Why Smart Divers Think With the Computer
It wasn’t long ago that dive computers as we know them today didn’t exist. You planned your dives using tables and tracked your time with a simple bottom timer. No NDL displays. No map of tissue loading. No on-the-fly adjustments of gradient factors. Just you, your depth, and a stopwatch. If it was dark, you may have even needed to shine your light on a monochromatic display just to read it. How uncivilized!
But thankfully, modernity came for the dive industry. Contemporary dive computers, pioneered and refined by Shearwater, have completely changed the way we plan and execute dives. With precise, real-time tracking of depth, time, gas mix, and ascent rates, they offer us revolutionary insights into our decompression status. For both recreational and technical divers, this technology has increased safety, reduced the complexity of planning, and allowed for more flexible dive profiles.
But there’s always a catch: the best dive technology is only as effective as the diver using it. While these tools are powerful, it is still vital to have a working knowledge of decompression theory and understand what you should expect your computer to tell you. Why? Because if something seems off, you need a way to recognize it; and one of the best ways to do that is to understand the ratios present in decompression profiles.
© Ewan Anderson
Understanding Ratio Deco
If you’re anything like me, and you learned to dive long enough ago, you may have panicked more than a little the first time you saw a decompression table in your Open Water class. All those numbers, letters, and arrows pointing in strange directions had me questioning just how much I really wanted to try this scuba diving thing. But with help from a patient instructor, I began to decipher what I was looking at. This was a decompression algorithm laid out in front of me and a powerful model I could use to manage inert gas stress. Seeing it written out on paper helped me grasp the algorithm’s purpose, but it was far too complex to memorize. This was not something that I could take into the water and apply in the real, dynamic environment of a dive. But what if there were a better tool that allowed me to make “battlefield calculations”? Some simple rules that were easy to remember and practice?
The ratios that emerge in decompression profiles give us a way to use one simple metric - average depth - to calculate our no decompression limit (NDL) and our decompression profiles.
© Ewan Anderson
Simple Tools, Real-World Use
Many US Navy divers learned the classic ratio called “the rule of 120”. This states that, when breathing air, average depth in feet plus bottom time may not exceed 120 to stay within NDLs.
Another popular ratio for the use of Nitrox 32 is the “30/30” rule where at an average depth of 30 meters you have 30 minutes of no decompression time; at 27 meters you have 40 minutes; at 24 meters you have 50 minutes, and so on for every three meters shallower.
If we compare the 30/30 rule with a standard Nitrox 32 table, we find that they match perfectly in the deeper ranges, but the ratio starts to become overly conservative as we move shallower. This is the reality of trying to place the straight line of a simple ratio like the 30/30 rule over the curve of a true decompression algorithm like the Bühlmann model, ZHL-16. The further we stray from a “set point” average depth (30 metres in this case) the less accurate our rule becomes. We should, however, consider this conservatism against the realities of gas usage, temperature tolerance, and other factors that may have us out of the water long before we hit NDL.
For the technical divers out there, put a dive to 45 meters into your favourite decompression planning software and select trimix 21/35 as your gas. You will find that you have a rough ratio of 1:1, where one minute of bottom time equals one minute of mandatory decompression. If you move your average depth deeper, you will have a predictably longer decompression. If you move it shallower, it will be shorter.
We see that this 1:1 ratio is effective between average depths of 39 to 51 meters, using trimix 21/35 or 18/45, for up to 30 minutes of bottom time. If average depth is three meters deeper or shallower than our 45 meter “set point” then we add or subtract five minutes of decompression time, respective. (If we stretch this ratio rule, either deeper or longer, we will eventually run into some practical limits: substantial differences between “ratio deco” and actual decompression algorithms). Play around with this in software and see how the ratio lines up with the actual calculations. You might be surprised by how closely they track each other.
