My experiences in rebreather diving

[francousteau]

~snip~ If you have a bail-out valve (BOV) on your mouthpiece, which has a second-stage reg as a working part of it, you may test it every so often before and during a dive, but not for any extended period of time. If you bail out to a separate bail-out bottle, then it will have a second-stage reg attached to it. ~/snip~

Well . . . . if you're using a Y block connecting 1 side to the Dil cylinder, the other to the offboard bail-out cylinder and the 3 leg to the BOV, along with an isolater, you are then able to stay on the BOV without switching out to a seperate 2nd stage. Using this setup, the BOV is isolated (via the isolator) from the DIL cylinder and feeds only from the offboard bail-out. I still have a 2nd stage with 5' hose coiled to the bail-out cylinder, but this is for an OOG diver primarily.

Franco

 

[Dive-aholic]

Got back in the water with the GEM again today for another 1.5 hour long dive. I put the MSR bags back in the CL cordura covers but got rid of the bungee that kept the covers small. WOB was just fine that way. There's plenty of room for expansion when those bungees are gone. It was nice to have the MSRs tucked into the covers rather than spread across my chest.

I took the GEM through a passage I knew would put me vertical going in and coming out and also had a section with low ceiling so I could test the WOB in different positions and see how good my buoyancy control was. The dive went great. There was no noticeable change in WOB no matter what position I put myself in. At times I was head down moving through a passage. At another time, I was head up coming up through a fissure. The way I have the unit rigged I can also get through some pretty small restrictions. Buoyancy control was no issue through the smaller areas. All in all, the dive went great!

My working RMV was a little higher this time - .137. But I also did a lot more depth change during this dive which would account for the higher consumption rate. My deco RMV was much lower, though - .044.

Sorry, Jax, still no pics. I should be diving with someone my next GEM dive so I will try to get some pics then.

 

[Doppler]

Interesting thread Rob... and very thoughtful writing. Thanks for sharing.

 

[muddiver]

This is a great post. It really cool to hear how someone with a different rig is experiencing the diving.

First, a HUD is a Heads-Up Display that sits out in front of your mask to tell you what your RB's oxygen sensors are reading. As you probably know, there is a closed breathing loop with a RB and you need some way of knowing what the percentage of oxygen is in the air you are breathing. When you are breathing air that is fine your HUD will generally flash green. If the air is hypoxic or hyperoxic then your HUD will flash red. There's more to it, but that's the general concept. FYI, the oxygen sensors also send the data to your handsets that let you read precisely what that percentage of oxygen is.

My RB has a HUD which not only shows the red and green but also vibrates when the sensors read air that is hypoxic or hyperoxic.

When you exhale into the RB's breathing loop, your breath is generally hypoxic. If you don't move that hypoxic air fast enough within the loop, so that it will mix with the other air in the loop, you can actually push that hypoxic air past the oxygen sensors. Naturally the sensors will read the hypoxic air and send an alarm to the HUD. In my case it will also cause the HUD to vibrate. If I check and find that everything is OK, but that I'm just not breathing fast enough (actually, not breathing steadily enough) then I adjust my breathing accordingly.

It doesn't take too long to figure out what the RB wants to see. You have to recognize that on a RB you are the "motor" that makes the breathing loop work. It's not hard, just something you have to do.

I hope that makes some sense and helps.

W ripley,
I plucked this portion of your post because something struck me and I wanted to point it out. You description is only from your prosective and using the model and brand of rebreather that you have. Not all rebreathers have a HUD, let alone a vibrating HUD. The unit I dive is manual and has three oxygen sensor displays that one monitors to make sure that the gas mix is correct.

Also, it would be interesting to know what brand/model your talking about because not all rebreathers have an issue with the exhaled gas circumventing the scrubber just because one is not breathing hard. I don't get how that works out.

 

[w ripley]

You description is only from your prosective and using the model and brand of rebreather that you have. Not all rebreathers have a HUD, let alone a vibrating HUD. The unit I dive is manual and has three oxygen sensor displays that one monitors to make sure that the gas mix is correct.

Also, it would be interesting to know what brand/model your talking about because not all rebreathers have an issue with the exhaled gas circumventing the scrubber just because one is not breathing hard. I don't get how that works out.

Muddiver - The description of the HUD was there to answer Jax's reply, and to also explain that mine vibrates. It started when I said, "My rig has a vibrating HUD."

You are right in that my experience is limited, as both of my rebreathers basically work the same. The first was a MEG (actually a MEG with Hammerhead electronics) and my current RB is a Hammerhead, which also has Hammerhead electronics. Both have a loop, 2 counterlungs, 3 O2 sensors and a scrubber. I have been diving around a number of other RB divers, so I've seen their units, but haven't dived them.

I'm not aware of any RB that will allow the exhaled gas to circumvent the scrubber, as mine certainly doesn't. I can only tell you what I have observed over several years. I do have a call into the manufacturer, to see if there is some other explanation. I will post that as soon as I hear it.

 

[Mr.X]

I'm not aware of any RB that will allow the exhaled gas to circumvent the scrubber, as mine certainly doesn't. I can only tell you what I have observed over several years. I do have a call into the manufacturer, to see if there is some other explanation. I will post that as soon as I hear it.

??? I don't know of any CCR which has a loop bypass. If exhaled gas within the loop gets past the scrubber - there's usually a unintended mechanical issue within the loop, or user set up error. Pre-dive checks are so important. It would be interesting to note what you've observed over the years?

