Howard, these questions are primarily for you. I have a few buddies with rebreathers so I've done more reading on this subject lately.
My understanding is that the only use/reduction of your oxygen supply is the metabolic rate of your body and that is the same regardless of depth hence a 19 cu ft oxygen bottle may last several hours even at 300 fsw. The main breathing gas/diluent can be air or some mixture of helium/air and this is used/lost only on your ascent. Deco is minimized since you are in effect breathing a perfect deco mixture all the way up (not practical on open circuit).
First of all am I essentially correct in my understanding? It also seems to me that after dives deeper than about 250 fsw the clear preference would be for closed circuit just due to the sheer amount of tanks that would be required for open circuit as you get much below that depth. I understand the argument for closed circuit regarding the cost savings of helium (after the rebreather is paid for) but I'm more interested in the efficiency of the rebreather over OC for deeper dives.
My questions are primarly just am I correct in my understanding of the issues.
Anyone else who wants to post anything informative regarding rebreathers can feel free to use this post to do so.
Rebreathers
Started by
gcbryan
, Aug 17 2006 05:06 PM
18 replies to this topic
#1
gcbryan
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Posted 17 August 2006 - 05:06 PM
#2
ScubaDadMiami
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Posted 17 August 2006 - 05:32 PM
Everything you say is correct. I will add one clarification in case anyone reading would misinterpret one point that you raise.
As the diver descends, the CCR counterlungs are much like a wing. The increasing surrounding pressure reduces the volume of the gas inside. This provides the diver with less gas to breathe.
To compensate for this, the diver adds some gas to the counterlungs (just like adding some gas to the wing during descent). Since pure oxygen, which is what one of the two cylinders of the CCR contains, would be toxic at a depth beyond 20 feet, the gas added comes from the second tank, which can have anything ranging from air to heliox in it (depending on the depth and exposure of the dive). This supplies the necessary inert gas to the mix that prevents an ox-tox hit on the bottom.
So, when you mention that the only inert gas used/lost would be during ascent, I would just like to let those out there reading this know that some gas is injected into the breathing loop during the descent. The inert part does enter the body. However, it comes back out during ascent. So, it is lost from the tank but not actually consumed by the diver during descent and on the bottom during the dive. If the diver remains at a constant depth, the diver will never again have to inject more inert gas into the loop. From that point forward, the diver injects oxygen as the oxygen in the loop is metabolized.
Yes, you are right, increasing the PPO2 does not mean that the body uses more oxygen with every breath. The body only uses what it uses. So, the diver exhales plenty of oxygen that is not used back into the loop, and then the diver can re-inhale this, saving the need for big tanks on long, deep dives. After several breaths, the diver just adds a tiny squirt of oxygen back into the loop to account for the oxygen that the body metabolizes. It's really cool!
When the diver ascends, just as gas in a wing increases its volume, the gas in the counterlung expands. So, eventhough people say that rebreathers are bubbleless, the truth is that the CCR diver does dump some expanding gas during ascent. This is the gas that is lost after it has been added by the diluent tank. Further, once the diver gets to 20 feet, the diver will then vent all inert gas from the loop, and then flush the loop with oxygen in order to do the last stops on pure oxygen. Every couple of minutes, the diver has to remember to flush the loop again in order to remove the inert gas from the loop that is leaving the body. This keeps the mix at 100 percent from that point forward.
As the diver descends, the CCR counterlungs are much like a wing. The increasing surrounding pressure reduces the volume of the gas inside. This provides the diver with less gas to breathe.
To compensate for this, the diver adds some gas to the counterlungs (just like adding some gas to the wing during descent). Since pure oxygen, which is what one of the two cylinders of the CCR contains, would be toxic at a depth beyond 20 feet, the gas added comes from the second tank, which can have anything ranging from air to heliox in it (depending on the depth and exposure of the dive). This supplies the necessary inert gas to the mix that prevents an ox-tox hit on the bottom.
So, when you mention that the only inert gas used/lost would be during ascent, I would just like to let those out there reading this know that some gas is injected into the breathing loop during the descent. The inert part does enter the body. However, it comes back out during ascent. So, it is lost from the tank but not actually consumed by the diver during descent and on the bottom during the dive. If the diver remains at a constant depth, the diver will never again have to inject more inert gas into the loop. From that point forward, the diver injects oxygen as the oxygen in the loop is metabolized.
