March 30th, 2006, 12:32 PM
Divers commonly hear leaders in the dive community say, “Be back on the boat with 500 psi.” What does this mean, exactly, and how do they get that number?
What they mean is that they want you to return to the boat NOT with an empty tank, but with 500 psi in reserve. This is a safety margin that allows divers to have something in their tanks in case of an emergency.
How do they get this number, and what sort of emergency could they be preparing you for? To fully understand the answers to those questions, a diver must be familiar with the concept of “Rock Bottom.” The explanation of the concept – which is a fundamental building block for all diving, but especially important in overhead, deep, or staged decompression diving – shows that while 500 psi may be common in shallow reef diving in warm waters, it is wholly inadequate for many dives commonly practiced by even the most casual of scuba divers. Scary? You betcha. Read on.
This article has been written to educate the general diving public about the concept of “Rock Bottom,” and to help others to increase safety margins to acceptable levels within the scuba diving community regardless of the level of intensity of the dive. While reading this, understand that this is a basic overview of the concept of “Rock Bottom,” and how it applies to you as a diver. For more information about “Rock Bottom,” and/or to improve your skills regarding dive planning and safety, contact your local GUE instructor. Additionally, questions regarding “Rock Bottom” can also be forwarded to us on the DIR Explorers forum. We welcome all discussions regarding “Rock Bottom” and scuba diving in general.
Plainly put, “Rock Bottom” is the minimum amount of breathing gas that a diver will need in an emergency to get both he/she and his/her buddy to safety. While calculated in cubic feet, it’s typically expressed in terms of pressure (psi or bar, depending on location) so that the information that it provides can be expressed in a term that will allow the diver to translate it on his pressure gauge. Typical “Rock Bottoms” are 750 psi and 1000 psi in most open water, non-penetration (no caves or wrecks), no staged decompression (“no-deco”) dives. “Rock Bottom” is information that a diver needs to be able to effectively plan a safe dive at any depth.
To be able to figure “Rock Bottom,” a diver must first know what sort of breathing rate he or she has. This is expressed in terms of “SAC,” or “Surface Air Consumption.” A diver with a “SAC” (pronounced “sack”) rate of 0.5 breathes half a cubic foot of breathing gas per minute at sea level. A diver with a “SAC” rate of 1.0 breathes one cubic foot of breathing gas per minute at sea level. These two numbers – 0.5 and 1.0 – represent the upper and lower limits of most experienced divers’ “SAC” rates. Typically, the diver with a “SAC” rate of 0.5 is in good physical condition, extremely relaxed in the water, and is a well-seasoned diver. It is not uncommon for first-time divers to have a “SAC” rate of 2.0 or higher.
Another concept that a diver must understand is one that is taught in every common basic Open Water scuba diving course: The concept of “atmospheres,” or “ATAs” (pronounced, “addas”). A diver must understand that at sea level, he is surrounded by one atmosphere of pressure, or one “ATA.” At 33 feet, he is surrounded by two “ATAs”, at 66 feet, three “ATAs” and so on. "ATAs" is simply a way of communicating the ambient pressure of the surrounding water on a diver, in terms of sea level air pressure. This is important because while a scuba diver descends to depth, ambient pressure compresses his breathing gas, making him consume many more molecules of that gas on each breath because the gas is compressed. Due to this phenomenon, a diver with a “SAC” rate of 0.5, for example, will consume half a cubic foot of breathing gas per minute at the surface, but will consume twice that at 33 feet, or 1.0 cubic foot of breathing gas per minute. The same diver will consume 1.5 cubic feet of breathing gas per minute at 66 feet (three "ATAs") and 2.0 cubic feet per minute at 99 (four "ATAs"), assuming that his breathing rate remains constant.
With this knowledge and understanding, a diver can accurately predict exactly how much gas he will need to survive an emergency situation underwater – exactly the point of figuring “Rock Bottom.”
Let’s say that a diver – we’ll call him Scuba Steve – and his buddy Diver Debbie decide to go for a recreational reef dive one day, and want to know exactly how much breathing gas they need to have in reserve for an emergency. The very worst that could happen in any scuba situation is the complete loss of breathing gas – that is, either Steve or Deb finds themselves suddenly out of gas completely, due to a failed life support component. It can be reasonably assumed that Steve and Deb have been watching their breathing gas supplies and have not voluntarily overstayed their welcome.
Of course, the failure of a life support component is in and of itself not life-threatening – after all, both Steve and Deb carry a backup, and thus are prepared to donate breathing gas to their buddy. At the beginning of the dive when both of our fearless divers have plenty of breathing gas, this isn’t a problem. However, as supplies get depleted as their dive continues, the possibility of a failed component has potentially more serious consequences – after all, each diver could easily watch how fast their supplies are being depleted, and simply time their ascent accordingly. Unfortunately, this does not allow for safety in the face of one diver suddenly losing his or her gas supply – the diver with the intact life support would need to donate gas, and suddenly gas supplies would be being depleted by both individuals – at twice the “SAC” rate of just one diver. Ultimately, this could mean that both divers would suddenly find themselves with not enough gas to safely surface, and they’d be forced to make a decision to either stop breathing or surface too fast – which would cause a whole plethora of new problems, including possible decompression sickness.