X

 

[ianr33]

Also, it would be interesting to know what brand/model your talking about because not all rebreathers have an issue with the exhaled gas circumventing the scrubber just because one is not breathing hard. I don't get how that works out.

No rebreather expert but I think "circumvent" is not he correct word here. Circumvent would imply bypass,whereas I think the issue being discussed was that with shallow breathing exhaled air will simply go backwards and forwards,being exhaled and inhaled again and never actually reaching the scrubber.

 

[Mr.X]

No rebreather expert but I think "circumvent" is not he correct word here. Circumvent would imply bypass,whereas I think the issue being discussed was that with shallow breathing exhaled air will simply go backwards and forwards,being exhaled and inhaled again and never actually reaching the scrubber.

Rebreathers as a rule use check valves to ensure that exhaled gas is moving towards the scrubber. I cannot stress the importance of the pre-dive check to ensure that everything is assembled properly and that in the protocol there is a two minute pre-breathe prior to splashing in. Warming up the scrubber, checking for leaks and making sure your sensors are reading properly, as well as verifying gas %'s are critical.

Additionally, it's important to use the term "gas" vs. "air" as the mixture in the loop is not technically air anymore.

X

 

[Dive-aholic]

Did my longest dive so far on the GEM today. 2 hours 42 mins. Estimated RMV .12. CLs are where they need to be which made it so I could finally reach inflators and exhausts with both hands. Still have to work a little with the left hand, but I couldn't even get to my shoulder exhaust with my left hand before. So this is a big improvement. Made some adjustments to the canister bolt snaps and it felt like the canister was riding much better on this dive. Didn't end up getting any pics this dive either. I was supposed to do a GEM dive at the beginning of the week but came down with a stomach bug that's been going on around work and that didn't happen. I hate getting sick on my days off! Checked out some smaller passages and felt the cooper hoses rubbing against the ceiling at times. So I need to get some protective covers for those. Buoyancy control was great, though. It's almost easier in a RB because I can breathe all I want without affecting it. That took some getting used to the first small passage I went into. I automatically started to breathe slower like I do in OC to keep from having any buoyancy changes and realized I didn't have to do that. So I focused on just breathing RB-style and it worked well. Overall a great dive!

 

[Dive-aholic]

It's been a while since I posted in this thread. It's not that I haven't been diving my GEM. I certainly have! There just hasn't been much to report. I've made a few adjustments to how I rig the unit on me and am still playing around with different ideas. One of the things that's an issue is the Cooper hose on the right side that comes off the canister. Because of the position of the canister, this hose tends to drop down alongside my arm rather than staying over my shoulder. I have a few ideas on how to correct this. I've decided to stop messing with things and to just dive the unit for a while.

I did buy Golem Gear hose covers to protect the hoses running along my back and over my shoulders and feel much better about going into smaller passages. Although I've been doing more deep stuff lately and not so much small.

I did a couple of dives to the flight deck of the Oriskany with the GEM and it worked great at depth. I found that my ppO2 drop isn't quite as much at depth as it is in shallow waters (shallow meaning above 100'). I have a theory about this. I posted this in a different thread, but will also post here in case any of you are not following that one.

Here is my theory. The drop in O2 percentage is partially due to some residual CO2 making it through the scrubber as happens in all rebreathers. This remains a constant number as long as the scrubber is working properly. The difference between shallow and deep depths (and in this I'm putting the line at 4 ata or about 100') is the ratio of CO2 left in the loop in relation to the increased density of O2 in the loop. Bear with me because I'm simplifying this and likely not using the correct terms. At 2 ata if you have 4 parts CO2 compared to O2 and N2 you will get a 4% (averaging the 3-5% drop) drop in O2 from your supply gas. So if you are using 36%, you will have a ppO2 readout of .64 on your ppO2 display and .6 on your dive computer. (Most dive computers will present a ppO2 readout to the first decimal place, so there is some significant rounding occurring.) Breathing that same mix at 4 ata will increase the density of the O2 while the CO2 density remains the same, so now you have 2 parts CO2. This results in a drop equivalent to 2% or a ppO2 reading of 1.36 on the display or 1.4 on your dive computer, which is close enough to be inconsequential in your dive planning as far as a decompression schedule. Again, this is my theory and I may not be correct in this. If I'm not, I will gladly take any critiques.

This past weekend I took the GEM on another 150' dive, this time in a cave and it performed well. A couple days later I went even deeper with it. I had the opportunity to dive the GEM in the cave that gave it's designer the inspiration to build it. He was there diving with us that day, too. Took it to 280' and it performed flawlessly. I have my GEM set up so I can do gas switches with it and I used a travel gas for a majority of the dive, switching to my deep gas just as I was beginning my descent down the pit. The GEM worked flawlessly. The deeper I got, the less the difference in the ppO2 between my display and my dive computer. One of the best parts of taking the GEM on this dive (besides the money I saved on helium by only breathing about 25 cf of 14/50, which came out to 1 cf/m!) was how warm it kept me in the 58 degree water. I had put an extra layer of undergarments under my dry suit, but I probably could have done without it. I had to towel off the sweat after the dive! I'm also thinking I need to trade my large trimix cylinders for smaller trimix cylinders. I don't need the 108s and 120s anymore, except maybe for the weight.

So I have some ideas on different ways to carry the GEM during my dives. I will be experimenting with them over the next few weeks. I'm also considering getting a gas switch block but haven't quite made up my mind on that yet.