I recently took the PADI Enriched Air Diver speciality course from Shellie and Curtis at Sin City Scuba in Las Vegas. Curtis reminded me that SCUBA skills, like any other skills, are “perishable”: you have to use them or lose them. So, in the spirit of trying to prevent the skills from perishing, I’m writing a little tutorial here on how to use the PADI nitrox tables. (You should seek qualified instruction before diving or using nitrox.)
I love this stuff because it’s essentially applied chemistry and physiology! I’ll use “ata” to refer to “atmospheres absolute”: on the surface, we experience 1 ata. Every 10 m / 33 feet, we experience an additional ata of pressure. For example, at 10 m, we’d have a pressure of 2 ata.
Unlike climbing, the diver can run into the limits of human physiology quite easily. There are three constraints that are relevant to the discussion:
Narcosis: Both nitrogen and oxygen can have narcotic effects at pressure. People report euphoria, cognitive impairment, sleepiness, hallucinations, terror, etc. The effect seems to start at around 100 feet (when breathing air), with one extra “martini” of impairment with every successive increment 33 feet. (Hence, recreational diving is not generally done below 130 feet.)
Nobody quite understands what the biochemical origin of the effect is, but it’s thought to have to do with the impairment of nerve impulses. This hypothesis is consistent with the fact that more nonpolar gases seem to be more narcotic. The Meyer-Overton study from the early 1900s showed that there is a linear correlation between the narcotic potential of a gas and its solubility in oil! (Real life is a bit more complicated than this, but it’s a good starting approximation.)
The Bunsen solubility coefficient for nitrogen and oxygen are similar: 0.052 ata-1 and 0.11 ata-1, respectively, suggesting that breathing different oxygen/nitrogen gas mixtures won’t change the narcotic potential very much. On the other hand, the coefficient for helium is very low: 0.015 ata-1, which explains its use a diluent gas for technical diving. Presumably, the solubility of helium in non-polar solvents is rather low because it is a relatively small, non-polarizable noble gas. In contrast, xenon, a highly polarizable noble gas, is extremely narcotic, with a coefficient of 1.9 ata-1. True anaesthetics are also soluble; for example, nitrous oxide has a coefficient of 1.56 ata-1, while chloroform, fluorohydrocarbons, and ethers can be 10-1000 ata-1. How cool is that?
Nitrogen Solubility: As you might expect, the solubility of gases increases with pressure. In general, nitrogen is much less soluble than oxygen (presumably because it is much less polar, and we are talking about solubility in water). This generally doesn’t pose a problem when descending, but can create problems on ascent when the gasses come back out of solution. These problems are known as decompression illness. A lot of effort has been expended on generating physiological models of how much nitrogen gets dissolved in a typical human body as a function of depth and time. In recreational diving, we generally don’t perform mandatory decompression stops (i.e., “no-decompression diving”). That means we have to limit how much nitrogen gas gets dissolved in our bodies, and diving with nitrox is a way to do that (see below).
Oxygen Toxicity: When breathing gases with oxygen partial pressures above 1.4 ata, even short exposures can have deleterious effects, mostly on your central nervous system (e.g., seizures, which would be catastrophic underwater). This is thought to be due to metabolic free radicals, but the exact biochemistry is unclear to me. The general recommendation is to plan dives so that the maximum partial pressure of oxygen never exceeds 1.4 ata. The tables I discuss below also provide some numbers up to 1.6 ata, for contigency (i.e., emergency) purposes. Generally, when dives are planned properly, one will run into nitrogen solubility problems before oxygen toxicity problems. (You can also just run out of breathing gas. No dive table will help you with that!)
“Nitrox” refers to nitrogen-oxygen mixtures that have more oxygen and less nitrogen than air (21% oxygen). Since nitrogen and oxygen are similarly narcotic, diving with nitrox will not prevent narcosis (by much). Rather, the purpose of diving with increased partial pressures of oxygen is to reduce the amount of nitrogen that can be dissolved into your body. This extends no-decompression limits at moderate depths. However, there is a tradeoff: oxygen toxicity becomes a problem when going deeper.
The purpose of reading these PADI dive tables is to let you stay within safety limits for nitrogen solubility and oxygen toxicity. Of course, you can use your dive computer to keep track of this stuff for you, but I think it’s really nice to be able to check its work, or have a backup if the computer stops working.
Below, I’ll go through three examples. Each example will have two dives separated by a surface interval. We’ll calculate both oxygen and nitrogen exposure (separately) using the following tables:
PADI air table (for calculating nitrogen exposure when breathing air)
- The enriched-air-nitrox (EAN) tables (for calculating nitrogen exposure when breathing nitrox):
- The DSAT tables (for calculating oxygen exposure and equivalent air depths, EAD):
Better versions of these tables are available from PADI for a small fee.
(The course I mentioned only covers up to 40% nitrox, which is just fine for recreational purposes. There are a lot of other details I am not covering here, like how to deal with equipment, analyze tanks, etc. This is not a replacement for a real course.)
Dive 1 Surface Interval Dive 2 time (min) 25 45 20 depth (feet) 100 0 90 gas mixture EAN32 air EAN36
- EAN32 is 32% oxygen/68% nitrogen
- EAN36 is 36% oxygen/64% nitrogen (and so forth)
Oxygen Exposure Analysis:
Dive 1 Surface Interval Dive 2 ppO2 (ata) 1.29 0.21 1.34 oxygen exposure (%) 15% 0% 15% total oxygen exposure (%) 15% 15% 30%
- ppO2 is the partial pressure of oxygen in ata
- oxygen exposure is given as a percentage of your maximum allowable 24-hour-long oxygen exposure
The purpose of this analysis is to ensure we don’t exceed the oxygen exposure safety limit.
