Yes, I'm wondering why there would be more CO2 molecules at depth? I understand that gas density increases as a function of depth (e.g. pressure)...but CO2 is a product of metabolism. Assuming the same workload, CO2 production should be the same at depth as it is at the surface, yes? Although you will fit those same CO2 molecules into a smaller space at depth, should the number of molecules be the same regardless of depth?
Why depth has an effect on sorb capability to scrub CO2
- Thread starter fsardone
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
Just curious, but if a guy consumes 1lpm of oxygen regardless of depth, how is he producing multiples of that in CO2 at depth say 5ata or 10ata? Lol
OP
fsardone
Solo Diver
There is always the same number of CO2 molecules.
This makes the scrubber less capable to grab them because there are more molecules [edit: OF OTHER GAS DUE TO INCREASED DEPTH] in the way of those few molecules of CO2.
This makes the scrubber less capable to grab them because there are more molecules [edit: OF OTHER GAS DUE TO INCREASED DEPTH] in the way of those few molecules of CO2.
Huh? Read what you just wrote.
broncobowsher
Contributor
The goal is to have no CO2 coming out of the scrubber. You exhale about .9L/min (I just use the simple math of 1L/min just like the O2 even though I know it is wrong).
Every breath you dump about the same number of molecules of CO2 into the scrubber regardless of depth.
The extra gas density makes it harder for the CO2 to find a home in the scrubber. Lowers the percentage of CO2 due to dilution but not the quantity of molecules.
Getting anything besides the slightest trace through the scrubber is considered a breakthrough and is to be avoided.
If the high gas density is insulating the CO2 away from the scrubber, going to a lighter (shallower) gas density will reduce the extra gas getting in the way of the CO2 finding its new home in the scrubber.
Every breath you dump about the same number of molecules of CO2 into the scrubber regardless of depth.
The extra gas density makes it harder for the CO2 to find a home in the scrubber. Lowers the percentage of CO2 due to dilution but not the quantity of molecules.
Getting anything besides the slightest trace through the scrubber is considered a breakthrough and is to be avoided.
If the high gas density is insulating the CO2 away from the scrubber, going to a lighter (shallower) gas density will reduce the extra gas getting in the way of the CO2 finding its new home in the scrubber.
OP
fsardone
Solo Diver
Are you talking to me?
Not sure because you did not quote but replied just after my message.
In case you were this is what I wrote ...
If you were referring to increased CO2 molecules the following is the message that introduced the (wrong) idea that the number of [CO2] molecules would increase with depth [WHILE THE TOTAL NUMBER ACTUALLY DOES].
Finally, if your comment above was addressed to me, no offence taken. Just to make clear I never thought depth has any influence on the production of metabolic CO2 and yes you get more use out of the scrubber when you decrease your dept. The last message of yours (here quoted) is mainly the reason why I decided to start this thread.
I DID CLARIFY IN CAPS THIS AND MY PREVIOUS MESSAGE, WHERE MIGHT LEAD TO MISUNDERSTANDING. I DO APOLOGIZE IF I WAS NOT CLEARER THE FIRST TIME OUT.
Regards
I’m not an expert in chemistry or physics but this is what I have figured out about scrubber efficiency. With some crude simplifications.
The most important factor to scrubber efficiency is temperature!!! Many chemical reaction rates double for every 10℃ temperature rise.
The chemical reaction of scrubbing CO2 out of the gas is an exothermic reaction. Meaning it produces heat.
Heat can be lost by three different means: convection, conduction and radiation
Convection:
At depth the gas is denser. The specific heat (energy needed to heat the gas, kJ/(kg*K)) increases with the density. The denser gas at depth takes away more heat from the reaction (by convection) and therefore decrease the reaction temperature and scrubber efficiency.
Nitrogen N2 got a specific heat of 1.04 kJ/(kg*K). Helium He got 5.19 kJ/(kg*K). So is nitrox better than trimix?