Thus, “Rock Bottom” was conceived. This is the amount of gas required by two divers to deal with the problem at hand and ascend safely while sharing gas on one tank. Both divers would need to calculate “Rock Bottom” and practice it so that if either system failed, both could rely on the working system during the ascent to the surface.
Back to Steve and Deb: They decide that they’re going to do a dive on a reef at 60 feet of depth, which we will round to three “ATAs.” Steve and Deb are both well-experienced, and thus know their “SAC” rates are around 0.5 cubic feet per minute. We know then, that they will be consuming about 1.5 cubic feet of breathing gas per minute each during this dive.
Logically, then, we can say that if one diver’s life support system failed, then they’d both breathe from the same tank… If we gave them one minute to steady themselves and begin an ascent, we know that they’d use 3.0 cubic feet of breathing gas out of that tank. Steve and Deb are safe divers, and so they ascend at 30 feet per minute, and thus would take two more minutes to surface. During this ascent time they’d be at an average depth of 30 feet (half of starting depth), or two “ATAs.” Two divers with a “SAC” rate of 0.5 at two “ATAs” for two minutes would consume two cubic feet of gas (2 divers * 0.5 SAC * 2 ATAs = 2 cuft). Thus, we know that total gas consumed would be 3 cuft + 2 cuft, or 5 cubic feet of gas. Let’s not forget, however, that divers in an emergency situation are known to double their breathing rates, and so we’ll plan to need 10 cubic feet of breathing gas.
Still with me? We’ve just decided that these guys will dive safely with 10 cubic feet of gas in reserve… That is, 10 cubic feet of gas is their “Rock Bottom.” If they’re diving single aluminum 80 tanks (which are standard), that’s 1/8th of their gas supply… Expressed in terms of psi, that’s 1/8th of 3000 psi, or 375 psi. Rounded up (which you want to do anyway because an aluminum 80 is actually a little shy of 80 cubic feet of breathing gas at 3000 psi), we can simply say that these divers have a “Rock Bottom” of 400 psi.
…So what does that mean? That means that they need to make sure that they never, ever touch that gas unless there’s a legitimate emergency. They conduct their dive completely on the remaining 2600 psi – all of it. When they surface, unless there’s been an emergency, they still have “Rock Bottom” in their tanks – 400 psi.
Let’s take Steve and Deb for another dive – this time to 100 feet of depth. If one of them had a system failure at 100 feet of depth, they’d spend one minute getting themselves situated, and they’d use 4 cuft of gas doing it. Then they’d ascend, taking three minutes – with an average depth over three minutes of 50 feet, or 2.5 "ATAs." They’d thus consume 3.75 cuft of gas during the ascent, for a total of 7.75 cuft of gas. Doubled (due to breathing rate), that’s 15.5 cuft of gas, or 20% of their tank (15.5/80). In terms of psi, that’s 581 psi… Round to 600. Remember, that’s a MINIMUM number, based on experienced, relaxed, and fit divers. Unfit or casual divers would require more, in accordance with their “SAC” rates.
The concept of “Rock Bottom” might need to take other activities into account as well – for example, most major agencies recommend a “safety stop” at 15 feet for 3 minutes if the divers have done multiple dives or a dive deeper than 60 feet. Using the same calculation, this would require an additional 9 cuft of gas, or 337.5 psi. Added to the minimum required “Rock Bottoms” as calculated above, and we get about 750 psi for the 60 foot dive and about 1000 psi for the 100 foot dive. Again, these numbers would need to increase if “SAC” rates were above what the most experienced divers commonly require.
…Which, of course, lends credence to the theory that “be back on the boat with 500 psi” – the theory of a 500 psi “Rock Bottom” - will not provide even experienced divers with enough breathing gas in an emergency situation, even when the dives are of the non-technical, “recreational” sort. A diver practicing the “Rock Bottom” of 500 psi concept will, worst case scenario, be forced to either drown or neglect his or her safety stop, putting him at risk for decompression sickness.
What’s worse, though, is this: modern computers do not and can not plan for “Rock Bottom” – that is, they can not foresee the sudden quadruple of "SAC" rates (two divers who have suddenly doubled their "SAC" rates, suddenly breathing off of one source) and do not have the capacity to allow for planned “Rock Bottom” times. Thus – “diving a computer” – that is, simply going up when the computer says to go up – is a dangerous practice that can leave a diver out of breathing gas at depth, especially when he’s already had to deal with one problem of someone else’s failed life support. This situation is known as “task loading” – the combination of catastrophic events which can lead to serious dive-related problems, even when the diver has enough skill to be capable of handling a single catastrophic event without an issue.
Studies in the realm of scuba diving accidents rarely note that one, single problem was the cause of injury or death. In almost every case, scuba-related accidents are caused by a series of events “snowballing” and creating a situation from which recovery is impossible.
For this reason, we maintain that no computer can take the place of a good, solid dive plan which includes a “Rock Bottom” calculation. Simply put, divers using dive computers often progressively increase their reliance on the device, while progressively neglecting their gas management and dive planning skills - unaware that the crutch they’re using is not taking all factors into account.
For more information regarding proper dive planning, “Rock Bottom” calculations, and other safe dive practices, please visit the DIR Explorers forum or contact your local GUE representative.