[WhackUmStackUm]

Since there seems to be some interest amoung Forum members, I started this thread to record the progress of the design and construction of my low budget rebreather project.

:nuke:I'm not recommending that anyout build one of these.:nuke: I just thought some folks might be interested in following my progress.


Initial design goals:

1. This will be a bail-out rebreather (or BOB). During deep dives I will carry it as a backup incase my main rebreather fails completely.
2. This rebreather will be used to support diving on deep reefs in open water. It may not be suitable for cave diving and other overhead environments.
3. It must be reasonably light-weight and low volume.
4. It must be inexpensive relative to commercial rebreathers. Target cost is $500 to $1,000, not including the dive computer.
5. O2 will be added manually on the first version, but version two will add O2 automatically.
6. Diluent gas will be added manually. Although this is easy to add on, I doubt I will need automatic dil addition since I will be ascending in a bail-out situation.
7. It must support diving to 400'.
8. It should be flexible enough to support different amounts of anticipated deco time (30-90+ minutes).
9. I will use off-the-shelf parts whenever possible.
10. Keep it simple by including only options that I will use. For example, I allways use the same PPO2 set points, so there is no need for me to be able to adjust them.
11. Since I will be switching from one rebreather to another in a bail-out situation, I plan to use an independent dive computer to track my deco obligation.
12. Displays need to be both BIG and simple.
13. No, I am not allowing others to take life insurance policies out on me.

Since no one in their right mind would build a rebreather from scratch, I decided to call this project "Whack BOB."

Cheers,
Whack 'um

 

[WhackUmStackUm]

Since you probably won't be shooting fish, or will you....

No spearfishing initially. However, there is a BIG warsaw grouper out there with my name on it.

...at 100 meters or more have you considered a chest mounted scrubber with a manual needle valve for O2 injection. A couple of 9 cf bottles of dil and o2 on your back should be more than enough to get back to the surface. With a little creative plumbing, between the little tanks and your standard tanks you would probably not have to abort a dive. A couple of sensors built into the inhale side of the scrubber to keep things compact.

Yes, I am considering the same type of setup.

Would a single counter lung be enough at that depth for dwell time? A scrubber shaped like my dolphin scrubber with its oval shape would probably work.

The counter lung configuration is something I will have to experiment with.

Will your (3D) printer make clear objects?

I am not sure. I have been building parts out of black and yellow ABS plastic filament. The way the plastic is melted and laid down in layers, I doubt a part would end up clear, even if you started with clear filament.

 

[coolbluestreak]

I'm following....
What are you looking to use for a computor, make model?

 

[WhackUmStackUm]

I will add to this list as needed during the project.

Please note these are quick and dirty definitions. If you would like the complete definitions, including special cases, feel free to Google the terms.


Rebreather-related terms:

• Open circuit: The term used to describe a normal scuba setup. You breathe air in from a tank and out into the water.
• Closed circuit: A breathing system which allows you to breathe the same gas over and over again.
• Semi-closed circuit: A breathing system which allows you to breathe most of the gas over and over, but some gas is exhausted into the water.
• Rebreather bailout or bailout: A completely redundant scuba system which allows a rebreather diver to fulfill his deco obligation and return to the surface should the rebreather fail in a catastrophic way.
• PPO2: Partial pressure of oxygen. There is a range of PPO2 levels, in your breathing gas, in which your body will function normally. PPO2 is equal to the % of O2 of your mix (fraction of O2 or FO2) times the numbers of atmospheres of pressure the gas is under when it enters your lungs. So the PPO2 of 30% (0.30) nitrox at the surface would be 0.30 (FO2) x 1 (ATM) = 0.30 (PPO2). At a depth of 33 feet (one more atmosphere) the PPO2 of the same mix would be 0.30 (FO2) x 2 (ATM) = 0.60 (PPO2).

 

[WhackUmStackUm]

I'm following....
What are you looking to use for a computor, make model?

My current plan is to go with a Liquivision X1. I need a trimix computer which supports constant PPO2 diving. My friends that have one, love it.

 

[TONER]

Brian
what are the major benefits of a rebreather over scuba how much training do you need is it as safe as scuba with proper training and how much for a good set-up thanks

 

[WhackUmStackUm]

Brian
what are the major benefits of a rebreather over scuba?

Good question!

Here are what I consider to be the benefits and drawbacks of diving a rebreather:


Cons:

1. Rebreather gear is expensive to buy and maintain relative to recreational open circuit systems. However, technical diving open circuit gear is expensive as well. Rebreather prices are coming down, but they are still costly.
2. Pre-dive checks and equipment cleaning are more time consuming.
3. Certification training can be expensive, depending on where you get it, and how far you have to travel.
4. A rebreather capable of deep diving (500'+) can be more hassle than it is worth when diving shallow spots of 60' or less.
5. As with any technical diving, staying down longer presents new challenges such as becoming chilled, muscle fatigue, chaffing because you are swimming longer, and wrinkling up like a prune!

Pros:

1. You can stay in the water much longer given the same volume of gas. A fresh setup on my rebreather is good for about 4 hours in the water, independent of depth. My rebreather uses two 15 cu ft tanks, one with O2 and one with air or trimix. When I go diving I like to spend as much time as I can in the water!
2. A closed circuit rebreather is almost silent. The wildlife experience is completely different. Animals treat you more like a fish, than an intruder.
3. I get about 20% more no-deco bottom time over nitrox. However, I almost always have deco to do. Why not stay longer? I have plenty of gas!
4. Because we carry many redundant systems, I feel that I have more options if/when there is an equipment failure. Honestly, this has more to do with a tech diving mindset than the rebreather. Open circuit tech divers also carry redundant systems.

Like open circuit technical diving, rebreather diving is not for everyone. For me the pros outweigh the cons. Diving is my main hobby these days, and I dive a lot.