No. You have to take density into account. N2 got a density of 1.165kg/m^3. He got a much lower density of 0.1664 kg/m^3. So if specific heat is converted to 1 atm cubic meters instead of kilograms the values turn around. N2 got about 1.2 kJ/(m^3*K) and He got 0.86 kJ/(m^3*K). Helium transfers 29% less energy from the reaction than N2.
As a less dense gas He also lowers the work of breathing (WOB).
Conduction:
Heat loss by conduction comes from the temperature gradient between the inside and outside of the scrubber. Heat is conducted from the warmer material to the colder material. An aluminum tube filled with soda lime is much worse than a plastic scrubber cartridge inside a scrubber canister. Thermal conductivity of aluminum is 205 W/(m*K) and of hard PVC-U plastic is only 0.16 W/(m*K).
An air gap between the cartridge and the canister insulates the soda lime better from the surrounding cold water. Thermal conductivity of air is as low as 0.0262 W/(m*K) at 1 atm. It gets higher with increasing depth/density. The best option is a radial in-to-out scrubber or a co-axial scrubber where the warm inhale gas flows between the canister and the cartridge.
Radiation:
All materials radiate thermal energy based on their temperature. The hotter an object, the more it will radiate.
The middle of the scrubber gets more thermal radiation from surrounding material than a position closer to the edge. This phenomenon is one more reason why the middle of the scrubber is warmer.
One interesting and easy thing to try is covering the inside of the scrubber canister or cartridge with heat reflective sheet with one sided adhesive (check for off-gassing!). I planned to test this with my next DIY unit. This could potentially even out the thermal profile though the scrubber from middle to the edge.
Scrubber breakthrough
One way to increase the scrubber efficiency is to increase the size. Tests show size versus time to breakthrough is not linear. A half size scrubber last for less than half the time. Doubling the size more than doubles the time. This may be caused by the long cone shaped reaction front. With a flat and short reaction front the size to time ratio would be more linear.
Average dwell time does not depend on geometry. Only on scrubber volume. Scrubber geometry and counterlung design affect the gas flow speed in the scrubber.
Scrubber breakthrough happens when the reaction front reaches the end of the scrubber. Time to breakthrough can be expanded by making the reaction front shorter and less cone shaped.
Gas flow speed and scrubber temperature affect the length of the reaction zone. Lower gas flow speed and hotter temperature makes the reaction front shorter and therefore increase the efficiency. More soda lime is used before breakthrough
The scrubber is hotter in the middle because of thermal conductivity and radiation. This makes the reaction front cone shaped in a round axial scrubber. Longer at the sides and short in the middle of the scrubber. The cone shape can be reduced by evening out the thermal differences with good thermal insulation and maybe with the heat reflective sheet.
The length of the reaction front caused by gas flow speed can be changed with scrubber geometry and counterlung design. A long and narrow scrubber tube has a higher gas flow speed because of smaller cross section area. The Mk15/16 rebreather got a donut shape axial scrubber design. The length (L) to surface area (A) ratio L/A is very low. The KISS rebreather got a long and narrow scrubber tube with high L/A ratio. The L/A ratio affect the gas flow speed and WOB.
To make things more complicated the cross section area also affect the temperature. With a larger cross section the heat is spread over a larger reaction front making it cooler. There is a usable range of L/A ratios and you have to make compromises between WOB, gas flow and temperature.
Counterlung design affect the flow speed. Average flow speed stays the same but peak flow speed is half with a dual counterlung design. With a single counterlung you push gas through the scrubber for half the time of your breathing cycle followed with an equal length pause. Gas moves through the scrubber at inhale or exhale cycle only. With dual counterlungs gas flows the scrubber at both inhale and exhale cycles with half the peak flow speed.
With a single counterlung you have an exhale counterlung if the rebreather is front mounted. You have an inhale counterlung for back mounted units. This is because you can use the hydrostatic imbalance (pressure difference caused by the water column height between the lung centroid and the air fill level in the counterlung) to your advantage to reduce WOB. The pressure differen assists the gas though the scrubber.
