GEM recreational rebreather

[Rhone Man]

Sounds interesting (although a 4 hour dive at 100 feet must involve a looooooooooooooooooooot of deco).

 

[DwayneJ]

Sounds interesting (although a 4 hour dive at 100 feet must involve a looooooooooooooooooooot of deco).

He said max depth 100' and the rest of the dive 65'.

V-Planner says is doable....

SCR
Decompression model: VPM - B
DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 1
Dec to 100ft (2) Nitrox 36 50ft/min descent.
Level 100ft 18:00 (20) Nitrox 36 1.45 ppO2, 75ft ead
Asc to 65ft (21) Nitrox 36 -30ft/min ascent.
*Level 65ft 220:00 (241) Nitrox 36 1.07 ppO2, 46ft ead <<< CNS high
*Asc to 10ft (243) Nitrox 36 -30ft/min ascent. <<< CNS high
*Stop at 10ft 4:00 (247) Nitrox 36 0.47 ppO2, 2ft ead <<< CNS high
*Surface (247) Nitrox 36 -30ft/min ascent. <<< CNS high
OTU's this dive: 280
*CNS Total: 98.1% <<< CNS high

and me on CCR...
Decompression model: VPM - B
DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 1
Dec to 100ft (2) Diluent 0.2 0.70 SetPoint, 50ft/min descent.
Level 100ft 18:00 (20) Diluent 0.2 1.10 SetPoint, 90ft ead
Asc to 65ft (21) Diluent 0.2 1.10 SetPoint, -30ft/min ascent.
*Level 65ft 220:00 (241) Diluent 0.2 1.10 SetPoint, 46ft ead <<< CNS high
*Asc to 10ft (243) Diluent 0.2 1.10 SetPoint, -30ft/min ascent. <<< CNS high
*Stop at 10ft 2:00 (245) Diluent 0.2 1.10 SetPoint, 0ft ead <<< CNS high
*Surface (245) Diluent 0.2 -30ft/min ascent. <<< CNS high
OTU's this dive: 284
*CNS Total: 102.6% <<< CNS high

... but its the very limits of the unit described and my own rEvo both in terms of personal O2 exposure and rebreather scrubber duration.

Dwayne

 

[Hatul]

What about the work of breathing? I read reports that the Poseidon MK VI is harder to breathe than open scuba.

Also it looks like the thing is strapped to a regular tank, so it would make sense to use a smaller tank if you want to cut down on weight?

What about total weight, buoyancy and the amount of lead you need to take?

Adam

 

[Dr. Doug Ebersole]

Here are my impressions after about 25 hours on the GEM.

The work of breathing with the unit is very good. Like all rebreathers the breathing is differerent than open circuit scuba -- not easier, not harder, just different. You have to retrain yourself to breathe more like you would breathe topside. Once people get used to not having a regulator propelling gas into their mouth, it becomes pretty natural.

As far as weight goes, you could DEFINITELY put it on a smaller tank to limit weight but still have enough gas for your dive, two tank boat trip, etc.

For weighting purposes I use about 5 extra pounds of lead compared to what I would use if I was diving the same cylinder on open circuit.

This past weekend I dove mine on two 80-90 foot dives in West Palm (not multi-level) and at the end of the two dives I still had 2400 psi in my aluminum 80. That was very easy drift diving so the gas extension was maximal as I wasnt' working very hard. Another two tank trip this weekend where I was working a little harder I was able to do two 60 foot dives (again not multi-level) and still have 1800 psi left in my aluminum 80.

So, what's the catch? The tradeoff is between gas extension and the actual nitrox mix you are breathing. When you use a semi-closed rebreather the % oxygen in the loop is always lower than the % oxygen in the cylinder. If you are getting a lot of gas extension then you are not adding a lot of fresh nitrox to the loop and your % oxygen in the loop will fall. This will shorten your no-deco time unless you plan for it and put a high nitrox mix in your cylinder. You have to realize, however, that your cylinder is your bailout so the higher the nitrox mix in the cylinder the shallower your MOD for the dive.

