Recreational Rebreathers?

[t-mac]

Hi there. My RDB and I are outfitting our boat to spend some time in the Caribbean and one consideration is maximizing our independence from shore facilities. We have chartered a number of places there, dragging along 4 tanks for our OC systems, which gives us a couple of long dives or 3-4 moderate dives each before we need to refill. Being there for a week, we are not usually too far from facilities and this isn't a big deal, but it would be ideal not to have to plan for air fills as often. One obvious solution is to outfit the boat with a compressor, which is something we will explore, but the other thought was to use rebreathers.

We are interested in them anyway, but the cost would only be justified for us if we dove as as regularly as we would if we were living aboard and island hopping. And so, I guess I would say that, aside from the quiet, which is attractive, gas storage and utilization are big factors. We are not as focused on some of the other benefits of rebreathers and really don't intend at this point to go beyond recreational limits. That leads us to consider rec rebreathers like the Hollis explorer. It is particularly attractive because it uses a single bottle of nitrox, which I would imagine makes fills relatively easy. Of course, given the smaller tank size, we would be able to transport more tanks and presumably (not sure of this) get multiple dives from each tank, giving us greater freedom between visits to fill stations. I realize maintenance, supplies, and potentially reliability are on the other side of this. We want to spend more time diving and don't want to spend all of our time maintaining, repairing and visiting dive shops.

I'd love to get some comments on whether you think this makes sense and help me to balance the considerations in choosing between a compressor (which has its own maintenance issues) and the rebreather option.

Thanks,

Tom

 

[DA Aquamaster]

OC and an air compressor

I had a compressor on my boat for several years. I still have the compressor (a surplus Stewart Warner that has the advantages of being easy to maintain and capable of continuous running), but I sold the boat (a nice boat with the disadvantage of needing continuous maintenance).

The compressor worked well to refill tanks between dives. Realistically, they don't require much maintenance overall, when you're just using them for your own fills, as the hours you put on them are low, and the daily maintenance is near zero, so they are very trip/vacation friendly if you do any upcoming service and maintenance before you leave.

The downside is that unless you bring along some O2 and a continuous blending stick, you'll be diving air. That's not a bad thing on shallow reefs, but if you're getting below about 50-60 feet, you'll miss diving nitrox.

However, even with that limitation, a portable compressor is in my opinion a great option for OC vacation diving off a boat for a pair of divers.

Rebreathers

We also currently dive KISS Sidekick side mount rebreathers, primarily for cave diving, but we'll probably use them on a fair percentage of open water dives as well, particularly as I also like video and photography.

The downside of any rebreather is that you need to do some daily maintenance, including breaking the unit down, rinsing the counter lung, hoses and DSV with Steremine, dumping the old sorb and then repacking the canister, and then putting it all back together. It's also beneficial to let the head (containing the sensors and connections) dry out overnight. You could fast track it blowing some air over the head, but you'd need at least a low pressure compressor for an air source.

It's not as bad as almost everyone suggests,it's not difficult, and it really doesn't take long, but start to finish it's still about 30 minutes. Then you've got about 10 minutes of pre-dive check activity for the first dive of the day and about half that for the second dive of the dive. In short, there are some wonderful advantages, but there's no free lunch and you'll have about 45 minutes less time a day for kicking back and relaxing.

To be fair though, that's about the same amount of time you'll spend refilling tanks with a compressor, so it's a bit of a wash.

Active Addition SCR, versus Intelligent SCR versus Manual CCR

With the Hollis explorer you'll save some time as it will use extend air cartridges that save about 10 minutes of time packing the scrubber - but they take up more space to store (before and after use) than absorbent, and they cost about 4-5 times as much per dive.

The Hollis Explorer is an "intelligent" SCR rebreather rather than an "active addition" SCR. With an active addition SCR, greater depth means higher flow rates (whether it's controlled by a fixed orifice or by electronics) so the deeper you go the less bottom time you get, just like OC. The Explorer is more efficient and it can manage a set point better so it offsets some of the deco disadvantages common to active addition SCR designs.

I certified on the Drager Dolphin/Ray/Atlantis (all similar active addition SCRs) in the 1990's and learned that I really didn't want to buy one due to all the active addition SCR disadvantages. Those rebreathers represented the first major effort at a recreational rebreather. It remains to be seen if the Explorer fares any better.

I have my doubts as a manual CCR (mCCR) like the KISS Spirit or Orca Spirit isn't any more expensive and offers significant decompression and gas efficiency advantages that you won't find in even an "intelligent" SCR.

