4. Rebreather (CCR) vs Open Circuit (OC)

One of the questions everyone considering buying a rebreather asks is: what are the advantages and disadvantages of a rebreather compared to an open circuit?

First, let's examine the differences between rebreathers and the open-circuit scuba diving we are familiar with;

Rebreather vs Open Circuit

I think many more points could be added here; perhaps I'll add them over time, or you can discover more as we delve deeper. For now, let's suffice with this and examine what we mean in more detail.

As we said initially, "dive times exceeding (almost) 4-5 hours, regardless of depth." In closed-circuit systems, apart from minor losses such as oxygen consumption, mask equalization, and gases injected into the BC or dry suit, our gas loss is almost negligible. Why do I say "almost"? Because during ascents and descents, there can be situations where we need to release gas from the circuit due to changes in pressure and volume. We will understand this in more detail later. Let's return to our topic; in short, our gas loss is minimized. Generally, closed-circuit systems contain one 3-liter diluent cylinder and one 3-liter pure oxygen cylinder. When you fill these cylinders up to 200 bar, gas consumption practically ceases to be a limiting factor in dive time. Here, the 3-hour lifespan of the carbon dioxide absorber chemical (according to the user manuals of most closed-circuit systems) becomes the limiting factor in terms of time.

Saving Time and Effort: Is Time Money?

I wish I could just get the idea out of my head that you could dive for hours with two 3-liter tanks, but why would I? Especially if you find a location without a dive center, where refilling tanks is practically impossible, and you want to go diving and discover new places. Normally, if you want to explore a place thoroughly, you'd either take plenty of 15-liter tanks or load a gasoline-powered compressor into the car. Back and neck problems would be a bonus. But is a rebreather like that? Just throw those tiny tanks in the trunk and hit the road. Of course, it's not all rosy; a rebreather is a huge device, after all. Still, you go to your new spot, dive for hours without the hassle of refilling tanks, go in and out without needing to change tanks. You can take 2-3 kg of soda lime (a carbon dioxide absorber chemical) with you and continue on your way. Are you going to stay for days? Take two more full 3-liter tanks, problem solved. Plenty of diving awaits you without wasting time refilling tanks. Was time money?

Time is money, and refilling tanks is money too. When we talk about deep diving, Trimix, and helium, open-circuit diving has no chance of competing in terms of cost, but let's consider the cost advantage of rebreathers even in normal air diving. Let's imagine a 5-hour open-circuit dive consists of 5 one-hour dives. Of course, depth, tank capacity, and individual usage will vary greatly, but let's take a practical approach and assume we use up one tank per dive. This means we'll need 5 tanks for such a dive, or if we have the option of having them refilled, we'll have them refilled 5 times. A single tank refill today costs approximately 450-650 TL (10-15 euros). 5 tanks means a cost of 50-75 euros. If you can get them refilled cheaper, factor that in. For this many dives using a closed circuit, the cost will be around 10 euros for chemicals, a 3-liter tank refill (let's say another 10 euros), and oxygen refills (which won't easily exceed 5 euros per tank). Adding it all up, the entire operation can be completed for 25-30 euros. Roughly speaking, we've solved the problem at half the cost. If we include trimix dives containing helium, I won't go into too much detail, but according to an analysis by NAUI in the USA, doing these dives with a rebreather instead of an average of 25 open-circuit trimix dives essentially saves the rebreather money.

When comparing costs, it's important not to only consider the cost of refilling the cylinders. The closed-loop unit will also have its own maintenance costs. This can vary greatly depending on the brand; for example, some brands require maintenance support at the authorized service center (main headquarters), while brands like JJ might suggest bringing it in after 10 years. When choosing a unit, you need to carefully evaluate the maintenance frequency and cost, especially if you live in Turkey and plan to ship and receive your device; you will need a good customs broker. Besides service maintenance, oxygen sensors, which are typically replaced every year or year and a half, will cost around €200 annually (approximately $60-80 per sensor for 3 sensors), depending on the number of sensors in your unit. Other maintenance costs are no different from standard scuba equipment; you won't encounter any different maintenance costs compared to a regulator or BC.