Yes, you are right, increasing the PPO2 does not mean that the body uses more oxygen with every breath. The body only uses what it uses. So, the diver exhales plenty of oxygen that is not used back into the loop, and then the diver can re-inhale this, saving the need for big tanks on long, deep dives. After several breaths, the diver just adds a tiny squirt of oxygen back into the loop to account for the oxygen that the body metabolizes. It's really cool!
When the diver ascends, just as gas in a wing increases its volume, the gas in the counterlung expands. So, eventhough people say that rebreathers are bubbleless, the truth is that the CCR diver does dump some expanding gas during ascent. This is the gas that is lost after it has been added by the diluent tank. Further, once the diver gets to 20 feet, the diver will then vent all inert gas from the loop, and then flush the loop with oxygen in order to do the last stops on pure oxygen. Every couple of minutes, the diver has to remember to flush the loop again in order to remove the inert gas from the loop that is leaving the body. This keeps the mix at 100 percent from that point forward.
"The most important thing is not to stop questioning." Albert Einstein
"For the diligent diver, closed circuit rebreathers are actually safer than open circuit scuba." Tom Mount
"For the diligent diver, closed circuit rebreathers are actually safer than open circuit scuba." Tom Mount
#3
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Posted 17 August 2006 - 05:52 PM
question on the DR CCR. I know that on some Rebreathers you fly it manuelly the whole time including the adding of gas, then there are some that the injection is controlled by a computer that you can over ride. Which version of this does the DR do does it inject of gas for you via the HH electronics or what?
just curious
just curious
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#4
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Posted 17 August 2006 - 05:57 PM
Great explanation Howard as always. Thanks.
Edited by gcbryan, 17 August 2006 - 08:01 PM.
#5
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Posted 17 August 2006 - 06:27 PM
Deco is minimized since you are in effect breathing a perfect deco mixture all the way up (not practical on open circuit).
Not sure if Howard wants to address this, but there is an issue here.
If we define a "perfect mix" as being 1.4 (or whatever would be appropriate for the length of the dive so as not to go over the oxygen clock) then the rebreather may or may not provide the perfect mix throughout the dive either by choice or by chance.
It is my understanding that upon descent or ascent, very few rebreathers can maintain the desired setpoint accurately. So if we decide to descend with a mix of 0.7 that mix will be somewhat variable during the descent. You would not want to set a hot mix like 1.3 on descent, have the unit not keep up with it's addition of diluent, and spike the o2 causing a hit. Likewise, you would not like to see the rebreather not keep up with the addition of oxygen at the proper time and see the mix go hypoxic.
Because rebreather dives tend to be somewhat longer than open circuit dives, and are often deeper as well, it is prudent to keep a moderate setpoint. Most of my friends are working with a 1.0 or 1.1 for their dives. So open circuit dives, though likely shorter, would be more efficient at depth if shooting for something near best mix.
All this also assumes we are talking about CCR and not SCR. SCR has some particular advantages over CCR in many circumstances, though deco efficiency is not one of them.
If I have misrepresented anything here I hope Howard or one of the other RB guys will correct me.
#6
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Posted 17 August 2006 - 06:30 PM
question on the DR CCR. I know that on some Rebreathers you fly it manuelly the whole time including the adding of gas, then there are some that the injection is controlled by a computer that you can over ride. Which version of this does the DR do does it inject of gas for you via the HH electronics or what?
just curious
The Dive Rite, the Megalodon, the Inspiration, the Evolution, the Ourboros, etc. all use computers to handle the mix. There is a new Megalodon that is beginning to ship that you fly manually, but the most popular manual rig is the KISS unit from Jetsam.
#7
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Posted 17 August 2006 - 07:56 PM
Deco is minimized since you are in effect breathing a perfect deco mixture all the way up (not practical on open circuit).
Not sure if Howard wants to address this, but there is an issue here.
If we define a "perfect mix" as being 1.4 (or whatever would be appropriate for the length of the dive so as not to go over the oxygen clock) then the rebreather may or may not provide the perfect mix throughout the dive either by choice or by chance.