Consumables

With a mCCR you'll use about 1 liter of O2 per minute. To put that in perspective, an Aluminum 13 at 3000 psi holds about 400 liters of O2 - enough for about 6 hours of normal diving. You'll also use very little diluent and we average about 300 psi of diluent from an aluminum 72 for 180 minutes of diving - that's about 7 cu ft per day. You can use nitrox, air, or trimix for diluent, depending on location, dive depths, etc. For a small back mount rebreather the diluent will also be in a small 13 to 19 cu ft aluminum tank or in a 2 or 3 liter steel tank, like the O2.

A single set of Dil and O2 tanks should be enough for two diving days, so the total gas supply for a 10 day trip for two divers doesn't take up much space. You'll also want some bailout gas, but for open water diving a single AL 30 or AL 19 for each diver will suffice for the entire trip.

In comparison, active addition SCRs generally extend the gas by a ratio of about 3 to 1, so figure 1/3rd the gas you'd use on OC. I'm not sure how much the Explorer might improve on this.

With SCR or CCR. 5.5 pounds of absorbent or a single Extend Air cartridge will last for about 4 hours in warm water under normal light swimming conditions, so for two divers for 10 diving days, you'd need 20 EACs or 110 pounds of sorb (it comes in 44-45 pound kegs).

 

[t-mac]

Thanks, DA Aquamaster. A lot to chew on. As you suggest, there is not boat that does not require constant maintenance, which is another motivation to keep diving gear maintenance down. It sounds like the bottom line is that the potential space saving and independence factors with rebreathers would be real and the maintenance is not as big a deal as I imagined. The other thing with the compressor is the space. I actually have a genset that I really don't use and may consider taking out and selling. That would free up a lot of space for compressor, tank storage or whatever. Again, thanks for the info and the perspective.

 

[The Chairman]

That was a lucid reply, DA Aquamaster!

 

[muzikbiz22]

T-Mac - You are on the right track ! I dive the Hollis Explorer often, and love it. The 40cf tank that is used with the unit, can last you a days worth of diving. I use 40%, and often get 3 each 1 hour dives. Once you learn how to work the DCP (manually), etc., it's great and fun. The scrubber is rated for 2 hours, so you will need to re-pack your scrubber, or bring a 2nd scrubber unit with you, and swap after dive 2. DA Aquamaster mentioned something about using Extend Aire cartridges. I am unaware of anyone using these, as well as any compatibility with the Explorer. Perhaps some divers are ??? Would love to know. He is correct in that they are more expensive, and they can last longer. You're better of with packing your own with "812"?. It's cheaper, and quite frankly, it's easy. DA Aquamaster does hit on a lot of great points !! At the end of the day, it's nice not having to do fills, wait on fills, not lugging "doubles", etc.

 

[DA Aquamaster]

I could be wrong on the EAC cartridges for the Explorer - I only looked at it briefly at DEMA and may have merged some things in my head.

Muzikboz, I am interested on the actual gas use with the Explorer as well as how the system actually works and how well it can maintain a PO2 set point - potentially offsetting some of the gas and deco disadvantages of an SCR.

I'm assuming that the drive gas also dictates a depth limit, and I'm curious about the gas percentages that can be used in one.

 

[muzikbiz22]

I could be wrong on the EAC cartridges for the Explorer - I only looked at it briefly at DEMA and may have merged some things in my head.

Muzikboz, I am interested on the actual gas use with the Explorer as well as how the system actually works and how well it can maintain a PO2 set point - potentially offsetting some of the gas and deco disadvantages of an SCR.

I'm assuming that the drive gas also dictates a depth limit, and I'm curious about the gas percentages that can be used in one.

I'll try to explain "normal" terms, as I am not the person to speak "technically". The range of working gas is 32 to 40 %. The higher the octane/mix, the better the unit runs. Higher mix/octane = more efficient, less injections, more time. Taking it up a level, this all works in conjunction with how you have the "DCP" set. It can be set to "Auto", or flipped to "Manual". That value range is 10 to 90. 10 being the unit uses less gas (less injections), and set to 90 the unit gives you more injections(more NDL time). I usually run mine at 10 for maximum economy. The setpoint is dynamic. The constant is the mix of gas, so the computer does the math based on that. I liken it to photography where the constant is the ISO, and the variables are the shutter speed and aperture. You're "dialing in" these different components to achieve the desired exposure, or in the case of the explorer the best combo of gas economy & NDL. You do have a depth limit, based on the MOD of the gas. For instance, if you need to hop on to the BOV, then you are breathing "straight off the tank". I would recommend downloading the manual from the Hollis sight, it's quite intuitive.

 

[DA Aquamaster]

Thanks, that helps. In some respects it is similar to the Dolphin, Ray, etc, in overall concept, except that they used different orifices for different gas mixes - 70%, 60%, 50%, 40% and 32% rather than using electronics to change the gas injection rate to modify the PO2. As you've noted the O2 percentage of the drive gas dictates the MOD for bailout even though the loop PO2 runs slightly lower at that maximum depth.

