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While the purpose of this site is to present some easy to use decompression tables, the entire page still must be read so that a diver gets an understanding of the assumptions and methodology that went into the tables.
The Beginning Tec 40 Diver
Decompression tables are given below for small decompression profiles such as PADI's Tec 40 certification level. This Tec 40 certification is a nice bridge between recreational diving and full technical diving. This can also be viewed as how all dives should be conducted, i.e. with a pony bottle for a reserve and proper ascents from all dives (since ascending from all dives is a decompression process). Since the Tech 40 class as well as the equipment configuration limits the depths and duration of the dives, the calculation of the decompression tables are also limited. If progressing through PADI's technical courses, using Departure's decompression software will be a more efficient method of calculations in those classes ... and of course it can also be used in the Tec 40 class from the beginning.
Two different sets of Tec 40 decompression tables are given. One has moderate deep stops and the first set has deeper initial stops - which is based upon newer research. Both are given since different instructors and divers believe in different decompression techniques. You will notice that the second set does not require as much decompression as the first set, but using this as a reason to stay down longer or decompress less is not a valid criteria on how to choose a table. For more custom tailored decompression dive tables, a diver may always use Departure's decompression software.
While PADI's Tec 40 certification wisely limits a diver to ten minutes of "required" decompression, this does not mean that a Tec40 diver is limited to only 10 minutes of decompression. Just as a safety stop after every recreational dive is beneficial, padding or adding extra decompression time is also highly recommended since it has the same goal of eliminating more nitrogen prior to surfacing. On the Tec 40 style decompression dive tables, you will notice that the far left column is a suggested safety stop procedure. But as you get into decompression, the decompression times can be less. This is because these are minimum decompression times and again, you should add extra time to them. You should also plan your Tech 40 dive to have enough gas to fully perform the stops with extra time being added.
Since you are now entering the realm of technical diving, you have learned that no table or software can guarantee or fully prevent the possibility of a diver getting decompression sickness - especially when a diver chooses to enter into the mandatory decompression stop environment. Therefore it is up to you as to how conservative you desire to dive - which is where decompression dive software can assist by allowing personal choices to be entered into the profile.
Starting with Time Limits
It is important to understand how decompression dive tables, including the following ones were designed. Hopefully you have read the Decompression Strategy article and the Ascending from a Dive article as a starting point for understanding decompression tables and ascending from dives. As you learned there, the obvious starting point is to determine how conservative (or aggressive) a dive's no-stop time limits are. The following table shows some relatively modern time limits, their consensus and how they compare as well as the time limits the Tec 40 created decompression tables were centered around.
As can be seen, the time limits these dive tables use should be considered maximums and therefore the decompression tables have no safety margin or conservative factor built in. Different divers want different conservation factors and therefore you must build in yourself how much extra decompression stop time you desire. This may be done a variety of ways, such as by treating your dive as being deeper or longer, by using "air" tables when diving on nitrox, or by using decompression dive planning software where you can input this information. This is the preferred method since you can enter your desired conservatism factors, time limits and gas consumption and it is additionally a great learning tool. See our page for Tec 40 type decompression software for more information.
Ascending through the stops
Now that it is understood the time limits the dive tables are modeled on (which controls the gas pressure/tension) upon surfacing, the next step is to understand the ascent through the stops. As can be seen below, there are multiple ways to model the actual decompression stops to the surface.
The above is simply to show the variety of ways a diver may choose to decompress and that the following decompression tables are not the only way to model decompression. Therefore a diver may wish to use diving decompression software for their Tech40 class and dives. But whether using the below decompression tables or decompression software, it is ultimately the diver's responsibility in choosing the decompression. It is beyond the scope of this article to discuss all the different decompression theories, strategies, and risk related factors that the diver should learn in their technical scuba classes. So the remaining portion of this page will be spent on simply presenting the decompression tables.
Decompression Table Design
While there are many ways to model and implement decompression dive tables, these were designed with simplicity in mind for the beginning technical diver and Tec 40 diver.