© Ewan Anderson
Ratio Deco as a Sanity Check, Not a Substitute
So, have we just argued away the need for that awesome new Teric you’ve been eyeing at the dive shop? Absolutely not. Ratio deco isn't meant to compete with your computer—it’s there to complement it. When you understand the pattern behind the calculations, you can use your computer more effectively and with more confidence.
Say your computer gives you 20 minutes of NDL, but based on your ratio estimate, you expected around 10. That gap should trigger questions. Did I input the wrong gas? Did I make a mistake with timing? Did I select an inappropriate gradient factor? As early IBM programmer George Fuechsel stated, “Garbage in, garbage out.” Knowing how to sanity-check your data puts you back in control.
In the recreational and technical courses I teach, I emphasize the importance of taking ownership of the dive by encouraging students to operate in “gauge mode” for a while. No NDLs, no deco ceilings, no time-to-surface number being handed to them on a platter—just depth and time. Students still plan every dive using decompression software, verify the ratio deco equivalent, then update throughout the dive based on those pre-dive models. Forcing them to do this helps students build their situational awareness and sharpens their instincts. When they return to full decompression mode on their computer, they’re not just following numbers on a screen—they're verifying, understanding, and anticipating.
© Chris Mullen
Humans + Computers > Humans or Computers Alone
I have found my Perdix to be incredibly reliable, but as the user manual states, no technology is infallible. Settings can be changed. Software bugs can rear their heads. Batteries can die. On the other hand, even the most experienced diver can miscalculate under stress or fatigue. That’s why the partnership matters.
The human brings judgment, adaptability, and context. The computer brings precision, real-time tracking, and consistency. Together, they create a system with checks and balances.
This synergy is what makes the modern dive computer so valuable. They’re not just machines delivering orders—they’re collaborators in the dive helping to guide decompression. But for that relationship to work, the diver needs to bring knowledge to the table.
Build a Dialogue, Not a Dependency
Modern computers offer a variety of sophisticated features to deliver critical information so you, the diver, understand your decompression status underwater. They're built to be trusted. But trust is earned, and smart divers verify.
By combining foundational decompression knowledge with modern computer guidance, you’re not just a passenger on your dive—you’re an active participant. You can:
• Cross-check displayed information with a mental model like ratio deco.
• Catch issues before they become problems.
• Make informed decisions if something changes mid-dive.
It's about building a dialogue with your computer, not a dependency on it.
Conclusion
Shearwater computers are powerful, simple, and reliable tools. But the smartest system is still a well-informed diver using that tool with purpose. Learning shorthand rules like ratio deco isn’t a rejection of modern tech, it’s how you make the most of it. Ratio deco is a mental model that gives you a deeper level of situational awareness, and the confidence to question things when they don’t feel right.
On your next dive, try comparing your ratio deco estimate to your computer’s display. See how closely they align, and more importantly, use this comparison to anticipate your computer’s display in five or ten minutes time. That habit builds sharper instincts and deeper trust in both your thinking and your tools.
Humans and computers working together will always outperform either working alone. Know your computer, know your plan, and know how to check it. That will help keep you safe and having fun underwater!
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Written By Jason Cook
Edited By Ewan Anderson
© Sabrina Figliomeni
Jason Cook is a CCR and technical diving instructor for Global Underwater Explorers based on Vancouver Island, Canada. He first tried diving at the age of 12, and has thousands of dives in oceans and caves across the world. As an educator he has worked with a wide variety of students from the civilian, defense, and scientific sectors.
Jason volunteers with the Underwater Archaeological Society of British Columbia, Project Baseline in Saanich Inlet, and has served on the board of the British Columbia Underwater Explorers. Notably, he was involved in the identification and documentation of the SS Admiral Knight in 2022, a significant maritime discovery of a vessel that sank off the west coast of Canada in 1919.
Before transitioning to diving, Jason had a career in the music industry as a record producer, recording engineer, and session musician, collaborating with many Juno and Grammy award winning artists across North America and internationally.
