Decompression Theory-Bubble Counter

Decompression Theory Part 3- Bubble Counters

A big selling point of technical diving is that there is always more to learn, absorb, practice and challenge oneself with.


How Logging Your Dives Can Make You a Better Diver

Here are a few items you can include in your logbook to help you stay organized and honest, track progress, and work on self-improvement as a diver.


Why are Prerequisites Important in Technical Diver Training?

Prerequisites can be found in the Standards and Procedures for any course you are interested in taking.

Things Divers Should Never Do

The following are sensible suggestions of things divers should never do, based entirely on common sense.

The Top 3 Finning Techniques and When to Use Each One

A diver should be able to move through the water using their fins as the exclusive means of propulsion to increase efficiency and minimize the impact to the environment.

Speeding up Decompression – Tips & Tricks

by Dr. Thomas W. Powell:

2 trimmed divers in deco

photo credit: Thaddius Bedford

For the technical diver, decompression is an event that cannot be avoided. To go deep, and stay longer on an open circuit rig, you are going to go beyond your no decompression limits. With this requirement being recognized, decompression periods can be tedious and long, depending on the type of diving performed by the diver. The reality is that an individual cannot reduce decompression stop times without altering a dive plan. To make decompression periods more enjoyable however, a diver can find various activities to pass the time in an efficient and useful manner.

Some ideas for expediting a decompression stop may include:

  1. The first action a technical diver can perform during a decompression stop involves working on skills. This is a good time to set a line marker as a visual reference and practice skills like hovering, trimming out, and even kick styles like the back kick. A decompression stop is a period during which a diver must stay at a certain depth on a certain gas. The objective is to stay at a specific depth, which makes hovering in a trimmed out position a great skill to practice. For this reason, the time should not be wasted, and it is a perfect opportunity to work on becoming a better diver.
  2. Second, a diver may enjoy certain modern advances in technology. This may seem excessive, but I was recently speaking to a dive professional and rebreather diver who watched his students on a decompression stop. They gathered iPads in waterproof cases, and from their decompression staging location, began to read. They had literally prepped their equipment to allow them to read books while waiting out a decompression cycle. This may seem to break from the idea of, “getting away from it all while underwater,” but it is an effective means of passing time during a decompression stop in an enjoyable manner.
  3. The third way for a diver to pass the time during a decompression stop is through exercise. Most divers recognize that strenuous exercise during a descent or dive can cause an elevated heart rate, increased blood pressure, and therefore and faster on-gassing. Conversely, the Divers Alert Network (2014) states that exercise during decompression stops can aid in the off-gassing of inert gasses. Essentially, light exercise can increase your body’s ability to decompress. Despite this factor, the exercise performed must be mild. The Divers Alert Network also states that too much exercise can cause bubble formation, similar to shaking a soda bottle. This bubble formation can be problematic, and put the diver at risk. A safe and effective example is swimming around the line while working on trim.
  4. Fourth, a diver can play a game with his or her buddy or team. This could be something pre-planned using wet notes or some form of game (like cards) left at the decompression stop location during descent. A game will focus the mind and pass time in an efficient manner. This type of activity can also allow for work on hovering skills. If the divers involved are trimmed out, holding cards, and monitoring the actions of others while maintaining depth, the divers are both having fun and practicing skills.
  5. This final method for speeding up decompression stops involves the ultimate activity for a technical diver. An individual can begin planning out a future dive. Essentially, the diver can even plan a future dive to the same location, planning times at depth, bailout procedures, and gas requirements on his or her wet notes. This action would actually make the best use of time and allow the diver to hone his or her skills at working tables and math problems on the fly, underwater.

Divers always seem to be looking for fun and excitement at every turn. This desire is magnified in technical divers. Decompression stops are a necessary break required any time a diver passes his or her no decompression limit, especially if this passage is performed on purpose in a technical plan. If a diver must take a deco stop, the time can be used in an effort to improve personal skills while enjoying his or herself. The reality is that a diver must determine what works best for his or her own personal needs. The goal is to be safe, enjoy the sport, and pass the time in the best manner possible.