To make things more complicated gas can drastically cool down in the counterlung. A single inhale counterlung may be better in some design because the warm exhale breath goes directly into the scrubber. The dual counterlung may cool the gas so much in the exhale counterlung it makes the scrubber less efficient. One good example is the Sentinel rebreather. It is known for good scrubber efficiency.
That old “Here on Zord…” explanation is BS. It’s a kindergarten type explanation with no scientific basis. The extra N2 or He diluent molecules do not work as an chemical inhibitor. The increase in gas molecule collisions under higher pressure works both ways and cancel each out. A CO2 molecule can collision and change path from or to soda lime. Extra N2 or He molecules do not interfere with the scrubbing chemical reaction or the reaction in O2 sensors other than by thermal transfer.
Check for interesting WOB & scrubber docs at Theory&Docs | DIY Rebreathers
If I missed any good documents please let me know.
Sorry for the long post
I left out some minor stuff.
The most important factor to scrubber efficiency is temperature!!! Many chemical reaction rates double for every 10℃ temperature rise.
The chemical reaction of scrubbing CO2 out of the gas is an exothermic reaction. Meaning it produces heat.
Heat can be lost by three different means: convection, conduction and radiation
Convection:
At depth the gas is denser. The specific heat (energy needed to heat the gas, kJ/(kg*K)) increases with the density. The denser gas at depth takes away more heat from the reaction (by convection) and therefore decrease the reaction temperature and scrubber efficiency.
Nitrogen N2 got a specific heat of 1.04 kJ/(kg*K). Helium He got 5.19 kJ/(kg*K). So is nitrox better than trimix?
No. You have to take density into account. N2 got a density of 1.165kg/m^3. He got a much lower density of 0.1664 kg/m^3. So if specific heat is converted to 1 atm cubic meters instead of kilograms the values turn around. N2 got about 1.2 kJ/(m^3*K) and He got 0.86 kJ/(m^3*K). Helium transfers 29% less energy from the reaction than N2.
As a less dense gas He also lowers the work of breathing (WOB).
Conduction:
Heat loss by conduction comes from the temperature gradient between the inside and outside of the scrubber. Heat is conducted from the warmer material to the colder material. An aluminum tube filled with soda lime is much worse than a plastic scrubber cartridge inside a scrubber canister. Thermal conductivity of aluminum is 205 W/(m*K) and of hard PVC-U plastic is only 0.16 W/(m*K).
An air gap between the cartridge and the canister insulates the soda lime better from the surrounding cold water. Thermal conductivity of air is as low as 0.0262 W/(m*K) at 1 atm. It gets higher with increasing depth/density. The best option is a radial in-to-out scrubber or a co-axial scrubber where the warm inhale gas flows between the canister and the cartridge.
Radiation:
All materials radiate thermal energy based on their temperature. The hotter an object, the more it will radiate.
The middle of the scrubber gets more thermal radiation from surrounding material than a position closer to the edge. This phenomenon is one more reason why the middle of the scrubber is warmer.
One interesting and easy thing to try is covering the inside of the scrubber canister or cartridge with heat reflective sheet with one sided adhesive (check for off-gassing!). I planned to test this with my next DIY unit. This could potentially even out the thermal profile though the scrubber from middle to the edge.
Scrubber breakthrough
One way to increase the scrubber efficiency is to increase the size. Tests show size versus time to breakthrough is not linear. A half size scrubber last for less than half the time. Doubling the size more than doubles the time. This may be caused by the long cone shaped reaction front. With a flat and short reaction front the size to time ratio would be more linear.
Average dwell time does not depend on geometry. Only on scrubber volume. Scrubber geometry and counterlung design affect the gas flow speed in the scrubber.
Scrubber breakthrough happens when the reaction front reaches the end of the scrubber. Time to breakthrough can be expanded by making the reaction front shorter and less cone shaped.
