No limits? No, limits…

What limits your time underwater while diving? Most certified divers would answer that it’s how much air is left in their tank, but would forget the other part of the equation. This other limiting factor that must be considered is nitrogen load, or the amount of nitrogen absorbed by the body during a dive.

All divers learn about this in their certification course, but many delete the mathematically traumatic experience from memory (remember the infamous “dive planning table”). Owing to its density and clutter, the table is an intimidating set of information to digest. Many divers complete their course without ever truly understanding its implications. To be fair, it’s a difficult concept to teach, which is why many dive instructors breeze over it, opting instead to stress using a computer.

The dive computer is one of the greatest innovations in recreational diving. Used properly, it simplifies decompression theory to a few basic rules. But it is also one of the most misleading, misunderstood and misused tools in the diver’s gear bag and even proper use doesn’t guarantee avoiding decompression sickness.

Recreational diving: Why we pay attention to nitrogen load…

The vast majority of divers are defined as recreational divers, which means they practice ‘no-decompression diving’. This means that the diver should stay within the established limits of depth and time set forth by the dive table or a dive computer. Doing so ensures the nitrogen absorbed by the body remains below the (*industry accepted) amount that would require a decompression (DECO) stop to safely release some of that nitrogen before surfacing. (I recently read that most dive computers on the market accept a 2-5% probability of decompression sickness…)

The amount of nitrogen absorbed increases with depth and time underwater, meaning the more time at depth, the more nitrogen absorbed. Upon ascent, the drop in pressure allows that nitrogen to dissolve back out of the tissues and to escape. This happens along a sliding scale of safety.

What is Decompression Sickness?

The safe method of nitrogen release is a relatively slow process in which the nitrogen dissolves out through the blood reaching our lungs, and it’s exhaled. The more risky way is a quick release in which the nitrogen in the system isn’t given time to be exhaled, so it forms bubbles and escapes anyway it can. This can lead to decompression sickness (DCS), known commonly as the ‘bends’. DCS can manifest as a variety of medical issues from an inflammation of the skin (skin bends) on the mild side of the spectrum, to severe neurological symptoms at the most serious end.

If we exceed the commonly accepted limits of how much nitrogen our bodies can absorb, then we are strongly encouraged to decompress during ascent to avoid decompression sickness (DCS). This means we do DECO stops (we stop on the way up to allow the nitrogen to dissolve out slowly). In my eyes, one of the great faults of the recreational diving training agencies (PADI, SSI, etc.) is the way in which these emergency protocols are covered in the introductory courses.

Most organizations treat decompression protocol as a black-area, basically saying “just don’t do it”. While this advice is sound, what they don’t teach well are the real implications of nearing and surpassing the limits, finding yourself in DECO, and how to handle it if you do find yourself there.

The truth about DECO is that in itself, it’s not necessarily dangerous. Plenty of divers regularly put themselves in decompression (think commercial divers) and never experience negative effects. The highest risk comes from ascending without completing the decompression protocol. This typically happens because people…

a) don’t realize they’ve put themselves into DECO

b) don’t understand how to follow the DECO protocols and/or ignore them or

c) don’t have sufficient air to complete the DECO protocols.

So how do we avoid DECO, and what do we do if we find ourselves there?

Avoiding DECO

If you are using a computer, it is very easy to avoid DECO; you just have to know what to look for. Dive computers serve many functions, but the most important one is to tell you how much time you have left before you need to start ascending. This time limit is calculated by the computer using current dive information (depth and time throughout the dive), recent previous dive information (depths and times of recent dives and surface intervals), and gas mixture. It is commonly referred to as the NDL (no-decompression limit) or the no-stop time.

While the computer is useful for communicating depth, dive time, water temperature and an array of other information, the NDL is the critical information, and the most important reason to use a dive computer. This number can be identified as the countdown timer on a dive computer typically denoted by the symbol ‘NDL’, ‘NO DEC’ or ‘NO STOP TIME’. As you descend, this number decreases, and as you ascend it increases. As long as this number does not wind all the way down to zero, you will stay out of DECO according to the algorithm employed by the computer you’re using. 

Prudent advice is to not “ride your NDLs”, meaning don’t wait until your NDLs read 5 minutes or less to start ascending. A good way to stay away from DECO is to always try to maintain at least 5-10 minutes of NDL on the computer. This way if you become distracted and forget to check, you still have a significant buffer until you reach 0. I like to teach this by comparing NDLs to how much air is left in your tank; we don’t wait until the tank is completely empty to start ascending, we give ourselves a conservative buffer. Sometimes mistakes happen though.

Managing DECO

So you ignore your computer long enough, and when you finally check, it shows something you haven’t seen before and is probably beeping or flashing. The computer is trying to communicate that you have surpassed your NDL (you have spent more time than you were allowed at your current depth). The first step is, don’t freak out. Stay calm, signal your buddy and divemaster, and show them your computer. One of the common signs for ‘DECO’ is to hold up your pinky finger and wiggle it, but you and your buddy must discuss these signals before the dive.

The next step is to follow the instructions given by the computer and begin your ascent. As you begin to ascend, remember to maintain a slow ascent rate of no more than 60 feet per minute (1 foot per second) and much slower if possible (a rough rule of thumb is to allow all of the smallest bubbles from your exhale to ascend more quickly than you).

Depending on your computer, it will denote the need for a decompression stop with the symbol ‘ASC TIME’, ‘DECO TIME’ or ‘STOP TIME’ followed by the number of minutes needed for the stop and the ‘CEILING’, or the depth below which you should spend that time. All computers are different; you must read the instructions and understand your computer before using it, even the parts about DECO.

After completing the steps prescribed by your dive computer or by the dive planning tables, you are then technically out of DECO. From there it’s prudent practice to refrain from diving again for at least 24 hours if not more.

DECO or not, DCS is possible

It is critically important to note that regardless of whether you go into DECO or not, you can still get decompression sickness; and if you have a mild case of it which you don’t get diagnosed, diving more can worsen the symptoms rapidly. If you recognize any of the these symptoms in the hours following any diving, you should assume that it is diving related and consult a physician with knowledge of hyperbaric medicine. Early detection leads to better care and helps to prevent worsening the condition.

• localized itching

• tingling

• numbness

• joint pain

• blotchy or mottled bruising

• confusion

• shortness of breath

Decompression Diving Glossary