How much training do you need?

Like any scuba certification, the requirements depend on the agency. IANTD requires classroom time, a test, about 500 minutes in the pool, and about 8 hours of rebreather diving in order to get an open water card (130' max). Most of the other agencies have either added recently, or will be adding soon, recreational rebreather certification classes.

Is it as safe as scuba with proper training?

Personally I would not switch to a rebreather if I was going stick to no-deco diving at recreational depths (< 130'). So let me rephrase the question as, "Is closed circuit technical diving as safe as open circuit techincal diving?"

Indeed, I feel safer on closed circuit than open circuit. Why? Because there is little risk of running out of air.

Think about it. How would your diving experience change if you were carrying 4 hours of air?

How much for a good set-up thanks

Retail prices vary depending on the bells and whistles you choose. Prices are also coming down steadily. Today most new closed circuit rebreathers cost between $5,000 and $10,000. You can sometimes find good deals on used gear on the Internet. I sold one that needed some TLC for less-than $2,000 this year. It was given to me in exchange for some work I did.

 

[Silent but Deadly]

I'll jump in on this. Yes a rebreather is a lot of maintenence. But, I can dive all day for about 10.00 a day, NOT a dive. I generally spend about 2 hours and some change diving a day, limited to the other guys out of air and tired of waiting on me. I have what I call a true recreational rebreather. Its completely self contained. I have 19cf of dedicated bailout and 19cf of dil, with a 13cf tank of O2 underneath the other tanks in a stand. I strap it on like a bc, turn on my O2 and go diving. I can be ready faster than the other guys usually. The downside is the manufacturers depth limit is 165' and I'm not trimix certified so most of my diving is relatively shallow. (compared to whackum) When you are pushing black snapper away instead of chasing them, you know something has changed for the better.

 

[Orion45]

Brian
what are the major benefits of a rebreather over scuba how much training do you need is it as safe as scuba with proper training and how much for a good set-up thanks

Are you thinking about switching to a rebreather?

 

[TONER]

i dont know enough about them i have always heard they were dangerous but before i moved down here i thought scuba diving was crazy Brians posts has sparked some intrest

 

[Evensplit]

Hollis is here this week doing an instructor certification class for the Prism2 rb. We've had lots of rb's come through the shop over the years, but this is the first full blown class we've had here. The future instructors must complete 100 hours on the unit before they get their instructopr credentials.

The best question I've heard so far was "what if you puke in the loop?" The look on everyone's face was priceless. "Well, it's kinda like shi**ing in your drysuit."

 

[WhackUmStackUm]

I broke the larger design tasks down into the following subsystems:

1. PPO2 measurement and display
2. CO2 scrubber
3. O2 addition
4. Diluent addition
5. Breathing loop (i.e mouthpiece, hoses, counter lungs and water management)
6. Physical layout

 

[Silent but Deadly]

Do you have a starting design that we can pick apart, make suggestions (or copy)? I think I would start with arranging everything since you are filling space you don't really have to spare. And then work on every component to make it the best design possible.

 

[WhackUmStackUm]

Hollis is here this week doing an instructor certification class for the Prism2 rb. We've had lots of rb's come through the shop over the years, but this is the first full blown class we've had here. The future instructors must complete 100 hours on the unit before they get their instructopr credentials.

The best question I've heard so far was "what if you puke in the loop?" The look on everyone's face was priceless. "Well, it's kinda like shi**ing in your drysuit."

lol - I wish I had been there to see their reaction. However, it is not difficult to deal with, in practice.

 

[Orion45]

However, it is not difficult to deal with, in practice.

Which one...the puking or the shi**ing?

 

[WhackUmStackUm]

Which one...the puking or the shi**ing?

Yes!

 

[TONER]

OK another dumb question
how long can you dive (rebreather) at 160' (depth at tenaco house) with minimal deco time? i am assuming you generaly have a deco stop?

 

[mike6043]

Have you looked at using msr bags for counter lungs?

 

[WhackUmStackUm]

OK another dumb question
how long can you dive (rebreather) at 160' (depth at tenaco house) with minimal deco time? i am assuming you generaly have a deco stop?

There are a number of variables. The PPO2 level you are diving, the amount and mix of the bailout gas you are carrying, the amount of helium in your diluent bottle, and a few other variables factor in. The higher the oxygen level (PPO2), within safe limits, the slower you will accumulate deco time. More helium generally adds to your deco time. The amount and type of your bailout gas(es) limit how much deco time you can do should the rebreather fail completely.

Here are some scenarios:

If you were diving my rebreather with my gradient settings (ignore this for now) and...
if you dive to 160' and...
you do not want to do any deco and...
you keep your PPO2 at 1.3, and...
you are using 21% oxygen and 20% helium in your diluent, then...
your maximum bottom time would be around 8 minutes.

This is not enough time to do much, so let’s plan on doing some deco.

If you want to only carry one 40 cu ft bailout bottle (with 1/3 reserve) and...
you keep your PPO2 at 1.3, and...
everything else is the same, then
your maximum bottom time would be a little over 16 minutes and with a bailout mix of 26/8 (02%/He%).

Better...but still too short for me.

If you were willing to carry both a 30 cu ft and a 40 cu ft bailout bottle, then...
your maximum bottom time would be a little over 26 minutes, with bail-out mixes of 26/8 trimix and 63% nitrox. In this case you would do 22 minutes of deco.

My dives typically run from 60 to 90 minutes, with 20-40 minutes of deco.

 

[TONER]

why do you keep your PPO2 so low (1.3) ? if i'm understanding the term right i set my computer to allow a 1.6 higher O2 means less nitrogen?
by the way those last numbers were very informative to me to help guage your alloable time thanks