Gas flow speed and scrubber temperature affect the length of the reaction zone. Lower gas flow speed and hotter temperature makes the reaction front shorter and therefore increase the efficiency. More soda lime is used before breakthrough
The scrubber is hotter in the middle because of thermal conductivity and radiation. This makes the reaction front cone shaped in a round axial scrubber. Longer at the sides and short in the middle of the scrubber. The cone shape can be reduced by evening out the thermal differences with good thermal insulation and maybe with the heat reflective sheet.
The length of the reaction front caused by gas flow speed can be changed with scrubber geometry and counterlung design. A long and narrow scrubber tube has a higher gas flow speed because of smaller cross section area. The Mk15/16 rebreather got a donut shape axial scrubber design. The length (L) to surface area (A) ratio L/A is very low. The KISS rebreather got a long and narrow scrubber tube with high L/A ratio. The L/A ratio affect the gas flow speed and WOB.
To make things more complicated the cross section area also affect the temperature. With a larger cross section the heat is spread over a larger reaction front making it cooler. There is a usable range of L/A ratios and you have to make compromises between WOB, gas flow and temperature.
Counterlung design affect the flow speed. Average flow speed stays the same but peak flow speed is half with a dual counterlung design. With a single counterlung you push gas through the scrubber for half the time of your breathing cycle followed with an equal length pause. Gas moves through the scrubber at inhale or exhale cycle only. With dual counterlungs gas flows the scrubber at both inhale and exhale cycles with half the peak flow speed.
With a single counterlung you have an exhale counterlung if the rebreather is front mounted. You have an inhale counterlung for back mounted units. This is because you can use the hydrostatic imbalance (pressure difference caused by the water column height between the lung centroid and the air fill level in the counterlung) to your advantage to reduce WOB. The pressure differen assists the gas though the scrubber.
To make things more complicated gas can drastically cool down in the counterlung. A single inhale counterlung may be better in some design because the warm exhale breath goes directly into the scrubber. The dual counterlung may cool the gas so much in the exhale counterlung it makes the scrubber less efficient. One good example is the Sentinel rebreather. It is known for good scrubber efficiency.
That old “Here on Zord…” explanation is BS. It’s a kindergarten type explanation with no scientific basis. The extra N2 or He diluent molecules do not work as an chemical inhibitor. The increase in gas molecule collisions under higher pressure works both ways and cancel each out. A CO2 molecule can collision and change path from or to soda lime. Extra N2 or He molecules do not interfere with the scrubbing chemical reaction or the reaction in O2 sensors other than by thermal transfer.
Check for interesting WOB & scrubber docs at Theory&Docs | DIY Rebreathers
If I missed any good documents please let me know.
Sorry for the long post
I left out some minor stuff.
I’m not an expert in chemistry or physics but this is what I have figured out about scrubber efficiency. With some crude simplifications.
The most important factor to scrubber efficiency is temperature!!! Many chemical reaction rates double for every 10℃ temperature rise.
The chemical reaction of scrubbing CO2 out of the gas is an exothermic reaction. Meaning it produces heat.
Heat can be lost by three different means: convection, conduction and radiation
Convection:
At depth the gas is denser. The specific heat (energy needed to heat the gas, kJ/(kg*K)) increases with the density. The denser gas at depth takes away more heat from the reaction (by convection) and therefore decrease the reaction temperature and scrubber efficiency.
Nitrogen N2 got a specific heat of 1.04 kJ/(kg*K). Helium He got 5.19 kJ/(kg*K). So is nitrox better than trimix?
No. You have to take density into account. N2 got a density of 1.165kg/m^3. He got a much lower density of 0.1664 kg/m^3. So if specific heat is converted to 1 atm cubic meters instead of kilograms the values turn around. N2 got about 1.2 kJ/(m^3*K) and He got 0.86 kJ/(m^3*K). Helium transfers 29% less energy from the reaction than N2.
As a less dense gas He also lowers the work of breathing (WOB).
