after the one-third-of-gas-used turnaround point, so long as the emergency happens after a good amount of gas has been consumed, obviously the dive would be aborted so this situation is avoided if the emergency occurs before significant gas consumption has taken place).
Diver B offers his primary regulator, the long hose. Both divers must now proceed single file back out of the wreck and to the tie in-point where their deco cylinders are secured, air sharing the whole way. But the tank attached to the long hose that has been donated only has a third of it's total capacity, and diver A is breathing fast because he was freaked out by that eel. The divers also kicked up some sediment on their way in that hasn't settled, so going back out is going slower. Diver B is breathing off his fresh tank hooked up to his secondary, short hose regulator, but diver A can't access that air supply because it is on a short hose and the divers are going single file.
Even worse, what if this happened on the way out when diver B just about almost finished off his long hose tank and his long hose was a few drags away from being a dead regulator?
This may be an overly contrived situation, but say something happens like a diver becomes entangled on the way out. The rule of thirds basically gives you a third of your air supply should something like that happen, but if you already breathed through the tank at the end of the long hose, you can't really air share at all.
Do sidemount divers breath on the short hose as their primary instead of the long hose? Do they manage their gas differently, such as switching to another regulator when a tank is at X% capacity or something?
