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What Is A Stage Tank And What Is It Used For?
The image we all have when we think of a diver is with a tank on his back. This is the case in most cases, but with the rise of technical diving, new equipment, techniques and procedures have had to be used to achieve longer dive times and dive deeper.
The first problem a diver faces when wanting to dive longer is the duration of the gas supply. If we dive with an open circuit, normal regulator that is, we take gas from the supply with one breath and we breathe it out into the water, the gas consumption increases proportionally with depth. At 10 meters depth, we have twice the pressure than at sea level and consume twice as much gas with each breath, for example. With this limitation, when divers need more bottom time or go deeper, they need more available gas supply.
The first option is obviously to increase the size of the tank on the back. If you dive with a single tank, we can increase the size from 12 liters, which is usually the most common tank, to 15 liters, to 18 liters and even to 20 liters. The 15-liter tank increases the amount of gas available by 25% compared to the 12-liter tank.
The problem of greatly increasing autonomy with a single tank is that in case of emergency and loss of gas, we only have one gas supply depending on our buddy to get to the surface. For this reason, and to be able to carry more gas more comfortably on the back, the twin-tanks were created, the most common being the 12-liter twin-tank, which is equivalent to a 24-liter single tank.
This configuration doubles the available volume of a regular tank in recreational diving with the advantage that in an emergency we can conserve gas in a tank using a valve that isolates the two tanks from each other, always leaving at least one operative with its regulator. There are also larger tank sizes, reaching 2×20 liters, making a total of 40 liters. These bottles are very heavy and difficult to handle for an average diver, both out of the water and in the water.
Even so, divers who do technical diving need more gas available for certain dives and for this reason they began to use additional tanks that could not fit on the back, and that is why the stage tank was devised. This tank is usually an aluminum tank that is not so heavy in the water as a steel tank, with 11-liter capacity provided with a harness with two clips, one near the neck of the tank and the other at approximately a third of the base.
This gives us two anchor points with which we can attach the tanks to our body harness that has D-rings at the chest and waist. This configuration is very comfortable because it allows us to manipulate the tank and its regulator with vision, leaving it behind if necessary, during parts of the dive, such as at a narrow cave entrance where we collect it again on exit, or before penetrating a wreck, reducing our volume to enter tight places. And also, it easily allows us to exchange tanks with a partner in case of emergency.
The procedure for using these tanks is to use a third of the tank and then leave it on the guideline in the cave or before penetrating a wreck, continue to breathe from the twinset on the back to a third, and return. When you reach the stage tank use another third of it until the exit. In this way we have the three tanks at the end of the dive with an emergency third remaining, which is how cave diving is planned.
Another procedure is also used, which is to use the stage tank up to half plus 15 bars and then the twinset to a quarter. This allows the maximum gas to be retained in the twinset that can be shared with a partner in an emergency, while the stage tank cannot be used by two people at the same time.
The stage tank, by definition, is an extension tank, but we also use the same configuration for decompression tanks, which are tanks with different gases than the ones we use during descent and the dive proper, to improve decompression efficiency during the ascent, and therefore we only use them during the ascent.
As they are configured the same, the stage tank and decompression tank are used interchangeably, although their function is completely different.
If you want to do a dive in a cenote, in Mexico, and penetrate 1000 meters into a shallow cave, you will have to use stage tanks, and we at Dressel Divers can teach you.