The 3 phases of a dive

There are three phases we go through during each dive: Compression, Isopression, and Decompression.

Keep reading to learn more about each of them, what happens during each phase and what you should look out for. At the end of the article, I also explain two additional phases that are often neglected but super important to plan.

Even though I do list a lot of potential dangers in this article, if you stick to the rules and techniques from your Open Water Diver course, you will be just fine! 😃

What are the 3 phases of a dive?

The three main phases of a dive are the Compression phase, the Isopression phase, and the Decompression phase. They are named after the pressure ratio we experience during each of them.

In the following, our trusty Social Diving will be with us to make it a little more visual.

In addition, I like to add the Preparation phase and the Debriefing phase to my personal dive planning because they are equally important but often forgotten or taken too lightly.

Why should you know about the phases of a dive?

“Plan the dive, dive the plan” is probably one of the first rules you heard during your Open Water Diver course, and rightfully so.

Only with a solid dive plan, you can ensure that you and your dive buddy stay safe throughout the dive and can enjoy your time underwater without problems.

Therefore, it is important that you understand the differences between them, know what happens to your body and dive equipment and how it all relates to your safety during a dive.

1 Compression Phase

• Duration: Short
• Tank pressure: High (200 to ~170bar)
• Descent rate: Medium to fast (~10-20m / minute)
• Biggest challenges: Proper equalisation technique
• Important reminder: Include a “bubble check” or “control stop” at 3-5 meters depth

Every dive begins with the compression phase. As we descend, the water pressure rises, thereby compressing our dive equipment and body.

This is governed by the law of Boyle-Mariotte which you can review in my diving gas laws guide.

Remember to always equalize properly to prevent barotrauma in your ears, mask, or other air-filled cavities in your body!

We start off by descending together with our dive group, dive buddy, or guide and try to get to the target depth as quickly as possible to conserve air and keep the Nitrogen load as small as possible.

As you learned in your beginner course, it is important to include a control stop, sometimes referred to as a “bubble check”, during which each dive buddy makes a 360° turn underwater so their buddies can check for air leakages, missing or faulty equipment and everyone can get their buoyancy right.

On most recreational dives, you should arrive at your maximum operating depth (MOD) with at least 170 bars left.

This might not always be possible, especially at oddly shaped reefs, but it is a good target number.

2 Isopression Phase

• Duration: Medium-long
• Tank pressure: Medium (180 to ~ 100bar)
• Ascent/Descent rate: Low to none
• Biggest challenges: Proper buoyancy, constant pressure monitoring, gas intoxication
• Important reminder: Don’t dive below your previously determined MOD (target depth) and check your pressure gauge regularly

During the Isopression phase, the pressure ratio remains constant and refers to the time you spent at the target depth without ascending or descending much.

The biggest challenges here are mostly related to proper diving technique, such as keeping good buoyancy, navigating underwater, and keeping an eye on your pressure gauge.

Your dive computer will display your current non-decompression time that you should never exceed.

As we spend time at depth, our breathing gas saturation rises and we may be exposed to Nitrogen narcosis when diving deep.

It is important to remember to keep a good margin of air for the following Compression phase to prevent “low on air” or even “out of air” emergency situations.

3 Decompression Phase

• Duration: Long
• Tank pressure: Medium-Reserve (100 to ~ 50bar)
• Ascent rate: As low as possible (10m/min below 10meters depth, 5m/min above)
• Biggest challenges: Slow ascent rates, barotrauma, DCS
• Important reminder: Ascend as slow as possible and do a “safety stop” for at least 3 minutes at 5 meters

On many dives, the decompression phase is the most critical phase as we ascend and need to adhere to our decompression procedures.

As you learned during your Open Water Diver course, we need to let our body lower its Nitrogen concentration slowly in order to prevent Decompression Sickness (DCS).

Therefore, the slower we ascend, the better.

Many studies show that 10 m/minute below 10 meters depth and 5 meters/minute above are optimal ascent rates during recreational diving.

This is also used on many dive tables, and recommended by training agencies like iac or CMAS.

Most dive computers implement dynamic ascent rates that allow for even faster ascents at deeper depths, however, I recommend sticking to those rates I mentioned above.

Preparation & Debriefing

I would like to remind you that the dive starts way before you ever get into the water and does not end when you took off your fins.

For a safe and fun dive, we need to plan ahead and debrief afterward.

Good dive preparation includes a briefing of the dive site, a weight check when diving at a new location, and important information such as dive time, maximum depth, and a review of hand signals and emergency procedures.

During the debriefing, we review the dive, how it went, and anything we noticed underwater that we want to improve the next time.

Of course, this is also a great moment for logging our dive. 😉

The more effort you put into these two phases, the better the dive will be!

Conclusion

I hope you liked this short refresher of the 3 phases of a dive.

The 3 phases of a dive are Compression, Isopression, and Decompression phase. Each of them comes with unique challenges and it helps to know what happens in each of them.

Don’t neglect proper planning before and a debriefing after the dive.

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Always dive with friends and happy bubbles. 😃

Cheers

Julius