It is my understanding that upon descent or ascent, very few rebreathers can maintain the desired setpoint accurately. So if we decide to descend with a mix of 0.7 that mix will be somewhat variable during the descent. You would not want to set a hot mix like 1.3 on descent, have the unit not keep up with it's addition of diluent, and spike the o2 causing a hit. Likewise, you would not like to see the rebreather not keep up with the addition of oxygen at the proper time and see the mix go hypoxic.
Because rebreather dives tend to be somewhat longer than open circuit dives, and are often deeper as well, it is prudent to keep a moderate setpoint. Most of my friends are working with a 1.0 or 1.1 for their dives. So open circuit dives, though likely shorter, would be more efficient at depth if shooting for something near best mix.
All this also assumes we are talking about CCR and not SCR. SCR has some particular advantages over CCR in many circumstances, though deco efficiency is not one of them.
If I have misrepresented anything here I hope Howard or one of the other RB guys will correct me.
I'm probably overstating things with the perfect mix comment but I would think over a deep, multi-level dive that the PPo2 would have a higher average on a CCR since there is a limit to how often and how many gas switches one can make on OC. On ascent OC would probably have a higher PPo2 just after a gas switch with CCR maintaining a higher PPo2 just before the next gas switch. Correct me if I'm off here.
Edited by gcbryan, 17 August 2006 - 07:58 PM.
#8
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Posted 17 August 2006 - 08:19 PM
Absolutely. I wish I could point you to the last copy of Advanced Diver Magazine which actually had an article comparing deco efficiency of CCR over OC, and did several examples which highlight EXACTLY what you describe here.
#9
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Posted 17 August 2006 - 09:38 PM
The DR allows for the diver to go with computer based control or manually. Believe it or not, I prefer manual, using the computer only as a back up. Here's why.
Let's say you want to keep an Oxygen Partial Pressure Set Point of 1.0 for your dive on the bottom. After descending (descent is another procedure), you set the computer to 1.0. As soon as the PPO2 drops to .99, the computer will fire a small squirt of oxygen into the loop, bringing up to about 1.1 or so. Then it will drop back over time. Sounds great so far, and it is especially if you are on a flat bottom. However, let's look at a wreck dive.
You are on the deck with the set point at 1.0 as above. As you breathe down the oxygen, the PPO2 drops to .99, and the solenoid fires some O2 into the loop, bringing the PPO2 up to 1.1. You decide to go over the side of the wreck to check out the propellors, increasing your depth by 30 feet. Now, your PPO2 has shot up to a dangerous level, and alarms are going off on the unit. To avoid this, you have to add some diluent to the loop as you are descending. You have done extra work, and you have wasted some gas (one of the big sins of CCR diving).
Now, let's replay the scene flying the unit using the computer as a backup instead of the primary controller. Instead of setting the Set Point to 1.0, you set it to .7 (you can make custom SPs, so it could be .8, .9 or whatever you want). You are on the deck of the boat, and you are puttting a squirt from the manual add oxygen button every couple of minutes. It's no big deal.
You decide to go check out the propellors. Your Heads Up Display and gauges show that you are starting to go below 1.0. You let it drop to perhaps .8. Then, you head over the side. Now, the O2 bumps up to perhaps 1.2. No big deal. You just give it a couple of minutes to lower back to 1.0. No real need to add gas (unless you descend enough that you need to add some volume to the loop). It is more efficient since you can anticipate your movement on the bottom, adding gas accordingly compared to the computer that will add it without knowing what you are planning to do next.
With CCR diving, PPO2 is not completey fixed and steady at a constant depth like it is with open circuit. On CCR, you average it out without letting it go above or below a range. So, you can set the computer to .7 but manually inject gas to let it go no higher than 1.1 but keep it no lower than .9. Your plan will be based on 1.0 and will be fine like this.
For descent, you start with a low partial pressure, typically .7 (but some use .4), and then you add gas to the loop from your diluent tank via an automatic depth compensating valve (or manually) during the descent, keeping the PPO2 from rising to a dangerous level.
