There are big rebreathers and little rebreathers. So why would you choose one over the other? The four vital elements that are affected when size changes in a rebreather are:
1. The buoyancy
2. The canister endurance
3. The work of breathing
4. Reliability of sensors
Example 1: A manufacturer makes a rebreather with a large canister in order to increase absorbent capacity and endurance (if it is designed it correctly). It now has a large volume (since the counterlungs themselves must be a minimum of 4.5l). I jump in the water and just can't sink. So while my unit might be fairly light on the surface, I need lots of lead to dive it. So the trade here is to accept that the extra endurance is what we want and choose (heavier) engineering materials to make the unit neutral in the water. The ultimate trade off here is traveling weight.
So if you want long endurance, be prepared to:
1. Wear lead
2. Pay for traveling weight
3. Pay for more absorbent
There is, however another twist to this part of story. If the rebreather is designed correctly, it can use smaller absorbent fills and still have long endurance. The trade off again is the size/volume that is needed to provide insulation, but at least you lose one of the downsides. The feature of a well insulated rebreather is that it doesn't suffer from massive endurance reduction with depth as most rebreathers do. The Sentinel (which has one of the smallest absorbent fills at only 2.2kg) does not suffer adversely with endurance reduction with depth. In addition the extra insulation ensures a very dry environment for the sensors, significantly improving accuracy and sensor life over smaller designs. The extra insulation is compensated for by a precise selection of engineering materials that makes the unit near-neutral in the water without additional weighting. Canister design is really a 'black art'. There are several basic concepts that fundamentally work (that is why there are so may 2.5kg axial canisters out there) but these are not really optimized. One or two more radical designs exist but as yet their operational rules do not seem to be based in much science and testing. Multiple canisters can either reduce the work of breathing (WOB) and at least double the duration (parallel systems) or double the WOB (at the canister) and increase the duration to an unpredictable limit (serial/inline). In a parallel system both canisters experience the same gas flow and both run for at least the full endurance of the single unit. The larger surface area of the two canisters may decrease the flow and increase the dwell time of the gas in the absorbent and have a beneficial effect. In the serial system, double the absorbent just increases the WOB and the first canister is preferentially exhausted. But if it is a small canister and/or you work hard and bypass it, then the second canister comes into play and hopefully stops any CO2, but now you don't know where you are from an endurance standpoint, since it is likely your endurance math hasn't taken into account partially using the second canister. If you swap the canisters around part way through a dive sequence, you are dealing with an even larger unknown quantity. So while smaller serial systems may seem specially attractive they can lead to WOB, unknown endurance and sensor care issues. Imagine you have two fuel tanks in your car. the first one gets used for crawling around town but when you need to go faster the 'go faster' inlet jet on the carb is needed and that grabs fuel from both tanks. You can't see what is in the tanks but you know one tank in normal use lasts about 'X' minutes. But now you are street racing! Both tanks are being used but you don't know how much. When you get to the gas station what do you do? Just fill the first in the hope that when you need to go faster again there will be enough left in the second? You don't want to lose the next race so you probably fill both! Hence in the end long/predictable endurance and a stable sensor environment will always affect buoyancy/weight.
Example 2 :
I want the best WOB but I don't want chest mounted counterlungs.
In fact to get things straight from the start; just because a unit has chest mounted lungs it doesn't mean it will have a low WOB. It may gain hydrostically but it may be like breathing through a sock - resistively speaking. So here again size matters. To keep the resistive circuit low you may need bigger mushroom valves, hoses, counterlung ports and counterlungs, but not too much absorbent as the more you add, the harder it gets to breathe. This obviously depends on the canister design. Many people suggest using a radial canister. That will reduce the resistive effort but it may also reduce the canister duration, especially under a high workload where the 'bed length' just isn't enough to cope with increased ventilation. While a well designed radial may deal with these issues, it is a myth that fitting a radial cures all. It is very difficult to design a radial that performs under all conditions. Back mounted counterlungs can also generate their own WOB issues. Long, thin counterlungs can generate a high resistive load in a vertical swim position (ascending/descending) as you are effectively sucking and blowing gas from a long bag with increasing pressure acting along its length. A centrally fitted lung(s) is far better at generating a lower WOB in all positions. Size in a rebreather also affects how it handles moisture, therefore protects the O2 and CO2 sensors. But don't avoid back mounted system because they look big on a diver. Consider the entire diver profile.
An over shoulder counterlung rebreather is a combination of something inflated on your front and a case on your back and the cases are often square in profile. Back mounted systems only carry volume on your back (and lungs don't collapse when you try to squeeze through a hole) and are often triangular in profile so less cross sectional surface area. If you look at the Henkel Restriction in Ginnie Springs with your back mounted rig on and think ' it isn't for me'......think again!
Small rebreathers have their issues and often do not provide the ideal environment for sensors, WOB or reliable canister life prediction.
Kevin Gurr - http://www.technologyindepth.com
Had the honor of being the first person (other than the designer/engineer Kevin Gurr) to dive the Hollis Explorer Rebreather. What a game changer! Incredible and surpassed my expectations!!