The difference however seems to be that the PO2 with the Dolphin, etc varied only with depth on any given dive with a given orifice, as the flow through the orifice was constant. That meant that with the 32% orifice, the 27 cu ft tank only provided about 45 minutes of gas, which really limited any decompression dives, and it meant you needed substantially hotter mixes to take advantage of the longer bottom times at shallower depths. It was pretty common to see Dolphins with a pair of 30s mounted on the sides of the cases feeding a common manifold to increase the available gas.

However the Explorer having the ability to use the fairly low O2 percentage drive gasses (32% and 40%) at shallower depths, but use the electronics to reduce the injection rate to balance NDLs with efficiency is an interesting approach. It doesn't really completely overcome the SCR disadvantages, but it does let you change the balance and vary the two in the direction that benefits you most on the dive.

----

I see the reference to "pre-packed scrubber" and that's what stuck in my head at DEMA, I just assumed an EAC.

I presume for a diver certified on the unit, the diver is packing his or her own scrubber, rather than using a pre-packed scrubber. The 3.3 pound scrubber isn't a bad size for two 1 hour recreational dives either, and with the Dolphin, they eventually came out with an insert to enable the diver to just half fill the scrubber, rather than use the full capacity - which better balanced the limited (stock) gas supply and scrubber life.

----

The manual for it is a lot more complicated than a KISS, and the electronics add a level of complexity and built in check list procedures that are not present with a KISS. It seems to have a lot in common with the Poseidon recreational rebreathers in that regard, with the system driving the check out and calibration process.

 

[Paul Haynes]

Hi there. My RDB and I are outfitting our boat to spend some time in the Caribbean and one consideration is maximizing our independence from shore facilities. We have chartered a number of places there, dragging along 4 tanks for our OC systems, which gives us a couple of long dives or 3-4 moderate dives each before we need to refill. Being there for a week, we are not usually too far from facilities and this isn't a big deal, but it would be ideal not to have to plan for air fills as often. One obvious solution is to outfit the boat with a compressor, which is something we will explore, but the other thought was to use rebreathers.

We are interested in them anyway, but the cost would only be justified for us if we dove as as regularly as we would if we were living aboard and island hopping. And so, I guess I would say that, aside from the quiet, which is attractive, gas storage and utilization are big factors. We are not as focused on some of the other benefits of rebreathers and really don't intend at this point to go beyond recreational limits. That leads us to consider rec rebreathers like the Hollis explorer. It is particularly attractive because it uses a single bottle of nitrox, which I would imagine makes fills relatively easy. Of course, given the smaller tank size, we would be able to transport more tanks and presumably (not sure of this) get multiple dives from each tank, giving us greater freedom between visits to fill stations. I realize maintenance, supplies, and potentially reliability are on the other side of this. We want to spend more time diving and don't want to spend all of our time maintaining, repairing and visiting dive shops.

I'd love to get some comments on whether you think this makes sense and help me to balance the considerations in choosing between a compressor (which has its own maintenance issues) and the rebreather option.

Thanks,

Tom

This 2 part article I wrote for a diving magazine regarding the whole concept and viability of the so called recreational CCR (rCCR) might be of interest and is available from the downloads section of my sites www.haynesmarine.com:

http://www.haynesmarine.co.uk/images/stories/Doomed To Repeat The Past - Part 1 Oct 12.pdf

http://www.haynesmarine.co.uk/images/stories/Doomed To Repeat The Past - Part 2 Oct 12.pdf

Here is another link to a related related rebreather article:

http://www.haynesmarine.co.uk/images/stories/A Survival Guide To Rebreather Diving Nov 2011.pdf

Rgds

Paul

 

[t-mac]

Thanks, Paul. Certainly these issues are not lost on me and prior to going down this path, all of this will be fully evaluated. Obviously, rebreathers are more complex and there are more insidious ways for them to hurt you, but at the same I have always found the citation of the death rate per user statistic to be misleading because people using rebreathers more often engage in riskier dives overall (e.g., Wes Skyles, Dave Shaw ...). Clearly rebreathers are riskier, but as you point out the technology has improved and my desire is to use them to dive perhaps less risky dives also mitigates this statistic, I believe. We do seem to be beyond the YBOD days. Your point about training is also important and tied into that is an understanding that training needs to be used regularly, which is another reason I will not go down this road until I can be comfortable with that too. Just because PADI offers a course, it does not mean anyone should take these issues less seriously. In any event, I do appreciate the cautionary note.

In light of your post, I guess it would be interesting for me to hear from others on whether the rise of the recreational rebreather, PADI training, etc. is likely to increase the bad statistics. Are manufacturers and training agencies giving us a false sense that rebreathers can be safely used by a more rec and less tec community?