Yes, regarding deco, it is highly efficient. Rather than maintaining a higher bottom set point and then ascending, lowering it until a gas switch like on open circuit, you keep it somewhat lower on the bottome. In fact, it tends to be lower because CCR divers can dive for so much longer than exposures of 1.4 are become dangerous. So, I will typically go with 1.0 on the bottom. I can keep that setting whether I descend or ascend.
Once I start my final ascent, I can maintain the same set point throughout the ascent. However, this will mean adding a lot of oxygen throughout the ascent stages, and then I will just have to vent that gas, wasting much of it. Instead, I will let the PPO2 drops as I ascend, and then I will bump it up to about 1.25 at my first deep stop, maintaining it throughout the rest of the stops as I ascend. Once I get to 20 feet, I flush the loop with pure oxygen, and I can get it up to 1.6 max (but usually let it go lower than this to about 1.35--what's a few more minutes one way or the other?).
Here is a profile for a dive that I am going to likely do next weekend:
Dec to 170ft (0) Diluent 15/30 0.40 SetPoint, 300ft/min descent.
Level 170ft 29:26 (30) Diluent 15/30 1.00 SetPoint, 106ft ead, 110ft end
Asc to 110ft (33) Diluent 15/30 1.20 SetPoint, -20ft/min ascent.
Stop at 100ft 1:00 (34) Diluent 15/30 1.20 SetPoint, 43ft ead, 67ft end
Stop at 90ft 1:00 (35) Diluent 15/30 1.20 SetPoint, 35ft ead, 61ft end
Stop at 80ft 1:00 (36) Diluent 15/30 1.20 SetPoint, 27ft ead, 54ft end
Stop at 70ft 2:00 (38) Diluent 15/30 1.20 SetPoint, 19ft ead, 48ft end
Stop at 60ft 3:00 (41) Diluent 15/30 1.20 SetPoint, 11ft ead, 41ft end
Stop at 50ft 3:00 (44) Diluent 15/30 1.20 SetPoint, 2ft ead, 35ft end
Stop at 40ft 4:00 (48) Diluent 15/30 1.20 SetPoint, 0ft ead, 28ft end
Stop at 30ft 6:00 (54) Diluent 15/30 1.20 SetPoint, 0ft ead, 22ft end
Stop at 20ft 22:00 (76) Diluent 15/30 1.20 SetPoint, 0ft ead, 15ft end
Surface (82) Diluent 15/30 -3.3ft/min ascent.
Now, lets compare that to the same kind of dive on open circuit:
Dec to 170ft (0) Trimix 15/30 300ft/min descent.
Level 170ft 29:26 (30) Trimix 15/30 0.92 ppO2, 108ft ead, 109ft end
Asc to 110ft (33) Trimix 15/30 -20ft/min ascent.
Stop at 100ft 1:00 (34) Trimix 15/30 0.60 ppO2, 60ft ead, 60ft end
Stop at 90ft 2:00 (36) Trimix 15/30 0.56 ppO2, 53ft ead, 53ft end
Stop at 80ft 2:00 (38) Trimix 15/30 0.51 ppO2, 46ft ead, 46ft end
Stop at 70ft 5:00 (43) Nitrox 50 1.56 ppO2, 32ft ead
Stop at 60ft 1:00 (44) Nitrox 50 1.41 ppO2, 26ft ead
Stop at 50ft 1:00 (45) Nitrox 50 1.26 ppO2, 20ft ead
Stop at 40ft 5:00 (50) Nitrox 50 1.10 ppO2, 13ft ead
Stop at 30ft 6:00 (56) Nitrox 50 0.95 ppO2, 7ft ead
Stop at 20ft 12:00 (68) Oxygen 1.60 ppO2, 0ft ead
Stop at 20ft 3:00 (71) Trimix 15/30 0.24 ppO2, 4ft ead, 4ft end
Stop at 20ft 7:00 (78) Oxygen 1.60 ppO2, 0ft ead
Surface (84) Oxygen -3.3ft/min ascent.
Times are almost identical. However, notice how even the curve is on deco in the mid-zones compared to open circuit. Not to mention that on one dive I have spent about $10 (at most) for gas, and I still have enough to do 3 more dives after. On the other dive, I have spent at least $75 for gas, and that's the end of my diving day.