Kevin Gurr dives his design - the new Hollis Explorer Sport rebreather at Blue Grotto in Florida.
It is an SCR but it sure does not feel like it. One gas. Tiny. Lightweight. Has gaseous CO2 monitoring and temp profiling of the sorb. Trims beautifully. I think I used around 10-15 psi per minute (from a minuscule tank) and it held set point beautifully, adding gas as needed with barely a trace of bubbles. I can't get over it! Great machine.
What is Optocon? Why is it important? Why Do I need it?
Two years ago, recognizing the vulnerabilities of hard wired electronics in rebreathers, VR Technology started developing a paradigm shifting system called Optocon. The goal was to remove potentially unreliable wiring and connectors within safety critical systems from our Sentinel rebreather and replace them with a much more robust and redundant system. With this change, we delivered a long-term, more reliable system as well as one that was far easier to replace and maintain in a simple modular fashion.
After countless hours of developing, testing and certifying this new technology under CE standards, I want you to understand why we tackled such a challenging issue, so I will try to explain why VR has bothered to spend so long developing the system and why, as a rebreather diver, it is important for you.
CLICK HERE to get the entire article as a .pdf
Upcoming Seminar at BTS I still have a few spots left in the Rebreather Fundamentals Workshop with Rebreather Pro Jill Heinerth - Sunday March 25 from 9am - 4pm at the Beneath the Sea Diving Symposium.
Meadowlands Exposition Center, Seacaucus New Jersey Classroom session: $95 will include many educational resources to take home. Tickets available online at: www.BeneathTheSea.org (space is limited) Enjoy a full-day academic session with CCR expert Jill Heinerth. Using fascinating stories and a multimedia presentation, Jill will cover all the essentials of rebreather diving.
If you are looking to purchase a rebreather, this is an opportunity to learn how to be a wise consumer and make the choice that is right for you. If you already own a rebreather and want a review of academics, new trends and the future of CCR diving, then this seminar will offer an opportunity to gather new information. If you want to prevent yourself from joining the ranks of 20 or so people who perish each year on rebreathers, this workshop will inform you about accident analysis and prevention. If you want a day with an expert who will carefully answer your questions and help you find solutions, then this is your best opportunity to spend a day with Jill. If you want to see how rebreathers are being used in exciting expeditions around the world, this full-day road show will entertain and motivate you to participate in CCR diving.
The workshop is non-denominational… all training agencies and all manufacturers are welcome. All levels of divers are welcome from recreational through technical divers.
Topics: Rebreather operations and procedures Basics, types, operation and essential care Selecting a rebreather: comparing apples and oranges Selecting an instructor Understanding CE testing Myths and misconceptions about rebreather diving Carbon dioxide: dangerous and ignored Reducing oxygen issues Planning for the future: deep mixed-gas operations Accident analysis and prevention: beating the complacency trap Traveling with rebreathers: tips and warnings Lots of time for questions...and answers
See you there -- Jill
VR Technology and Hollis Gear have teamed up to launch the worlds the most revolutionary piece of diving equipment since the BC.
The Explorer sport rebreather.
In 1992 VR Technology launched the world’s first multi-gas dive computer. Since that time VR Technology has lead the charge in revolutionizing diving equipment and achieved no less than 21 “worlds Firsts” in underwater technology.
Hollis Gear has unrivaled manufacturing and distribution capability and has built one of the best brands in the SCUBA Industry.
“The strengths of both companies were effectively utilized to produce a product superior to what could have been achieved individually. I can’t say too much today, but the Explorer will revolutionize the way we think about sport diving.” —Kevin Gurr
The Explorer sport rebreather will make its début at DEMA on November 2, 2011 at the Hollis Booth # 1747 and boasts technology never before seen on any underwater apparatus. The unit is neither a fully closed circuit rebreather nor a pure semi-closed system, but an intelligent hybrid that utilizes the best of both worlds. It’s compact, lightweight and extremely easy to use. The Explorer is unique in using a single gas; Nitrox, and is electronically controlled to achieve an optimal balance of PPO2 and dive time. Plug and Play absorbent cartridges, easy guided setup with “go or no go” mechanism that insures a cartridge is fitted, CO2 tracking, and 2 hour design duration make this a dream for any recreational diver. The Explorer diver will benefit from breathing warm gas, have lots of bottom time, and since it does not produce any invasive bubbles, see more sea life than ever before. Yes, this ESCR (Electronic Semi-Closed Rebreather) is designed to meet the PADI Type R requirement.
The Explorer package is supplied complete with regulator, cylinder, BCD and onboard decompression computer. It is ready to dive out of the box and at an amazing price given the technology and cost of alternative systems in the market.
Today, we feature a guest post from technical diver and rebreather designer Kevin Gurr. Kev is the guy behind the VR line of dive computers and, of course, the Sentinel CCR, closed circuit rebreather. This is a rather lengthy article, but well worth the read. You can click here to view or download it in .PDF format.
Jill Heinerth dives Kevin's VRx and Sentinel Expedition CCR regularly on exploration and photography/film making projects.
