Most scuba divers will have an Open water or maybe an Advanced Open water certification allowing them to dive to depths up to 30 meters. All of these dives are classified as recreational dives, meaning that the diver should not exceed the No-Decompression Limits (otherwise known as NDL or no-stop limits) set by either a dive computer or dive table. So, if that’s a no-decompression dive then what IS a decompression dive?
What is a Decompression Dive?
A decompression dive, put simply, is a dive that is planned with the intent of exceeding the no decompression limit of a dive computer or dive table. These types of dives are usually classified as technical diving, whereas staying within the NDL limits is called recreational diving. Decompression dives will start at a depth of below 40 meters, as the maximum depth of recreational diving is 40 meters.
Read More: How Deep Can You Dive? What Prevents Divers from Diving Deeper than 40 Meters?
Table of contents
- What is a Decompression Dive?
- What is a Decompression Stop?
- The Difference between Safety Stop and Decompression Stop
- When and Why do you need to do a Decompression Stop?
- How to Calculate Decompression Stops
- How do you Decompress when Diving?
- What Happens if a Diver doesn’t Decompress?
- What do Divers do during Decompression?
- How Deep can you Dive without Decompression?
- Training for Decompression Diving
- My Dive Computer says “DECO” what do I do?
- Conclusion
What is a Decompression Stop?
Due to the excess amount of nitrogen accumulated during a decompression dive, it is no longer safe to ascend directly to the surface at any point during the dive, as is the case with a recreational dive. To allow the body time to release the excess nitrogen from the body (and prevent decompression sickness) a decompression stop must be completed.
These are just like a safety stop in recreational diving. Decompression stops may need to be completed at various depths and for differing amounts of time depending on length and depth of dive.
Read More: What is Decompression Sickness?
The Difference between Safety Stop and Decompression Stop
- Safety stop;- A safety stop, while recommended on dives, is not required to be completed. If there is an emergency situation that requires an ascent directly to the surface during a dive, it is completely safe to do so without a safety stop. Recreational dives are considered no-stop dives as you do not need to ‘stop’ on your ascent. However, a safety stop is always planned to be executed at the end of a dive at around 5 meters for 3 minutes before surfacing.
- Decompression stop;- A decompression stop is performed when a diver has exceeded a NDL. Whereas, a safety stop is usually 3 minutes long, a decompression stop will be longer and mandartory. Depending on the dive, multiple stops may be required and at different depths.
When and Why do you need to do a Decompression Stop?
We use the Recreational Dive Planner (RDP) or dive computers to calculate the no-decompression limit, and as recreational divers, we adhere to the limits. A decompression stop or ‘deco stop’ must be completed any time a diver exceeds a NDL, whether planned or not.
Due to the amount of nitrogen in the body accumulated on longer deeper dives, a longer stop is needed to allow time for excess nitrogen to leave the body.
How to Calculate Decompression Stops
When planning a dive that involves going into deco, it is very important that decompression stops are calculated, to ensure a safe enjoyable dive.
As mentioned, going into deco usually involves either deep dives, long dives or both. These types of dives fall into the category of technical or tec dives.
Due to a number of variables on these types of dives, such as the type of gas used (nitrox, helium, 100% O2) the length and depth of stop can be very different on each dive. Diving software has been developed to assist in calculating and planning deco stops. This is something that would be explained in great detail once any technical diver training is undertaken.
During a recreational dive we would not need to calculate a deco stop as we would plan to stay within our NDLs.
However, if a no-decompression limit is accidentally exceeded, then an emergency decompression stop would be required.
Below is a table showing the recommended deco stop times based on the recreational dive planner. These numbers may vary if using a dive computer.
| Exceeded NDL by | Minimum Decompression Stop | Time Before Next Dive |
|---|---|---|
| Less than 5 Mins | 8 Mins | 6 Hours |
| More than 5 Mins | 15 Mins | 24 Hours |
How do you Decompress when Diving?
Compression and Decompression is all based upon absorbing and releasing gases (such as nitrogen) with increased or decreased pressure changes caused due to changes in depth. The more depth you go to the quicker you absorb gases, so the method to release these gases is to ascend to a shallower depth.
You can see from the following table that the deeper we are the less NDL we have, This is because the deeper we are the more pressure we are under, and therefore absorb nitrogen quicker. These numbers are taken from the recreational dive planner
| DEPTH (meters) | NO DECOMPRESSION LIMIT (NDL) (minutes) |
|---|---|
| 10 | 219 |
| 12 | 147 |
| 14 | 98 |
| 16 | 72 |
| 18 | 56 |
| 20 | 45 |
| 22 | 37 |
| 25 | 29 |
| 30 | 20 |
| 35 | 14 |
| 40 | 9 |
| 42 | 8 |
To decompress, we need to reduce the amount of pressure that is acting on our bodies. This is done by ascending to a shallower depth on the dive, then staying at this depth until enough nitrogen has been released before making your way to the surface or next depth to complete further deco stops.