Conduction:
Heat loss by conduction comes from the temperature gradient between the inside and outside of the scrubber. Heat is conducted from the warmer material to the colder material. An aluminum tube filled with soda lime is much worse than a plastic scrubber cartridge inside a scrubber canister. Thermal conductivity of aluminum is 205 W/(m*K) and of hard PVC-U plastic is only 0.16 W/(m*K).
An air gap between the cartridge and the canister insulates the soda lime better from the surrounding cold water. Thermal conductivity of air is as low as 0.0262 W/(m*K) at 1 atm. It gets higher with increasing depth/density. The best option is a radial in-to-out scrubber or a co-axial scrubber where the warm inhale gas flows between the canister and the cartridge.
Radiation:
All materials radiate thermal energy based on their temperature. The hotter an object, the more it will radiate.
The middle of the scrubber gets more thermal radiation from surrounding material than a position closer to the edge. This phenomenon is one more reason why the middle of the scrubber is warmer.
One interesting and easy thing to try is covering the inside of the scrubber canister or cartridge with heat reflective sheet with one sided adhesive (check for off-gassing!). I planned to test this with my next DIY unit. This could potentially even out the thermal profile though the scrubber from middle to the edge.
Scrubber breakthrough
One way to increase the scrubber efficiency is to increase the size. Tests show size versus time to breakthrough is not linear. A half size scrubber last for less than half the time. Doubling the size more than doubles the time. This may be caused by the long cone shaped reaction front. With a flat and short reaction front the size to time ratio would be more linear.
Average dwell time does not depend on geometry. Only on scrubber volume. Scrubber geometry and counterlung design affect the gas flow speed in the scrubber.
Scrubber breakthrough happens when the reaction front reaches the end of the scrubber. Time to breakthrough can be expanded by making the reaction front shorter and less cone shaped.
Gas flow speed and scrubber temperature affect the length of the reaction zone. Lower gas flow speed and hotter temperature makes the reaction front shorter and therefore increase the efficiency. More soda lime is used before breakthrough
The scrubber is hotter in the middle because of thermal conductivity and radiation. This makes the reaction front cone shaped in a round axial scrubber. Longer at the sides and short in the middle of the scrubber. The cone shape can be reduced by evening out the thermal differences with good thermal insulation and maybe with the heat reflective sheet.
The length of the reaction front caused by gas flow speed can be changed with scrubber geometry and counterlung design. A long and narrow scrubber tube has a higher gas flow speed because of smaller cross section area. The Mk15/16 rebreather got a donut shape axial scrubber design. The length (L) to surface area (A) ratio L/A is very low. The KISS rebreather got a long and narrow scrubber tube with high L/A ratio. The L/A ratio affect the gas flow speed and WOB.
To make things more complicated the cross section area also affect the temperature. With a larger cross section the heat is spread over a larger reaction front making it cooler. There is a usable range of L/A ratios and you have to make compromises between WOB, gas flow and temperature.
Counterlung design affect the flow speed. Average flow speed stays the same but peak flow speed is half with a dual counterlung design. With a single counterlung you push gas through the scrubber for half the time of your breathing cycle followed with an equal length pause. Gas moves through the scrubber at inhale or exhale cycle only. With dual counterlungs gas flows the scrubber at both inhale and exhale cycles with half the peak flow speed.
With a single counterlung you have an exhale counterlung if the rebreather is front mounted. You have an inhale counterlung for back mounted units. This is because you can use the hydrostatic imbalance (pressure difference caused by the water column height between the lung centroid and the air fill level in the counterlung) to your advantage to reduce WOB. The pressure differen assists the gas though the scrubber.
To make things more complicated gas can drastically cool down in the counterlung. A single inhale counterlung may be better in some design because the warm exhale breath goes directly into the scrubber. The dual counterlung may cool the gas so much in the exhale counterlung it makes the scrubber less efficient. One good example is the Sentinel rebreather. It is known for good scrubber efficiency.
