Closing the Gap Between One and Two! The Era of Dual Dive Computers Has Arrived!

the Editor says: Once you start diving, a dive computer becomes an essential piece of gear — not only does it help you plan your dives more safely, but the wealth of information on the display also makes every dive more convenient. The golden rule of dive safety is redundancy, and knowing how to choose the right dive computers to pair together is a genuinely practical skill. This article is sourced from the personal Facebook page of Instructor 丁楓峻, who shares his experience pairing the Shearwater Teric, Garmin MK1, and Atmos Mission One dive computers.

You Still Need a Backup Dive Computer

The PADI (certification agency) Open Water Diver (PADI/SSI cert) course can be taught using only dive computer content — learning to read NDL, surface interval time, and how to use Dive Plan and Log functions — without necessarily covering the RDP table algorithm or eRDP operation. In fact, PADI (certification agency) has already discontinued the eRDP, and I have never once seen a recreational diver bring an RDP or eRDP on a dive. Dive boats can be rocking and wet, and nobody wants to handle paper materials or spend mental energy crunching numbers and algorithms. Besides, when you've traveled somewhere far and spent good money on a luxury liveaboard, you're not going to want to give up any dives easily. If a dive computer malfunctions and forces you to surface and wait several hours for your residual nitrogen to clear before switching to a fresh backup, the time and money you lose might well have paid for another dive computer outright.

As they say, every minute underwater is precious! Of course, you could just wear one computer and mentally estimate how many minutes to subtract from your NDL — but that kind of calculation is never going to be accurate. So in practice, the best way to handle a dive computer failure, loss, or dead battery is to wear two dive computers as mutual backups.

For students who are genuinely interested in technical diving, there's plenty of time to learn the RDP / eRDP after mastering dive computer operation and theory. Alternatively, you can dive straight into studying Buhlmann ZHL-16C + Gradient Factor decompression theory through your dive computer and then explore other decompression algorithms such as VPM.

Backup Gear Is the Most Important Risk-Management Tool

Carrying backup equipment and always having a Plan B in mind is the most important principle for staying safe underwater. Aside from wetsuits and fins — which nobody brings a spare of — almost every other piece of gear can have a backup: a mask tucked into a thigh pocket, an octopus / alternate second stage, a snorkel rolled up in a thigh pocket, a mechanical pressure gauge (SPG) plus a wireless transmitter, back-mount combined with a sidemount pony bottle, and so on. Wearing a backup dive computer is no less reasonable than any of these. Factor in your dive buddy's backup gear and strict adherence to the buddy system, and you can dramatically reduce the chance of an accident. So — why not wear a backup dive computer? Why did so few people used to wear one?

Rental dive computers at dive shops commonly include models such as: Suunto Zoop Mares Puck Deepblu Cosmiq+

There was even a dedicated Oceanic BUD backup dive computer — a unit with no strap at all, just a clip for attaching directly to your BCD — priced under US$300. These budget-friendly computers are not hard to get hold of. The issue is that they tend to be bulky or limited strictly to diving, with no everyday wearability. For a holiday diver who might only dive once or twice a year, that kind of investment is hard to justify.

The Large-and-Small Dual Dive Computer Pairing

For divers who dive more frequently or at a more advanced level, investing in a large-and-small dual dive computer setup is a very sensible choice. The larger display can show a wealth of information, while the smaller one serves as a backup — and the smaller unit may use a traditional LCD screen, meaning batteries don't need to be changed as often. One important consideration is whether the backup computer uses the same algorithm as the primary; Buhlmann vs. RGBM, for example, are not the same. This is why people generally choose computers from the same brand:

Buhlmann ZHL-16C => Shearwater => Large Perdix AI (user-replaceable battery) + Small Teric RGBM => Suunto => Large EON (charger-charged) + Small D-series (coin cell battery)

That said, large-screen dive computers aren't suited for everyday wear. The current thinking, therefore, is: can both computers be worn daily? Can the smaller-screen computer be customized to display only the information you consider most important? And can the two computers show different information — so that even with a small screen it doesn't matter, because you've effectively split what a large screen would show all at once across two wrists?