Dr. Thomas W. Powell – Owner/Instructor Trainer (SDI/TDI/ERDI) – Air Hogs Scuba, Garner, NC

Choosing the Best Decompression Gas

By Jon Kieren

gas blending room

There are many factors that need to be considered when choosing decompression gasses for a dive. The dive profile, logistics, environment/site conditions, and personal preference all come into play; how do these factors affect our decision? First, we need to take a brief look at why we use different gasses for decompression to begin with, and then how the factors previously listed affect our gas choices. For big dives with extensive decompression obligations, it’s often a balancing act between oxygen exposure and off gassing.

Why switch gas anyway? This takes a brief lesson in decompression theory to explain; we’ll focus mainly on the off gassing portion of the dive. The rate of off gassing is related to the partial pressure within the tissues of the body and the partial pressure of the gas being breathed. When the partial pressure of the inert gas (mainly nitrogen and helium) in the lungs (the gas we are breathing) is LOWER than the partial pressure of the inert gas absorbed in our tissues, the gas will move from the area of high pressure (our tissues) to the area of low pressure (our lungs) and be expelled when we exhale.

There are two ways we can reduce the partial pressure of the inert gas in our lungs. First, is by ascending and letting Boyle’s law take over. As the gas expands as we ascend due to reduced ambient pressure, the partial pressure of the gas drops. This works but is not the most effective method. If we ascend too far or too fast and the ambient pressure decreases too rapidly, bubbles can form causing decompression sickness. The second method of reducing the partial pressure of the inert gas in our lungs is to reduce the fraction of the inert gas in our breathing mixture. In order to reduce the fraction of inert gas in the mix, we increase the fraction of oxygen. By switching to an oxygen rich gas on the ascent, we reduce the partial pressure of the inert gas in our lungs and increase the rate and efficiency of off gassing. So, more oxygen=less inert gas=faster/more efficient deco. Got it?

Okay, so if a higher fraction of oxygen is better for decompression, why don’t we just use 100% oxygen for the entire ascent? It would sure reduce our decompression times by a significant amount, wouldn’t it? Well, unfortunately we have to be cautious of the pesky oxygen free radicals caused by breathing high partial pressures of oxygen. If these oxygen free radicals are left to cause damage faster than the body can repair it, oxygen toxicity can become a serious concern. In short, the higher the oxygen content in the breathing gas, the shallower it must be breathed. As an example; for sport and technical diving applications, the maximum operating depth of oxygen is 6 metres/20 feet; and the maximum operating depth of 50% nitrox is 21 metres/70 feet. Here’s where we begin our balancing act.

We now need to consider the other factors that will affect our gas choice. First of all is logistics. What gasses are actually available? Many technical dive facilities have their decompression gasses pre-mixed, so you may be limited to what they have available or are willing to blend (gas blending can be a time consuming process). Also, there are many places in the world where 100% oxygen is not available, or can only be filled to roughly 150 bar/ 2000psi, depending on the fill station’s equipment. Once you know what your options are, you need to look a bit closer at the environment you’ll be diving in and how you will conduct your last decompression stop.