Let's say you want to keep an Oxygen Partial Pressure Set Point of 1.0 for your dive on the bottom. After descending (descent is another procedure), you set the computer to 1.0. As soon as the PPO2 drops to .99, the computer will fire a small squirt of oxygen into the loop, bringing up to about 1.1 or so. Then it will drop back over time. Sounds great so far, and it is especially if you are on a flat bottom. However, let's look at a wreck dive.
You are on the deck with the set point at 1.0 as above. As you breathe down the oxygen, the PPO2 drops to .99, and the solenoid fires some O2 into the loop, bringing the PPO2 up to 1.1. You decide to go over the side of the wreck to check out the propellors, increasing your depth by 30 feet. Now, your PPO2 has shot up to a dangerous level, and alarms are going off on the unit. To avoid this, you have to add some diluent to the loop as you are descending. You have done extra work, and you have wasted some gas (one of the big sins of CCR diving).
Now, let's replay the scene flying the unit using the computer as a backup instead of the primary controller. Instead of setting the Set Point to 1.0, you set it to .7 (you can make custom SPs, so it could be .8, .9 or whatever you want). You are on the deck of the boat, and you are puttting a squirt from the manual add oxygen button every couple of minutes. It's no big deal.
You decide to go check out the propellors. Your Heads Up Display and gauges show that you are starting to go below 1.0. You let it drop to perhaps .8. Then, you head over the side. Now, the O2 bumps up to perhaps 1.2. No big deal. You just give it a couple of minutes to lower back to 1.0. No real need to add gas (unless you descend enough that you need to add some volume to the loop). It is more efficient since you can anticipate your movement on the bottom, adding gas accordingly compared to the computer that will add it without knowing what you are planning to do next.
With CCR diving, PPO2 is not completey fixed and steady at a constant depth like it is with open circuit. On CCR, you average it out without letting it go above or below a range. So, you can set the computer to .7 but manually inject gas to let it go no higher than 1.1 but keep it no lower than .9. Your plan will be based on 1.0 and will be fine like this.
For descent, you start with a low partial pressure, typically .7 (but some use .4), and then you add gas to the loop from your diluent tank via an automatic depth compensating valve (or manually) during the descent, keeping the PPO2 from rising to a dangerous level.
Yes, regarding deco, it is highly efficient. Rather than maintaining a higher bottom set point and then ascending, lowering it until a gas switch like on open circuit, you keep it somewhat lower on the bottome. In fact, it tends to be lower because CCR divers can dive for so much longer than exposures of 1.4 are become dangerous. So, I will typically go with 1.0 on the bottom. I can keep that setting whether I descend or ascend.
Once I start my final ascent, I can maintain the same set point throughout the ascent. However, this will mean adding a lot of oxygen throughout the ascent stages, and then I will just have to vent that gas, wasting much of it. Instead, I will let the PPO2 drops as I ascend, and then I will bump it up to about 1.25 at my first deep stop, maintaining it throughout the rest of the stops as I ascend. Once I get to 20 feet, I flush the loop with pure oxygen, and I can get it up to 1.6 max (but usually let it go lower than this to about 1.35--what's a few more minutes one way or the other?).
Here is a profile for a dive that I am going to likely do next weekend:
Dec to 170ft (0) Diluent 15/30 0.40 SetPoint, 300ft/min descent.
Level 170ft 29:26 (30) Diluent 15/30 1.00 SetPoint, 106ft ead, 110ft end
Asc to 110ft (33) Diluent 15/30 1.20 SetPoint, -20ft/min ascent.
Stop at 100ft 1:00 (34) Diluent 15/30 1.20 SetPoint, 43ft ead, 67ft end
Stop at 90ft 1:00 (35) Diluent 15/30 1.20 SetPoint, 35ft ead, 61ft end
Stop at 80ft 1:00 (36) Diluent 15/30 1.20 SetPoint, 27ft ead, 54ft end
Stop at 70ft 2:00 (38) Diluent 15/30 1.20 SetPoint, 19ft ead, 48ft end
Stop at 60ft 3:00 (41) Diluent 15/30 1.20 SetPoint, 11ft ead, 41ft end
Stop at 50ft 3:00 (44) Diluent 15/30 1.20 SetPoint, 2ft ead, 35ft end
Stop at 40ft 4:00 (48) Diluent 15/30 1.20 SetPoint, 0ft ead, 28ft end
Stop at 30ft 6:00 (54) Diluent 15/30 1.20 SetPoint, 0ft ead, 22ft end
Stop at 20ft 22:00 (76) Diluent 15/30 1.20 SetPoint, 0ft ead, 15ft end
Surface (82) Diluent 15/30 -3.3ft/min ascent.
