Choosing a Rebreather
Choosing a rebreather can be a difficult process. Before investing in a rebreather, there are many things to consider. A more sophisticated rebreather doesn’t mean it’s a better rebreather. Just like you purchased your open circuit gear, you must determine how you plan on using the rebreather. The difference between the available rebreathers on the market today is huge with respect to cost, training time, complexity, testing standards, and usability.
Rebreathers can cost $2,000 to over $10,000, which means that you don’t want to jump right in and buy the first one you see. You need to make sure that the rebreather you purchase meets your needs, operates the way the manufacturer says it does, and is reliable and durable enough to last you many years.
Determining the type of rebreather you need is the most important start. There are oxygen closed-circuit rebreathers, semi-closed circuit rebreathers, and mixed-gas closed-circuit rebreathers to choose from. Understanding the benefits, limitations, costs, and use of each of these will help you to choose the right rebreather that best fits your needs. When looking at these different rebreathers, it’s important to note the cost and training requirements, as well as your objectives and the dive environment you plan on using the rebreather in.
Once you’ve determined the type of rebreather you want to purchase, you must select a particular model or manufacturer. When doing so, it’s important to pay attention to the following: the testing standards, performance standards, certification, future service availability, and if there are spare parts available for the unit. These criteria are important in finding the right rebreather that is safe, works well, and is reliable. The test data is the most common way to find out how the rebreather performs. Several tests are used to define the performance of a rebreather:
· Work of breathing tests
· Canister duration time tests
· Temperature performance tests
· Gas control tests
· Reliability tests
· Computer tests
A “work of breathing” test is used to determine how difficult respiration is through the breathing loop. The movement of gas through the breathing loop is powered by your inhalation and exhalation. The amount of effort is takes you to breathe through the system can impact you significantly while diving, so you don’t want a hard breathing rebreather. The test depicts how easy it is to breathe through the breathing loop at a series of depths and with the rebreather in different positions. A series of graphs depicts its performance. Basically, the easier it is to breathe through the system, the better the rebreather.
The scrubbing agent that removes the CO2 from your exhaled gas resides in a canister on the rebreather. There are different sizes of canisters that manufacturers use on their rebreathers, which means different amounts of scrubbing agent can be used from rebreather to rebreather. The duration a canister lasts is the canister duration. Canister duration is an important factor when deciding on a rebreather. The canister duration time must be enough for the type of diving you plan on doing. When a scrubber fails to remove the CO2 from your exhaled gas, “breakthrough” can occur, meaning that dangerously high levels of CO2 can build up in the breathing loop. When this occurs, the scrubber must be replaced. Canister duration tests are performed at different temperatures and with different diver effort levels. Basically, you must find a rebreather with a canister duration time that meets the needs for the type of diving you have planned. So, the larger the canister doesn’t necessarily mean the better the rebreather.
The scrubbing agent that removes the CO2 from your exhaled gas is affected by temperature. If the ambient temperature causes the scrubber to fall below a critical temperature, the absorbing reaction becomes inefficient and the canister duration time decreases substantially. The scrubber requires a certain temperature to operate properly, so diving in very cold water can affect the operation of the scrubber. Some manufacturers insulate their scrubbers to prevent this from happening. Cold-water performance is an important factor to look at when shopping for a rebreather.
Gas control tests are also important to focus on when shopping for a rebreather. Maintaining a safe and stable oxygen level is very important. A gas control test is used to measure the frequency and amplitude of the fluctuation in the PO2 in the breathing loop. The test variables include: depth changes, respiration rate, and automatic versus manual gas addition. A rebreather that is capable of maintaining a stable PO2 within the breathing loop is an important factor to look for.
The reliability of the rebreather you decide on is extremely important and should be a big factor when doing your research. You don’t want to buy a rebreather that has a lot of problems and fails to operate regularly. Manufacturers should be putting their rebreathers through reliability testing to determine theoretical mean time between failures. Testing is always subject to the individuals conducting the tests and where the tests are performed. Because some manufacturers perform in-house tests and test for different things, comparing test data from manufacturer to manufacturer can be difficult. Understand that a manufacturer wants to show you that their rebreather works the best, so testing results can sometimes be skewed in favor of the manufacturer, not the consumer. Also, because there are no “standards” for performance and protocol testing among rebreather manufacturers, it is difficult to compare and get a complete understanding of how different rebreathers perform. Because of this, it’s important that any rebreather you’re looking at be tested with an outside testing group, such as the U.S. Navy Experimental Diving Unit (NEDU). Currently there are no minimal standards testing in the U.S., although there is in some other areas. In Europe, there is the CE (Conformite Europeene) that tests to performance standards and if the item meets their minimal standards, it receives the CE Mark. However, CE testing may be different than some of the tests mention here, so complete evaluation of all testing data should be performed before deciding on a rebreather.
If you’re looking at mixed-gas closed-circuit rebreathers with built in computers, these computer systems along with the transducers that supply the data to the computer must also be tested and evaluated for performance and reliability. Computer programs supplying display information, decompression information, EADs, etc. must be tested for accuracy and reliability. The transducers that supply computers data must also be tested for longevity and accuracy. You don’t want transducers failing or delivering inaccurate data.
The rebreather market today is in a state of flux. It’s also a very small market that, for the consumer, is in its early beginnings. As a consumer, you should beware of the stability of a manufacturer before making a purchase. Look for a manufacturer with stability, cash flow, a good, long track record, and a commitment to supporting the products they build. You don’t want a here-today, gone-tomorrow company. You’ll need support, replacement parts, and help when something goes wrong with the unit.
Source:
Boxanic, Jeffrey E. Understanding Rebreathers. Flagstaff, AZ: Best Publishing Company, 2002.