Many divers will vary the depth they plan to conduct their final decompression stop based on the environment they will be diving in. In a perfect world, we would always conduct our last stop at 3 metres/10 feet. Unfortunately, this is not a perfect world. Rough seas and overhead environments may make it difficult or impossible to conduct your last stop at 10 ft, so it may need to be conducted a bit deeper at 6 metres/20 feet. Conducting this last stop on 100% oxygen could now be problematic as you will be exposed to a much higher partial pressure of oxygen for the duration of the final decompression stop. Add rough seas to this in open water, and it could be very difficult to remain at a safe depth on oxygen. This is an instance where reducing the oxygen content may be wise. While a lower fraction of oxygen will not be quite as effective as a decompression gas on this final stop, it can significantly reduce the diver’s oxygen exposure. If you are making multiple gas switches in order to maximize the partial pressure gradient for the entire ascent, you will also need to look at the environment to decide what gasses to carry. A good example of this would be a cave dive. If you were planning your dive to switch to 50% at 21 metres/70 feet, but you know that there is a restriction in the cave at 21 metres/70 feet making it difficult to conduct a proper gas switch, you have a few options. First, would be carry the same gas, but decide to switch to it at a shallower depth where there is not a restriction. This would work fine, but would not be as effective for your decompression. You could also choose to bring a different decompression gas. A leaner nitrox mix could be switched to a bit deeper, but would not be as effective for the shallower stops. A richer nitrox mix would be more effective in the shallower stops, but you would not be getting the advantages of a decompression gas until later in the decompression schedule. Using desktop/mobile decompression software makes running these alternative options quick and easy so you can see immediately how your choice will affect your decompression plan.

After looking at all of the scenarios above, sometimes it just comes down to personal/team preference. Many divers and dive teams choose to use a standardized set of decompression gasses. This policy helps keep things simple and consistent. If a diver always carries 50% and oxygen for decompression, then they are always making gas switches at 21 metres/ 70 feet and 6 metres/20 feet. This standardized method streamlines the dive planning considerably, is consistent, and works well for many applications.

While this is not a complete discussion on decompression gas planning, it’s a good example as to what type of considerations we need to take into account when choosing our deco gasses. These points, along with others, are covered in depth in the TDI Decompression Procedures, Extended Range, Trimix, and Advanced Trimix courses and course materials. For more information on these courses, please visit TDI courses section

How To: Label Your Nitrox Tank

by Lauren Kieren

Nitrox Tank Wrap

Labeling and Identifying Nitrox Cylinders

Any time you fill a tank with nitrox, it must be identified as such. This will help to prevent accidents in the event that someone uses a tank filled with nitrox without taking the proper precautions. The industry standard for tanks filled with nitrox is to mark the tank with a tank wrap, as well as identify the mixture with a label or tag.

A nitrox tank wrap is an adhesive decal, generally 10-13 cm. (4-5 in.) in width, that is designed to completely encircle the diameter of the tank. The decal is usually printed in yellow and green with the word Enriched Air and/or Nitrox printed continuously on it in bold green or yellow letters. This tape makes it quick and easy to spot a nitrox bottle in a group of tanks.

TDI Contents LabelIt is standard practice to identify the actual nitrox mix currently contained within the cylinder and note it on a Contents label. This information can be found by analyzing, or watching someone analyze, the cylinder for oxygen content using a properly calibrated and functioning analyzer. At a minimum the information recorded on the label should include; oxygen content, maximum operating depth (MOD) of the gas you will be breathing, the name / initials of the person who analyzed the mixture, and the date it was analyzed.

After you analyze the cylinder and find the oxygen content, the rest comes pretty easy. The only additional “how to” necessary is calculating the Maximum Operating Depth (MOD).

To calculate the MOD for a specific ppO2 and percentage of oxygen (FO2) the following formula is used:

MOD = 10metres X [(ppO2/FO2) – 1]

For example, if the gas contains 36% oxygen (FO2 = 0.36) and the maximum ppO2 is 1.4 bar, the MOD (m) is 10 metres x [(1.4 / 0.36) – 1] = 28.9 metres

MOD = 33feet X [(ppO2/FO2) – 1]

For example, if the gas contains 36% oxygen (FO2 = 0.36) and the maximum ppO2 is 1.4 bar, the MOD (fsw) is 33 feet x [(1.4 / 0.36) – 1] = 95.3 feet.

The additional information on the Contents label concerning nitrogen and helium is primarily intended for use by certified TDI Technical Divers trained to dive with helium in their breathing gas mixtures.

Finally, it is important to note that although the industry standard is described and pictured here, some countries require different looking labels by law. A proper nitrox diver course will teach you these procedures and allow you to practice until you are comfortable analyzing and properly labeling a nitrox cylinder.