The experienced divers were probably using kit called a rebreather, imagininatvely named because it allows you to rebreath1 the air that you expel. This reduces the volume of gas you need quite considerably, but even so, they needed lots of bottles all along the route, and this was the main reason why they needed so many divers. The core rescue team was 13 civilian and 5 Thai navy divers but there were a further 70+ divers in support of this team. Then there was the team of caverns who were supporting the divers, then the team supporting the caverns, ad infinitum it would seem.
1. At sea level Air is 21% O2 78% N2 1% other. The body uses approx 5% O2 per inhalation (ie 25% of the available O2) and replaces it with CO2 in the exhalaled volume. Thus you only need 5% of the volume of O2 that you would need as air. Rebreathers use science to remove the CO2 from the air inject more O2 and recycle it to the input port.
Yes, but it would certainly only be for the experienced divers.......
The science of removing CO2 is to use soda lime, which also only lasts 90 minutes, typically, and isn't as easy to stick a pressure gauge on. You wouldn't go leaving a rebreather for someone to pick up later, as it's difficult to know what state it's in, while an air cylinder with a contents gauge is easy to understand.
A diver using a rebreather in a cave where bits are flooded and breathing gear is needed, but bits aren't, will have to shut off the rebreather, and if the soda lime use is being timed, remember to turn that off when not in use, and them remember to turn it all back on when going underwater. On scuba gear, the diver can drop the regulator at any time, an pick it up with no switches, things to remember etc.
When a reabreather soda lime is all used up, the diver will slowly get symptoms of CO2 poisoning, and it may not be obvious to him or anyone else. If a rebreather stops adding O2 to make up what the diver used, or the electronics aren't turned on, the diver will usually pass out without symptoms. Depending on the depth, if the rebreather adds too much O2, the diver can be poisoned by the O2 and get convulsions underwater.
When a diver, or one of the casualties is using normal scuba gear, with our without a face mask, if the exhaled air bubble are coming out every 5 - 15 seconds, everything is fine. The gas mixture is what it was on the surface, and it can't have changed. Divers can easily hear the inhalation and exhalation of other divers, so diagnosing normal scuba gear on another diver or a casualty is easy and second nature to an experienced diver. When the air runs out, inhalation becomes progressively harder, and the diver notices. If the scuba gear goes wrong, either inhalation is impossible or there are bubbles everywhere. Either way, the diver and anyone around notices.
Also rebreathers are bulky, and not really possible to use unless the diver is close to the bags that hold the air from his lungs before feeding it back to him, so using one in tight cave is far more difficult than using a long hose on normal scuba set. The low pressure hoses have be to 40 mm diameter to allow the air to be moved by the diver's lungs and are therefore relatively thin walled and vulnerable to puncture in tight condition, while the intermediate pressure hoses on normal scuba gear are around 5 mm internally, reinforced and quite robust.
OK, I've only briefly used a rebreather, and I don't know except from media images what the cave was like, but for anyone not trained for diving, and caving, and cave diving, and rebreather diving and rebreather diving in caves, they present a whole raft of interesting ways to kill the user without warning.
At shallow depths a rebreather is of far less advantage than when deep. Go deep, start using funny gasses that aren't just air, and where decompression is a factor, and the reabreather has massive benefits. The 90 minutes is constant at all depths, instead of dropping by 4 times at 30 m like conventional gear. It's also hugely advantageous to be able to hold 25 kg of soda lime in a nice dry car in a plastic box, just like you can't with 25 kg of compressed air.