Dive Computers — One Link in Taiwan's Most Efficient Ocean Supply Chain

As analyzed above, the reluctance to buy two dive computers in the past was largely because they could only be used for diving, making it hard to justify the expense. But if dive computers come down in price and expand in functionality, buying one becomes as natural as buying a piece of clothing you can wear every day — and if two computers complement each other's features, the case for buying both becomes even stronger.

Taiwan has a solid information and communications technology industry and manufacturing base, and the Northeast Coast is close to Taipei, making it well-positioned to develop recreational diving equipment for R&D, testing, design, and manufacturing — with a complete supply chain and cost advantages over European, American, and Japanese brands. Taiwan also has a long history in recreational diving, not as long as Japan's, but well ahead of mainland China's. As a result, brands like Deepblu, Atmos, and Crest have successively launched MIT wrist-mounted small-screen dive computers with excellent value for money, while the high-end Garmin Descent MK1 is also manufactured in Taiwan (Further reading: The World's Most Powerful, Feature-Complete Dive Computer – Garmin Descent Mk1).

The Teric + Atmos Pairing in Practice

I use the Shearwater Teric + Atmos Mission One, mounting the Teric on my camera's buoyancy arm and wearing the Atmos on my wrist (you can also mount both the Teric and the Atmos on the camera arm). The great advantage of the Teric is its fully customizable display. For example, knowing that the Atmos main screen already shows MOD / Gas mix / water temperature / compass / NDL / depth, I can configure the Teric's main screen to show a completely different set of parameters. I set the Teric Home Screen Value to 3 and the Layout to Standard, then configured it to display TTS / T1 & GTR / SAC / Time of day. The reasoning: the Teric supports a wireless pressure transmitter, so it can show GTR and SAC — two parameters the Atmos cannot provide — meaning you can spread the maximum amount of data across two computers without it becoming overwhelming.

Diving is a lot like flying a plane — the number of parameters you need to be aware of is far greater than when driving a car. Driving on land is a 2D activity; no matter what happens, you stay on the road with no changes in depth or altitude. Even in dense fog or heavy rain with very poor visibility, you can just switch on your lights, slow down, or pull over — there's no immediate danger. But flying a plane or diving in low visibility may mean relying entirely on your instruments to tell you your heading and depth. On top of that, divers often have to perform other tasks — such as photography — which makes it all the more important that multiple data points are presented in the clearest, most concise way possible.

Solving the Problem of Not Wanting to Write — or Not Writing Enough in — Your Dive Log

In the past, keeping a dive log felt like a chore: you had to look up the name of the dive site and jot down what you saw and what happened while it was still fresh in your memory. And even if you wrote everything out by hand in meticulous detail, searching back through many dives later was a real hassle without any digital record.

The Teric doesn't have integrated GPS, but the Atmos does. I can use the Atmos's nearby dive sites feature to pull up a site name and GPS coordinates, or define a custom site based on the dive guide's briefing. I then use the Shearwater Cloud app on my phone to edit the site information in the Teric, quickly jot down what I saw using the tag feature, and then sync to the cloud and fill in more detail on the computer later (typing at length on a phone is rarely appealing).

Conclusion

I strongly recommend wearing two dive computers. No single dive computer is perfect — compact, lightweight, fully featured, outrageously capable, and cheap all at once is simply not possible. But with a well-chosen pair, you get complementary features and mutual redundancy. When pairing, consider price, screen size, battery charging and replacement method, feature complementarity, and decompression algorithm.

With a higher budget, you might go for a premium pairing like the Shearwater Perdix AI ＋ Shearwater Teric / Garmin MK1, or the Suunto EON + D5. For better value for money, pair with a Taiwanese home-grown brand, such as Teric + Atmos, or a combination from Atmos / Crest / Deepblu.

Further reading:

• 丁楓峻 Instructor's personal Facebook page

• The World's Most Powerful, Feature-Complete Dive Computer – Garmin Descent Mk1

• Garmin Descent Mk1: A New Style Statement for Divers

• ATMOS ＭISSION ONE – The Most Anticipated Dive Computer of 2019