Your rebreather already has a screen on your wrist or in front of your face. It shows your setpoint, your oxygen partial pressure, your depth, your time. So a fair question comes up the moment a diver moves from open circuit onto a loop: if the handset already tells you everything, why carry another computer at all?

The short answer is that most closed-circuit divers do carry a separate one, and for good reason. Your rebreather controller manages the loop you’re breathing right now. A backup computer plans the dive you fall back to if that loop quits. Those are two different jobs. One device rarely does both of them well.

What does your rebreather’s handset actually track?

Your handset watches the loop in real time. It reads the oxygen cells, holds your chosen setpoint, drives the solenoid that adds oxygen, and logs the dive as it happens. It is a life-support controller first. It isn’t built to plan an open-circuit escape across two or three bailout gases.

The reason it fixates on oxygen is blunt. Too little and you black out; too much and you risk a seizure underwater. Acute oxygen toxicity, a clinical reference from the National Center for Biotechnology Information notes, predominantly affects the central nervous system, presenting with seizures, visual disturbances, nausea, dizziness, and altered consciousness. Your controller exists to keep you off both ends of that range, dive after dive.

The handset also records what the loop did minute by minute, which is why reading the numbers your controller captured after you surface is part of honest CCR practice. That log tells you how the unit behaved. It doesn’t plan your next dive for you.

Why isn’t the controller enough on its own?

Because electronics fail, and when they do, you bail out. A flooded handset, a dead battery, or a cell reading you no longer trust can push you off the loop and onto open-circuit gas. At that moment you need a device that still knows your tissue loading and can plan an ascent on whatever is in your bailout cylinder.

Skip that math and you gamble with decompression sickness, which a StatPearls clinical review describes as a condition that occurs when depressurized gas (usually nitrogen) exits the solution phase in tissues and obstructs the circulation by forming bubbles. A computer tracking that obligation is what walks you up in stages instead of guessing.

How you plan that fallback depends on how much open-circuit bailout gas the dive actually demands, and your backup has to model the ascent on exactly that gas. A pretty screen is no help if it can’t switch to the cylinder on your back.

What a backup computer has to handle on a CCR

Once you accept that the backup exists for the worst moment of the dive, the shopping list gets specific. A recreational open-circuit computer that only knows one gas will leave you exposed. Three capabilities matter more than the rest.

  • Closed-circuit modeling at a fixed PO2 for the loop, plus open-circuit gas switching for the bailout you might actually breathe.
  • Trimix and multiple gas mixes, since deep loop dives almost never run on plain air.
  • Independent power and a display you can read fast, because you’ll be reaching for it under stress.

That gas flexibility earns its keep on the deep end, where the planning behind a deep trimix dive leans on the computer to juggle diluent, bailout, and decompression at the same time. The shallower and simpler your diving, the more forgiving the choice becomes.

Should your backup run in CCR mode or gauge mode?

It depends on how much you want it to model closed-circuit decompression. Run it in full CCR mode and it tracks deco at your setpoint, shadowing your controller so the two agree. Run it in gauge mode and it becomes a depth-and-time backup while you follow tables or your primary. Most divers pick one on purpose. Few should pick by accident.

Conditions change the math too. Something as ordinary as diving a rebreather at altitude shifts your decompression, and your backup needs the right profile loaded before you ever splash. Set it on the boat, not at depth.

Matching a computer to the diving you actually do

The honest way to choose is to start from your deepest planned dive and your bailout gases, then work backward to the feature set. A diver running shallow, single-gas loop dives needs less computer than someone staging trimix bailout at ninety metres. Buy for the dive you’re training toward, not just the one you did last weekend.

As an AP Diving rebreather specialist, Silent Diving stocks dive computers alongside the loops, cells, and cylinders they run on, so you can match a computer to your actual setup instead of a generic scuba rig. If you’re weighing options, it helps to compare dive computers for technical and rebreather diving side by side before you commit to one.

Where should you start when picking a computer?

Start with the failure you are insuring against. Your controller runs the loop; the second computer runs your escape. Once you frame it that way, the questions get easier: which gases must it switch to, how deep will it plan, and can you read it in a hurry? Answer those three and the shortlist writes itself.

Match the computer to your deepest planned dive and your bailout gases, then compare dive computers for technical and rebreather diving and talk to the team about how each one pairs with your loop. The right answer is the one that still has your back when the electronics on your chest don’t.

Frequently Asked Questions

Can I dive a rebreather with only the handset?

Some divers do, but it leaves no plan if the electronics fail mid-dive. A second computer is what plans your open-circuit ascent when the loop is out of the picture. Most training and experienced CCR divers treat carrying one as standard practice, not an upgrade.

Does the backup computer have to be CCR-capable?

Not strictly. A CCR-capable unit models closed-circuit decompression at your setpoint, which keeps it in step with your controller. A gauge-mode backup can work if you plan your decompression another way, but you take on more of that responsibility yourself.

Should both computers use the same algorithm?

Many divers match conservatism so the two devices broadly agree. When the algorithms differ sharply, you can surface facing two different ceilings and have to decide which to trust. Matching them removes one decision from an already busy moment.

Do I need air integration on a CCR computer?

It’s a convenience, not a requirement. Air integration lets you watch bailout or diluent cylinder pressure on the same screen, which some divers value. Plenty of others run analog pressure gauges and never miss it. Decide based on how you’d rather monitor gas.

Wrist unit or heads-up display for the backup?

Both work; it comes down to your setup and how you like to read data. The rule that matters is simple. You should be able to find and read the number you need without a fight, in cold water and low visibility, exactly when the dive is going sideways.