Gas analysis – the crucial first step
Before using a Nitrox cylinder, analyse its contents as taught in your training. The actual oxygen percentage may differ from the stated value, sometimes significantly.
Why is gas analysis so important?
- Variations in the fill: Even professional dive centres can make gas-blending mistakes.
- Impact on planning: An incorrect oxygen percentage leads to incorrect MOD and decompression calculations.
- Safety: An oxygen percentage that is too high or too low can create risks.
How is gas analysis carried out?
Gas analysis uses specialised portable oxygen analysers (also called O₂ meters). These instruments measure the oxygen percentage in the breathing gas with high accuracy.
Practical steps:
- Calibrate the analyser: Check and calibrate it with a suitable reference gas, following the manufacturer's instructions.
- Prepare the cylinder: Prepare the cylinder and measurement setup according to the manufacturer's instructions and your training.
- Connect the sensor: Use the appropriate sampling adaptor and the gas flow specified by the manufacturer.
- Read the measurement: Wait for a stable reading and record it.
- Document the result: Record the analysed value on the cylinder, your diving equipment and in your logbook.
A dive computer's cylinder pressure display does not measure oxygen content. It does not replace gas analysis.
Set your dive computer correctly
Once you have analysed your gas, your dive computer must be set with the correct gas information. This is critical for accurate decompression calculations.
What do you need to enter?
- Oxygen percentage (%): The measured value, not the value stated on the cylinder!
- Mix name: For example, EAN32 or the analysed percentage (e.g. 32%).
- MOD setting: Depending on the dive computer, the MOD may be preset or entered manually.
- Conservatism setting: Many divers choose a slightly more conservative setting for an additional safety margin.
Common mistake: Leaving the computer on its default EAN32 setting without checking the actual analysed value. This can lead to incorrect decompression calculations.
Dive planning, step by step
A sound dive plan considers several factors:
- Dive objective and depth: What will our maximum depth be?
- Check the MOD: Is our planned depth shallower than the MOD, with a margin to spare?
- Plan the dive time: How long will we spend at depth?
- No-decompression or decompression dive: Does the plan stay within the limits permitted by your training and planning system?
- Repetitive dives: Are we planning several dives? This changes the calculations.
- Dive computer planning mode: Available planning functions depend on the model; follow the manufacturer's instructions.
Worked example – a Nitrox dive with EAN32
Scenario: You want to explore a coral reef using EAN32.
Planning steps:
- Gas analysis: Measured value: 32% O₂ ✓
- Check the MOD: Calculated MOD at 1.4 bar: 33.75 metres. This does not replace an individual depth and dive plan.
- Dive objective: The planned depth must stay within all training, gas and equipment limits.
- Planned dive time: Determine this from the actual profile and an appropriate planning system; this example specifies no allowable dive time.
- Computer planning mode: A specific no-decompression limit depends on the calculation model, settings and previous dives, among other factors. This example cannot be used to derive one.
- Safety stops: Include safety stops according to your training, plan and conditions. They do not replace any required decompression stops.
- Documentation: Record the dive plan in your logbook.
Understanding no-decompression limits
The no-decompression limit is the maximum time you can spend at a given depth without requiring a decompression stop. Exceeding it means decompression stops are required on ascent.
One major benefit of Nitrox's lower nitrogen content: at the same depth, you can often have longer no-decompression limits than with air.
Example comparison: At the same depth and under comparable conditions
- With air: The available no-decompression time depends on your planning system and previous pressure exposure.
- With EAN32: A longer no-decompression limit may be possible. No specific time is given without a defined calculation model and complete input data.
Any potential benefit in no-decompression time must always be considered alongside oxygen limits and all other diving limits.
Repetitive dives with Nitrox
Planning becomes more complex with several dives in one day: nitrogen from the first dive has not yet been fully eliminated.
Key considerations:
- Surface interval: Determine it from your training, actual dive profiles and planning system. No universal minimum interval is given here.
- Residual nitrogen: The computer accounts for nitrogen remaining in your body from the first dive.
- Reduced no-decompression time: Previous dives affect the remaining no-decompression time; the effect depends on the profile and surface interval.
- Plan conservatively: Be particularly conservative when planning the last dive of the day.
Nitrox dive safety checklist
Before the dive:
- ☐ Analyse the Nitrox cylinder's contents
- ☐ Set the dive computer to the correct gas mix
- ☐ Keep the planned depth below the MOD, with a safety margin
- ☐ Prepare and record the dive plan
- ☐ Ensure your buddy knows the gas and the plan
- ☐ Check your equipment
During the dive:
- ☐ Respect the depth limit
- ☐ Check your dive computer regularly
- ☐ Communicate with your buddy
- ☐ Follow the agreed abort and emergency procedures
After the dive:
- ☐ Record the dive in your logbook
- ☐ Note the gas, analysed percentage and dive depth
- ☐ Allow the surface interval required by your profile and planning system
Special situations and safety
What if something goes wrong?
- Signs of oxygen toxicity: Warning signs and emergencies require the procedures taught in your training. This overview does not replace situation-specific emergency guidance.
- Dive computer problems: If the gas setting is wrong or the computer fails, follow your trained abort procedures and the manufacturer's instructions. Changing a setting afterwards does not automatically make previous calculations reliable.
- An unexpectedly shallow depth limit: Establish the MOD and other limits before diving. If conditions differ, follow your trained abort and emergency procedures.
Why training matters
Theoretical knowledge alone is not enough. A recognised PADI Nitrox course or ProNRC Nitrox course provides essential theoretical foundations. Any further assessments required by the organisation must be clarified before the course starts. No practical dives are included in the online service offered. Training topics include:
- Understanding gas analysis procedures
- Understanding dive computer gas settings
- Respecting the limits of your training and experience
- Understanding the importance of practical emergency training
- Receiving personal feedback from professionals
Important limits and responsibilities
These articles provide educational information and do not replace professional training. Actual dives should always:
- Be made with a certified dive buddy
- Be planned using a modern, reliable dive computer
- Follow local rules and regulations strictly
- Be approached responsibly and cautiously
Conclusion
Good Nitrox dive planning combines knowledge, careful preparation and practical training. Following these steps diligently helps you use Nitrox safely and get more from your diving.
Start with our PADI Nitrox online course or ProNRC Nitrox online courseto deepen your knowledge and work towards Nitrox certification.
Read more about the basics: What is Nitrox? and Oxygen limits and MOD.