Now, lets compare that to the same kind of dive on open circuit:
Dec to 170ft (0) Trimix 15/30 300ft/min descent.
Level 170ft 29:26 (30) Trimix 15/30 0.92 ppO2, 108ft ead, 109ft end
Asc to 110ft (33) Trimix 15/30 -20ft/min ascent.
Stop at 100ft 1:00 (34) Trimix 15/30 0.60 ppO2, 60ft ead, 60ft end
Stop at 90ft 2:00 (36) Trimix 15/30 0.56 ppO2, 53ft ead, 53ft end
Stop at 80ft 2:00 (38) Trimix 15/30 0.51 ppO2, 46ft ead, 46ft end
Stop at 70ft 5:00 (43) Nitrox 50 1.56 ppO2, 32ft ead
Stop at 60ft 1:00 (44) Nitrox 50 1.41 ppO2, 26ft ead
Stop at 50ft 1:00 (45) Nitrox 50 1.26 ppO2, 20ft ead
Stop at 40ft 5:00 (50) Nitrox 50 1.10 ppO2, 13ft ead
Stop at 30ft 6:00 (56) Nitrox 50 0.95 ppO2, 7ft ead
Stop at 20ft 12:00 (68) Oxygen 1.60 ppO2, 0ft ead
Stop at 20ft 3:00 (71) Trimix 15/30 0.24 ppO2, 4ft ead, 4ft end
Stop at 20ft 7:00 (78) Oxygen 1.60 ppO2, 0ft ead
Surface (84) Oxygen -3.3ft/min ascent.
Times are almost identical. However, notice how even the curve is on deco in the mid-zones compared to open circuit. Not to mention that on one dive I have spent about $10 (at most) for gas, and I still have enough to do 3 more dives after. On the other dive, I have spent at least $75 for gas, and that's the end of my diving day.
"The most important thing is not to stop questioning." Albert Einstein
"For the diligent diver, closed circuit rebreathers are actually safer than open circuit scuba." Tom Mount
"For the diligent diver, closed circuit rebreathers are actually safer than open circuit scuba." Tom Mount
#10
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Posted 17 August 2006 - 10:05 PM
First of all am I essentially correct in my understanding? It also seems to me that after dives deeper than about 250 fsw the clear preference would be for closed circuit just due to the sheer amount of tanks that would be required for open circuit as you get much below that depth. .
You should still have enough gas to get you back should the RB fail and you need to go to bail out.19cuft doesn't go far at 250-300 ft.
#11
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Posted 17 August 2006 - 10:26 PM
Dec to 170ft (0) Diluent 15/30 0.40 SetPoint, 300ft/min descent.
#12
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Posted 17 August 2006 - 11:00 PM
Dec to 170ft (0) Diluent 15/30 0.40 SetPoint, 300ft/min descent.
Not up to the challange Perrone
Nothing a heavier weightbelt can't fix 
#13
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Posted 17 August 2006 - 11:09 PM
I couldn't drop that fast with a scooter.
#14
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Posted 17 August 2006 - 11:14 PM
I couldn't drop that fast with a scooter.
Thats why the HEAVY weightbelt....
#15
gcbryan
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Posted 18 August 2006 - 12:53 AM
First of all am I essentially correct in my understanding? It also seems to me that after dives deeper than about 250 fsw the clear preference would be for closed circuit just due to the sheer amount of tanks that would be required for open circuit as you get much below that depth. .
You should still have enough gas to get you back should the RB fail and you need to go to bail out.19cuft doesn't go far at 250-300 ft.
Then in essense to dive a rebreather you have to carry all the gas that you would need for OC in addition to the rebreather? If you rebreather failed at the deepest part of the dive and at the max deco obligation?
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