That old “Here on Zord…” explanation is BS. It’s a kindergarten type explanation with no scientific basis. The extra N2 or He diluent molecules do not work as an chemical inhibitor. The increase in gas molecule collisions under higher pressure works both ways and cancel each out. A CO2 molecule can collision and change path from or to soda lime. Extra N2 or He molecules do not interfere with the scrubbing chemical reaction or the reaction in O2 sensors other than by thermal transfer.
Check for interesting WOB & scrubber docs at Theory&Docs | DIY Rebreathers
If I missed any good documents please let me know.
Sorry for the long post
I left out some minor stuff.
What he said....
This reminds me of my time in construction. I built A LOT of houses before I changed careers. It was always fun finding a new architect or engineer with LOTS of book knowledge, but not a single bit of practical application. They would draw or engineer things that simply were not possible. Sure, their math worked in their minds, but when the wood met steel, concrete or wood, you just couldn’t fit it all together.
We’re in the same realm here. There’s what the scientist says, and then there is what happens when we do a 300’ dive with TRT of 10 hours.
All of those other gases: Are not reacting with the sorb. Only the CO2 is. And sure, they might be passing through the sorb, but the efficiency of the sorb is not being diminished until it has been exhausted by ONLY CO2.
Reminds me of a production meeting I sat in on a creation for a new show going out. A brand new, fancy pants, Quebcoise engineer plopped down a stack of drawing for an incredible circus apparatus. He was beaming. He'd created the pinnacle of designs. Boy was he proud. It was a combination of like, 4 acts that could all take place at once. It was gonna cost 6 figures to fab, and would need about 50 people to install, and it needed to be setup in 15 minutes.
A crusty old rigger studied the plans for a minute, mulling them over, occasionally scratching his chin, letting out a series of "hmmm's...." He takes out a sharpie, carefully orients the plot, and draws a line on it. He studies it carefully and then draws a series of interconnected ovals at about a 30 degree angle to the line, ever so slightly touching each other, the last gracefully intersecting with the line. Pleased with his work he takes a step back and says to the engineer, "son, do you know what that is?" The engineer looks confused, studies the drawing, gets out a scale rule, starts to shift the plot, turning it 90 degrees to the left, 90 degrees to the right.... The rigger stops him after an uncomfortable minute and a half and says, "lemme save you a little bit of time and a lot of heartache. It's three eggs stacked on top of each other, standing on a hillside......"
The engineer went white as a sheet, then red as a lobster, packed up his drawings, and promised to return the next day with something a little more feasible.
Scrubber design and temperature are the two big variables. We didn't see significant impact of depth in saturation diving systems on scrubber efficiency. These chambers are basically giant multi-diver surfaces based and powered rebreathers. This is true even in chambers operating to 500M/1,650'.
The major difference is scrubber capacity was designed for at least 24+ hours for four people in deck chambers and gas is circulated by powered blowers. Ambient flow rates through the scrubber varied a little due to gas density, but blowers are in the 1/4+ HP range so depth had very little impact.
Another rebreather analogy is the surface-based closed circuit diver gas recirculation systems. Differential pressure through the absorbent housing is not a problem so they tended to have small diameter and tall vertically-mounted cylinders.
The complication with CCRs is they are lung powered and don't maintain the same position. Constantly changing orientation and much larger surface area to flow ratios (in order to minimize breathing resistance) dramatically increases the risk of channeling.
The major difference is scrubber capacity was designed for at least 24+ hours for four people in deck chambers and gas is circulated by powered blowers. Ambient flow rates through the scrubber varied a little due to gas density, but blowers are in the 1/4+ HP range so depth had very little impact.
Another rebreather analogy is the surface-based closed circuit diver gas recirculation systems. Differential pressure through the absorbent housing is not a problem so they tended to have small diameter and tall vertically-mounted cylinders.
The complication with CCRs is they are lung powered and don't maintain the same position. Constantly changing orientation and much larger surface area to flow ratios (in order to minimize breathing resistance) dramatically increases the risk of channeling.
Similar threads