Here Comes Some Science, You’ve Been Warned!
Little bit of physiology now.
When we descend and stay at depths underwater, our bodies are in the phase of ‘on-gassing’ and absorbing nitrogen. The extra pressure is forcing nitrogen bubbles into the blood system once we have inhaled them through our lungs.
You can compare this in terms of how fizzy drinks are made; the syrup of the drink is your blood and the carbonated bubbles are nitrogen. These bubbles are then forced into the syrup with pressure where they stay until the pressure is changed by opening the drink.
So what is happening in the body as we decompress (or off-gas) and slowly ascending, can be compared to opening a fizzy drink bottle very slowly. This allows the gas to leave slowly and controlled, and once enough gas has escaped it is then safe to take the lid off fully without spilling the drink.
This is similar to what is happening to our bodies while we decompress. The gases are leaving our system in a safe and controlled manner before we exit the dive. If we decompress too fast or not at all, that can lead to diving injuries.
What Happens if a Diver doesn’t Decompress?
Failing to decompress can lead to what is called DCI, a Decompression Injury.
There are many symptoms that can be displayed if a person is suspected of having DCI. The cause of DCI will be the fact that there is an excess amount of nitrogen still left in the body.
While on the way to the surface, even at a slow ascent speed, the body does not have enough time to release all the excess nitrogen, which may cause bubbles to form in the blood system or other parts of the body. Depending on where the bubbles form can cause different problems.
Symptoms of DCI
- Headache
- Nausea
- Chest Pain
- Breathing Problems
- Unconsciousness
- Joint Pain
- Extreme Fatigue
- Rash on the skin
- Paralysis
- Death
With most of these symptoms the cause is too much nitrogen in the blood system or the bubbles are blocking blood flow. Some symptoms, such as joint pain or paralysis mean that the bubbles have formed at a joint or in the spinal cord.
While this all seems very scary, quick recognition, administering emergency oxygen and seeking medical assistance mean that usually these problems are not long lasting. To prevent any of this happening, always follow your dive plan and stay within the limits of that plan. Don’t push your limits as this is usually when accidents happen.
Read More: Why is Decompression Sickness called ‘The Bends?’
What do Divers do during Decompression?
Some decompression stops can take many many hours to complete. Sadly while underwater, we don’t have the luxury of WIFI internet, so binge watching your favourite Netflix show just isn’t an option to pass the time.
Divers have now found many different ways of entertaining themselves on long deco stops, from playing hangman on a slate, having a few rounds of noughts and crosses (tic tac toe) or even playing games on diver computers.
Depending on the dive location, a deco stop can be a really good time to look for all the small macro life on the reef or to spend some time turning your brain off from real world stress and just enjoy the calm relaxing feeling of being underwater.
Some of the more technological advances mean that it is also possible to now have underwater MP3 players, so audiobooks and music are also good ways to pass the time, or even a game of chess on a magnetic board. As you can see there are many different ways to make those long deco stops a little more interesting.
How Deep can you Dive without Decompression?
The recommended maximum depth for recreational diving is 40 meters / 130 feet. Diving to this depth would require training by way of a specialty deep diver course. Staying within this depth range means there are no decompression stops required at any point.
If you do want to dive deeper than this depth, this is where you will be entering technical diving and when making decompression stops will be mandatory.
Training for Decompression Diving
Just as with recreational diving, technical diving has many different agencies to choose from. Padi Tec Rec, SDI, TDI and RAID are just a few. Each of these technical agencies will provide courses for various different depths such as 40m, 50m, 60m.
Part of this training involves carrying more than one tank on a dive, either on a sidemount setup or a twinset. Due to extended decompression stops (some lasting for hours) more gas is needed to allow for the time to complete the stops.
Next up for decompression diving would be using Nitrox (enriched air – where the percentage of oxygen is higher than 21%) all the way up to 100% oxygen or even using trimix (a blend of breathing gas consisting of oxygen, helium and nitrogen).
Using these gas mixes reduces nitrogen loads and helps to off gas quicker at deco stops. If you’re interested in learning about decompression diving, consult your local technical diving centre for more information.
My Dive Computer says “DECO” what do I do?
First thing, like in any situation in diving, is to not panic.
The benefit of diving with a dive computer is that it will tell you what you need to do! The computer is saying deco as you have accidentally exceeded the NDL limit and now require a deco stop. Luckily for us, with technology the computer will tell you what depth you need to ascend to, and for how long you need to stop there.
To fully understand your computer and all the information being given to you, it’s important that you always fully read your manual before use.
Conclusion
At the start, we asked what is a decompression dive? Hopefully from reading this article you now have a much better understanding of a deco dive and why deco stops are important. Going beyond a NDL isn’t a problem as long as you have the training, equipment and planning to successfully complete the deco stop on the dive. Being able to undertake decompression dives will open up a whole new world of diving to you and unlock parts of the ocean you were unable to reach before.