To get the required numbers, look at the DSAT tables. First, look at the “equivalent air depth table for enriched air.” At 100 feet, EAN32 gives an oxygen partial pressure of 1.29 ata. At 90 feet, EAN36 gives a partial pressure of 1.34 ata. Then, look at the “oxygen exposure table for enriched air.” At an oxygen partial pressure of 1.3 ata, being exposed for 25 minutes (rounding up) is 15% exposure. Similarly, exposure to 1.4 ata (again rounding up) for 20 minutes is another 15%, making for a total of 30%. (There doesn’t seem to be any credit given for surface intervals.)
Dive 1 Surface Interval Dive 2 no-decompression limit (min) 30 n/a 21 residual nitrogen time (min) 0 n/a 19 actual bottom time (min) 25 45 20 total bottom time (min) 25 n/a 39 ending pressure group O G S
- The no-decompression limit (NDL) is the maximum amount of time that can be spent at the given depth using the specified gas mixture.
- The residual nitrogen time accounts for dissolved nitrogen from previous dives. This is added to the actual bottom time to get the total bottom time.
- The amount of dissolved nitrogen in your body is specified as a “pressure group,” with earlier letters like A and B representing less dissolved nitrogen.
The purpose of this analysis is to ensure we don’t dissolve too much nitrogen into our body, which would come out of solution on ascent and could cause decompression injuries. When you perform the analysis I go through here, you will see how close you are to the edges of the table, where the safety limits are clearly marked.
Dive 1: Consulting EAN32 Table 1, we see that diving for 25 minutes to 100 feet takes us to pressure group O (after rounding up). At this depth, a safety stop is required (which is why the column is gray).
Surface Interval: Go across Table 2 horizontally. After 45 minutes, the new pressure group is G. (G comes earlier than O in the alphabet, indicating less dissolved nitrogen now that some time has passed.)
Dive 2: Switching to EAN36 Table 3, we see that diving to 90 feet starting from pressure group G gives a residual nitrogen time of 19 minutes and a no-decompression limit of 21 minutes. (Curtis says it’s a good habit to switch to the new table at this step.) The actual bottom time is 20 minutes, making for a total bottom time of 39 minutes.
Switching to EAN36 Table 1, we see that diving to 90 feet for 39 minutes takes us to pressure group S. This is one minute short of the NDL, which means we are diving within the table limits. However, this section is gray, which means we will need to make a safety stop (which you should do anyways).
I’m providing two more examples here, but with less detail so you can try working it out for yourself. Note that you will need the EAN32 and air tables for this problem.
Dive 1 Surface Interval Dive 2 time (min) 24 1:05 20 depth (feet) 110 0 70 gas mixture EAN32 air air
Oxygen Exposure Analysis:
Dive 1 Surface Interval Dive 2 ppO2 (ata) 1.39 0.21 0.66 oxygen exposure (%) 20% 0% 5% total oxygen exposure (%) 20% 20% 25%
Dive 1 Surface Interval Dive 2 no-decompression limit (min) 25 n/a 25 residual nitrogen time (min) 0 n/a 15 actual bottom time (min) 24 1:05 20 total bottom time (min) 24 n/a 35 ending pressure group P E Q
Sometimes, the nitrox blend you get won’t be a convenient 32% or 36%. This could just be due to the blending process, or perhaps you want a special blend for a particular depth. (There are some simple formulas for calculating the optimal blend for a given depth, but I won’t be discussing them in this post. It just involves finding the mixture whose maximum operating depth equals your desired depth.)
For these cases, including the problem below, you will need to use the equivalent air depth table so that you can use the air table as if it were a nitrox table. Notice that the equivalent air depth (EAD) is less than the actual depth, which is the whole point of using nitrox!
So, find the equivalent air depth for each part of the dive, and then use the air table as we did above.
Dive 1 Surface Interval Dive 2 time (min) 28 1:00 30 depth (feet) 100 0 80 gas mixture EAN34 air EAN40 equivalent air depth (feet) 78 n/a 53
Oxygen Exposure Analysis:
Dive 1 Surface Interval Dive 2 ppO2 (ata) 1.37 0.21 1.37 oxygen exposure (%) 20% 0% 20% total oxygen exposure (%) 20% 20% 40%
Dive 1 Surface Interval Dive 2 no-decompression limit (min) 30 n/a 38 residual nitrogen time (min) 0 n/a 17 actual bottom time (min) 28 1:00 30 total bottom time (min) 28 n/a 47 ending pressure group P E S
Nitrox is a useful tool for increasing potential bottom times at moderate depths. It increases the risk of oxygen toxicity, and this risk must be managed. Nitrox does not significantly reduce the risk of narcosis (and is generally not used at depths where this is an issue).
The PADI recreational dive planner and associated tables can be used to evaluate the oxygen and nitrogen limits for a prospective or historic dive. This process is a useful alternative to using a dive computer and is generally helpful for understanding what dive computers are doing.
If you are interested in learning more, contact the good people at Sin City Scuba! They are extremely friendly, very well-trained, and will be happy to answer all of your questions.