From what we've discussed above, it's possible to say that closed-loop systems, contrary to popular belief, are not a significantly more expensive investment in the long run. Although the initial investment cost may seem high, it's a device that pays for itself in the long term; the choice is yours…

Traning is a must;

Training is one of the sweetest parts of the job. We're talking about an interesting product here that you can't just buy with your money. To buy a closed-circuit diving unit, you need to receive training specific to that unit. Even if you're using a different brand of closed-circuit unit, you need to take training that explains its specific usage for each brand. Regarding training, you also need to check if certification systems (TDI, IANTD, SSI, etc.) are authorized to provide training for the brand you want. Or, conversely, if you specifically want a certificate from a particular certification system, it's a good idea to check which brands of closed-circuit diving units are certified. We'll go into more detail about closed-circuit training stages later, but briefly, once you've received training for a brand, you can't dive to any depth you want. To dive deeper, using trimix mixtures, you need to complete different training stages. Generally, after completing each training stage, you also need to perform dives at specific times to move on to the next level. Some units don't electronically allow you to dive above your training level. An instructor needs to authorize the electronic controller of your unit. In short, when choosing a training unit, you need to decide which certification system, which instructor, and which certification system you will receive training from.

This entire educational journey can be an advantage for some and a disadvantage for others; it's a matter of personal perspective. New education, new adventures, and new teachings might seem like a gain for some, while for others they might be seen as a waste of time and energy. If you're going to switch to a closed-loop system, you need to decide which side you're on and be prepared for a pleasantly long educational and practical journey.

Be careful, you might get bent!

When it comes to diving, anyone you ask will tell you about the risk of decompression sickness. Let's talk a little about how the risk of developing this condition differs in closed-circuit systems compared to open-circuit systems. Let's state from the outset that we are talking about a system that is safer than open-circuit systems in terms of decompression sickness. Let's see how that works.

Fully sealed electronic diving systems attempt to maintain the partial pressure of oxygen at a specific level throughout the dive. This target partial pressure is called the "setpoint." In practice, two setpoints are used: high setpoint and low setpoint. Generally, 1.3 atm is used as the high setpoint and 0.7 atm as the low setpoint. During a dive, the low setpoint is used in shallow water, and then the high setpoint is used. When the high setpoint is set to 1.3 atm, the system strives to keep the partial pressure of oxygen in the mixture the diver breathes at 1.3 atm. The constant "partial pressure" value means that a different mixture of gases is breathed at each depth. From another perspective, at each depth, you breathe the maximum amount of oxygen you can tolerate while maintaining a certain risk of oxygen toxicity. This also means you breathe the lowest possible amount of inert gas (helium and/or nitrogen). This minimizes the risk of decompression sickness.

In summary, fully enclosed electronic closed-loop diving systems carry a lower risk of decompression sickness compared to open circuit systems, and have longer NDL (No Deco Limit) times.

Cold and Dry Air

In open-circuit diving, the gas in our tanks, made of materials with high thermal conductivity such as steel and aluminum, works its way through our lungs in the coldness of the seawater. At the same time, the gas is compressed into our tanks after being filtered by compressor filters, and we breathe dry, ice-cold gas throughout the dive. In closed-circuit diving systems, the situation is slightly different. Although the gas we breathe still comes from steel or aluminum tanks and is still dehumidified, this time it doesn't come directly from these tanks, but from breathing bags called counterlungs. The breath we return to the system is now moist, and instead of being warmed by body heat, it remains "warm" in the system thanks to the exothermic reaction that occurs as it passes through soda lime. Of course, seawater continues to cool it, but it retains its heat better than materials like steel or aluminum tanks.

It is claimed that breathing a warmer and more humid gas throughout the dive reduces fatigue. Speaking of fatigue, another claim is that breathing high oxygen levels throughout the dive also reduces dive fatigue. Unfortunately, this is just a simple, unproven urban legend; however, it's quite clear that closed-circuit diving systems cause less fatigue.

Fighting for a liter of gas!

In fully sealed systems, one of the most unfortunate situations is gas leakage. Even the smallest gas leak can become a major problem in tiny tanks. Even without a gas leak, if you're supplying your buoyancy vest from your diluent tank, you can quickly deplete it with constant refilling and emptying of the BC during ascents and descents. You'll also notice how even mask equalization reduces the gas volume in counterlungs, creating the need to add more gas.

Open or Closed Finally?

While discussing the advantages of closed-circuit and open-circuit diving systems, I may have been a little biased, but considering benefits like gas savings, longer dive times, and lower fatigue, it's fair to say that the enjoyment of diving reaches a different level. Of course, open-circuit systems are what we've been accustomed to for years and stand out with their much lower complexity, while closed-circuit systems have the potential to open new horizons in diving.

Nevertheless, be careful; closed-circuit systems are no joke, and proper training and discipline save lives. Considering that experienced divers who switch to closed-circuit systems may become overconfident and push their limits after a while, which is another factor inviting accidents, we must remember that depth limits should not be pushed without plenty of practice. We will discuss these issues in more detail later.