In practical terms, the FIO2 in the loop will be about 5% lower than that in your cylinder. I was diving EAN40 in my cylinder this weekend which allowed me to bailout to open circuit if needed but still stay with my open circuit buddies who were on EAN34 and have TONS of gas left over at the end of the dive. On my computer, the "working" dive gave me an FIO2 in the loop of EAN37 while on the drift dive the loop FIO2 was EAN32.

Hope that wasn't confusing. Just my 2 cents as a longtime Drager/Azimuth SCR and Kiss CCR diver/instructor and now newbie GEM diver/instructor.

Doug

 

[TSandM]

For weighting purposes I use about 5 extra pounds of lead compared to what I would use if I was diving the same cylinder on open circuit.

Ouch. This make it much less attractive to us PNWers.

 

[Hatul]

Here are my impressions after about 25 hours on the GEM.


So, what's the catch? The tradeoff is between gas extension and the actual nitrox mix you are breathing. When you use a semi-closed rebreather the % oxygen in the loop is always lower than the % oxygen in the cylinder. If you are getting a lot of gas extension then you are not adding a lot of fresh nitrox to the loop and your % oxygen in the loop will fall. This will shorten your no-deco time unless you plan for it and put a high nitrox mix in your cylinder. You have to realize, however, that your cylinder is your bailout so the higher the nitrox mix in the cylinder the shallower your MOD for the dive.

In practical terms, the FIO2 in the loop will be about 5% lower than that in your cylinder. I was diving EAN40 in my cylinder this weekend which allowed me to bailout to open circuit if needed but still stay with my open circuit buddies who were on EAN34 and have TONS of gas left over at the end of the dive. On my computer, the "working" dive gave me an FIO2 in the loop of EAN37 while on the drift dive the loop FIO2 was EAN32.

Hope that wasn't confusing. Just my 2 cents as a longtime Drager/Azimuth SCR and Kiss CCR diver/instructor and now newbie GEM diver/instructor.

Doug

If I understand correctly, the Gem takes the Nitrox from your tank, and after you breathe it recycles it by scubbing the CO2, but not adding any O2. So does that mean that the FIO2 that you breathe in is always changing?

About the Kiss Sport, does that require trimix (helium) if you're diving to 165 ft?

Adam

 

[Codiak]

Ouch. This make it much less attractive to us PNWers.

It's the price of having counter lungs.

Bouyancy changes too... No just sucking it to go up, or exhaling hard to go down.

 

[Dr. Doug Ebersole]

Yes, the GEM injects nitrox, not oxygen, into the loop. The FI02 in the loops will always be lower than that in the cylinder and will change depending on your workload, forcefulness of exhalation, etc. As with all rebreather diving, the number one rule is "Always know your PO2". That having been said, it is usually about 5% (give or take) lower than your cylinder. So, for an easy recreational dive with EAN 36 in your cylinder, you could set a nitrox computer at about EAN30% for the dive. However, realize that your MOD is that for the EAN36, not EAN30, as if you bailout to open circuit you will be breathing EAN36 directly out of the cylinder.

The Sport Kiss can be used with air diluent or trimix. For depths of 165 fsw, I would use trimix as the diluent, say 17/35 or something similar, to lessen narcosis. However, I'm sure there are people diving it to 165 fsw on air. Note that if you dive to a PO2 of 1.3 on a CCR with air diluent, your PN2 at 165 fsw (6 ATA) is 4.7 ATA or an END of 162 fsw. Personally, I'd rather have some helium and an END of around < 120 fsw. Note this is just physics and applies to all rebreathers. Not just the Sport Kiss.

 

[SFLDiver3445]

Just my 2 cents as a longtime Drager/Azimuth SCR and Kiss CCR diver/instructor and now newbie GEM diver/instructor.

Doug

And don't forget rebreather underwater photographer extrodinaire!!!

 

[rjack321]

Isn't the GEM keyed to respirations so any variance in fO2 from the mouthpeice set SCR drop is basically due to workload?