The decompression tables were designed for single "mild" decompression profiles with the diver having 2 cylinders (and regulators) with one being a "standard" recreational size and the second being smaller such as a 40 (or 30) cubic foot pony bottle. The decompression tables take this into account by giving the amount of gas needed to do the profiles with different breathing rates (SAC/RMV). This allows the diver to reference the gas required and make sure there is enough gas to: 1) perform the entire dive on the main cylinder in case of a failure in the second cylinder, and 2) to be able to perform the entire ascent from the bottom on only the 2nd cylinder in case of a failure of the primary cylinder.
The gas needed is given for breathing rates between an SAC/RMV of 0.7 to 1.0. The gas required is shown as a total requirement as well as being broken down into the amount needed for both the bottom (↓) as well as the entire ascent (↑). It is probable the bottom gas requirement and the ascent gas requirement will be greater than the total needed given due to rounding each individual gas up to the nearest cubic foot. You will notice that decompression tables are only give for deeper dives. This is because at shallower depths, the diver's gas supply often (but not always) limits the dive and not the no-stop time limits (when using nitrox 32% or richer). But of course safety stops should always be performed and the maximum time should always be calculated to make sure decompression is not accidentally entered into.
In general, the decompression stops assume the diver is performing the stops on the same gas that was used at the bottom. This is for simplicity to not present an overwhelming number of tables as well as there is often no difference in the times calculated when using a richer nitrox gas (that is also good at the bottom, i.e. 32%) and just using the bottom gas in the calculations. While there could be a difference if switching to 32% for the stops from an air bottom gas, it is assumed that if 32% is being used for the deco gas then nitrox (and not air) would also be used for the bottom gas - which brings us back to not having an appreciable difference in stop times. But of course a diver is always welcome to use Departure's decompression software to produce their own decompression tables taking into account the gas switch.
Reading the Decompression Tables
Reading the decompression dive tables should be obvious. At this level of diving, it is assumed that nitrox will be used. So first, make sure that your are using nitrox with at least the amount of oxygen listed for the decompression tables. You may of course use a richer mix as long as you stay within acceptable oxygen partial pressures. The nitrox mix used for the decompression tables stops were calculated on a bottom mix giving a ppO2 of approximately 1.3. This was done for several reasons. One is that many technical divers use a lower ppO2 due to often staying at or near their max depth for their entire bottom time as well as due to the increased density of gas at deeper depths contributing to CO2 production. Another is that you will achieve a slight level of decompression conservatism if you use a richer mixture. But again, you can plan your actual dives with Departure's decompression software using your exact mixes.
Now simply find your depth and planned time. You can now find the time required at each stop as well as your "run time." Run Time is the time your watch or timer will give you when leaving that particular depth. You will notice the times given start at the bottom so you can see the time you must leave the bottom by. You may always leave the bottom sooner, but you may not leave your decompression stop until the full time is reached. The run time of the first stop may also appear a little long at first glance, but it also includes the ascent time (at 30'/min) from the bottom to the first stop in addition to the time of the stop.
As mentioned earlier, two sets of decompression tables. Both are considered "deep stop" tables, but the first set as deeper initial stops than the second based upon more recent research. Again, it is up to you to do your own research and choose which set of tables and conservatism you choose to use.
Tables will be posted any day now so check back daily (July 2, 2011)
The Tec 40 class from PADI
If you have gotten this far, then you are probably familiar with the Tec40 class. But for those that are not, the following is a brief description of the Tec 40 class.
The Tec40 is the entry point into the technical range as the Tec 40 class provides a transition from recreational to technical diving. Although the use of full tech gear (doubles and wings) is preferred, it does allow modified use of recreational gear in some situations, provided the diver has two separate regulators, with one of the first stages fitted with a long hose (for example, a main cylinder and pony cylinder combination). The intended working limit for a diver at this level is 40 meters/130 feet with up to 10 minutes of decompression. They may use any nitrox mix with up to 50% oxygen. To start the class, the diver must be the equivalent of a PADI Advanced Open Water diver (or equivalent) with a nitrox certification rating and have deep recreational diving experience.