Decompression dives allow experienced scuba divers to explore deeper sites and wrecks, but require specialized training, equipment and careful planning.
This comprehensive guide will provide you with a detailed overview of everything you need to know before attempting decompression dives.
What is a Decompression Dive and Why is it Risky?
A decompression dive is any scuba dive where you descend deeper than 60-100 feet for longer than 10-15 minutes.
Under these conditions, enough nitrogen from your breathing gas dissolves into your body tissues that it cannot safely be released by just ascending normally to the surface.
Instead, staged decompression stops are required during your ascent to allow the excess nitrogen to gradually offload safely.
Without these stops, the dissolved nitrogen forms bubbles as you ascend, which can cause decompression sickness (DCS), known as “the bends“. DCS symptoms range from joint pain and rashes to paralysis and death.
Decompression dives carry inherent risks, including:
- Decompression Sickness (DCS) – Caused by nitrogen bubble formation from too rapid ascents. It can be mild to life-threatening.
- Nitrogen Narcosis – Impaired judgement and coordination from nitrogen under pressure. Worsens with depth.
- Oxygen Toxicity – Convulsions and blackouts from too much oxygen at high partial pressures. A risk on long, deep dives.
- Gas Supply Issues – Running out of breathing gas due to higher consumption at depth. Requires larger tanks and planning.
- Equipment Failures – Higher risk of issues like regulator free-flows at extreme depths. Redundant backup systems are essential.
- Emergencies – On deep dives, problems must be solved quickly with little margin for error. Training and skills are critical.
Proper training, gases, equipment, and procedures can effectively minimize these risks, but decompression diving remains one of the highest risk activities in recreational scuba. Never attempt it without proper training and experience.
Decompression 101: How and Why Nitrogen Builds Up
To understand why decompression dives require special precautions, you need to first understand how nitrogen uptake and release works in the body.
Nitrogen is an inert gas that makes up about 80% of the air we breathe at the surface. Under the high pressures underwater, significantly more nitrogen from your breathing gas dissolves into your body tissues.
The deeper you go, the more nitrogen is absorbed.
This dissolved nitrogen is then carried by the blood flow to tissues throughout your body – muscles, fatty tissues, organs, bones, and blood. Different tissues absorb nitrogen at different rates based on their perfusion.
As long as you maintain depth, the nitrogen uptake reaches an equilibrium between absorption and release.
On ascent, the reducing water pressure means less nitrogen can be held in the solution, similar to opening a soda can.
If you ascend too quickly, the excess nitrogen comes out of the solution as bubbles rather than diffusing naturally.
These bubbles in the tissues and blood can block blood flow and damage cells, causing DCS. This is why controlled, staged decompression stops are so important.
Decompression Stops Clearly Explained
Decompression stops serve to eliminate excess nitrogen gradually and safely during dives where significant gas loading occurs.
There are two types:
- Should be made during all dives for prudence at 15-20 ft for 3-5 minutes.
- Help decompress fully when near no-stop time limits.
Required Decompression Stops
- Mandatory when no decompression limits (NDLs) are exceeded.
- Done at specific depths/times to keep nitrogen loading within safe margins.
- Calculated by dive computers and tables based on depth, time, and gases.
- Failing to make required stops risks DCS illness.
On a decompression dive, you first descend to your maximum planned depth. You then ascend gradually making all required decompression stops as calculated.
These stops are typically made for between 1-15 minutes at depths between 15-60 feet, but vary per the dive profile. Never omit required stops and follow your computer or tables precisely.
The deepest stops help flush nitrogen from fast tissues like blood and muscles. The shallow stops help release nitrogen from slower tissues like fat and bones.
Decompression stops are mandatory and substantially increase dive times. But they allow divers to return safely after pushing depth and bottom time limits.
Decompression Dive Planning Essentials
Decompression dives demand greater planning, gas supplies, and experience. Key factors to consider include:
- Increased breathing rates at depth consume more gas.
- Deco stops increase dive times, using more gas.
- Multiple decompression tanks may be needed for longer dives.
- Choose tank sizes to allow ample reserves at all depths.
- Limit depth and bottom times based on dive computer or tables.
- Plan conservatively – ascend early to minimize decompression obligations.
- Allow adequate surface intervals between repetitive dives to offload nitrogen.
- Check tables or computer to determine required stop times/depths.
- Add reserves for contingencies – delays, currents, low gas, etc.
- Inform shore contacts of expected surface interval.
- Record runtimes and gas consumption to refine future dive plans.
Backup Gas Supplies
- Carry redundant decompression gas supplies in case of primary regulator failure.
- Stage additional decompression gas tanks along the ascent line where required.
- Plan sufficient reserves to cover longer run times, delays, and emergencies.
- Review responses e.g. blown safety sausage deployments, buddy breathing ascent.
- Ensure the ability to deploy surface marker buoy from any depth.
- Have oxygen and first aid kit on site for treating DCS.
- Know emergency contact procedures if a diver is lost or incapacitated.
Mandatory Decompression Diving Equipment
Decompression dives require specialized equipment configurations to stay safe.
Multiple Large Gas Tanks
- Standard 80 cf tanks drain quickly at depth. Larger 120+ cf sizes are required.
- Multiple staged decompression tanks may be needed to complete longer profiles.
- Use multiple first stages with isolator valves for gas switching during ascent.
- Must be capable of calculating required decompression stop depths and times.
- Set conservatively and follow stops precisely – never omit required stops.
- Have backup/redundant computer in case of failure.
- Carry backup regulators and hoses in case of primary failure at depth.
- Ensure all regs are environmentally sealed against free flows.
Reels, Lift Bags, Sausage
- Use ascent reels and surface marker buoys for easy ascent line deployment.
- Lift bags and Sausage for emergency buoyancy if needed.
- Audible and visual emergency signals like whistles, strobes, and dyemarkers.
- Allows for getting nearby divers’ attention quickly if an issue arises.
- Oxygen kit, first aid, and fluids on site for managing DCS.
- Boat tenders should be aware of dive plan and prepared to assist divers.
- Emergency contacts designated in case injury incapacitates divers.
Common Decompression Diving Mistakes to Avoid
While following proper procedures is critical on all dives, it becomes even more critical when doing decompression dives.
Some common mistakes to be aware of and avoid include:
- Skipping decompression stops or reducing stop times
- Ascending too fast and exceeding maximum ascent rates
- Poor gas planning leads to low tank reserves during decompression
- Failure to monitor depth and time closely
- Repeating dives without full offgassing surface intervals
- Diving too deep or too long beyond the experience level
- Not monitoring gas supply levels closely during the ascent
- Diving with faulty dive computers or tables
- Ignoring DCS warning symptoms and failing to abort dives
- Diving without redundant air supply and safety equipment for emergencies
- Panicking and bolting for the surface if issues arise
- Pushing limits on depth, bottom times, and conducting repeat dives
Decompression diving leaves far less room for errors of judgement. Maintaining depth discipline, monitoring gas closely, and making safety the priority on every dive is critical.
Decompression Diving Certification and Training Tips
Given the knowledge and skills required, specialized training is needed to dive beyond recreational no-stop limits.
1. Get Proper Deco Certification
- PADI, NAUI, TDI, IANTD, and others offer decompression diving courses.
- Courses cover planning, physiology, tables/computers, gas mixes, and procedures.
- Courses may require prerequisite experience levels e.g. PADI Deep Diver.
2. Start Slowly and Within The Limits
- Begin with conservative dives under close supervision.
- Limit depth and times until your skills and reactions are proven.
- Gradually extend the depth and bottom times over multiple dives.
- Abide by computer/tables conservatively – follow required stops meticulously.
3. Practice Emergency Procedures
- Rehearse gas-sharing ascents, buoy deployments, and managing freeflows.
- Practice removing and replacing gear at depth in case of issues.
- Learn how to provide first aid and oxygen for dive injuries.
4. Refine Buoyancy and Gas Skills
- Deco diving allows little margin for error – skills must be solid.
- Practice controlled ascents and descents to improve depth control.
- Monitor gas closely and switch tanks early to avoid surprises.
5. Maintain Fitness and Avoid Fatigue
- Long decompression dives can be physically and mentally exhausting.
- Arrive well-rested and hydrated. Avoid pushing limits when tired.
- Maintain fitness – diving is strenuous at depths.
Decompression Dive Planning Example and Tutorial
To tie the concepts together, let’s walk through planning a sample 130 foot decompression dive to explore a shipwreck.
Depth and Time Limits
- 130 foot maximum depth, bottom time 25 minutes allowable with gas mix.
- Adds 35 minutes of decompression time. Total 60 minute planned dive.
- Use EAN32 for the descent and bottom portions of the dive.
- Switch to EAN50 deco gas during initial decompression stops.
- Finish decompression on pure oxygen between 20-10 feet.
- 2x 120 cf tanks of EAN32 bottom mix – allows 50% reserve.
- 1x 80 cf EAN50 stage tank for stops between 70-30 feet.
- 1x 40 cuft oxygen for final stops.
- Follow the computer which requires stops as follows per the planned profile:
- 70ft for 8 mins
- 60ft for 10 mins
- 50ft for 10 mins
- 40ft for 8 mins
- 30ft for 8 mins
- 20ft for 5 mins
- 15ft for 8 mins
- 10ft for safety stop 3 mins
Ascent and Descent Rates
- Maximum descent rate 100 feet/min to limit nitrogen loading.
- Maximum ascent rate 30 feet per minute during decompression.
- Normal ascent rate 60 feet/min after all decompression stops completed.
- Have oxygen, and first aid available on site.
- Ensure DCS contingencies are understood by all dive team members.
- Ensure all signaling devices, buoys, and backup regulators are functioning.
- Review emergency ascent procedures and assigned roles.
Following this level of careful planning including gas volumes, conservatism on time limits, strict decompression stops, and emergency readiness helps minimize the heightened risks of decompression diving.
Never take chances or push limits on these types of dives.
While pursuing deeper dives can unlock amazing new underwater possibilities, they require assuming substantially higher risks. Specialized training, equipment, planning, and strict adherence to procedures are mandatory. Never attempt to self-train or cut corners.
By working carefully within your experience level under supervision, you can progressively gain the skills to explore the frontiers of decompression diving safely. But always remember that the ocean remains an unforgiving environment. Staying conservative, vigilant, and focused on safety is critical to enjoying these demanding dives.