Technical diving with oc versus ccr gas requirements

[ccrprospect]

Thanks OP for raising this issue. Question is - given all the bailout requirements, how much additional bottom time (on average) does one gain on CCR at 100m versus open circuit? If you plan for CCR failure at the bottom part of the dive, that is..

 

[kensuf]

Thanks OP for raising this issue. Question is - given all the bailout requirements, how much additional bottom time does one gain on CCR at 100m versus open circuit? If you plan for CCR failure at the bottom part of the dive, that is..

The advantages of the CCR aren't necessarily in additional bottom time or reducing the gas volume you need to carry. The primary advantages are:

1. You have reduced the time pressure while solving problems as long as the loop is functional. Of course, at 100m depth, every minute you spend dorking around with problems results in several minutes of decompression.
2. The logistics surrounding "re-load time" between multiple dives are reduced -- instead of having to refill a bunch of bottles you're only refilling DIL, O2 and maybe a drysuit bottle + re-packing a scrubber. THIS really is huge.
3. Physiological advantages of warm moist air on the loop vs cold dry air.
4. The airline miles you rack up by buying a CCR and training on your credit card. These airline miles help you travel to exotic places, unless you're an American living in a plague filled country.
5. The street cred you get by telling all of your friends you own a CCR. *

* items 4 and 5 are open for debate.

 

[GF99/99]

on a CCR, no less than 2cfm for 10mins at the bottom to deal with hypercapnia. 1cfm for everything else. NO LESS unless you are actively diving open circuit. A lot of CCR divers stop diving OC regularly and still use 0.5-0.7 to plan dives because that's what they used to do. They get used to breathing as much as they want and can't keep it below 1cfm

If my google conversion math is correct this is pretty much on par with what I use for planning. I usually use 60l/min for first 10 then 35l/min till the 50% gas switch and then drop it down to 20l/min for deco gasses. So to make life easy in Multideco I plug in 45l/m (to average out between the 60l/min and 35l/min) for bottom and 20l/min for deco.

I am just always curious to see what others use.

Yes I 100% agree that CCR divers SAC rates are horrible. Or at least mine is. lol

 

[PfcAJ]

Thanks OP for raising this issue. Question is - given all the bailout requirements, how much additional bottom time (on average) does one gain on CCR at 100m versus open circuit? If you plan for CCR failure at the bottom part of the dive, that is..

You need to head up at or before you hit your bailout limits. Difference between oc is that you don’t need to carry additional reserves to share deco gas. For instance, it’s not uncommon to carry 1.5x the required deco gas on oc. If a bottle fails, you share with your buddy and do 1.5x the time. With a rebreather, you only need 1x the deco gas. Losing a breather puts you on oc. Losing that bottle puts you on your buddy’s bottle while he’s on the breather. You’re covered.

So if you’re carrying 2hrs of deco gas, you need to be ascending at or before you have 2hrs of deco ahead of you.

your bailout gas is almost always one of the main limiting factors. The scrubber on many units will last longer than most people want to be in the water on one dive.

 

[ccrprospect]

You need to head up at or before you hit your bailout limits. Difference between oc is that you don’t need to carry additional reserves to share deco gas. For instance, it’s not uncommon to carry 1.5x the required deco gas on oc. If a bottle fails, you share with your buddy and do 1.5x the time. With a rebreather, you only need 1x the deco gas. Losing a breather puts you on oc. Losing that bottle puts you on your buddy’s bottle while he’s on the breather. You’re covered.

So if you’re carrying 2hrs of deco gas, you need to be ascending at or before you have 2hrs of deco ahead of you.

your bailout gas is almost always one of the main limiting factors. The scrubber on many units will last longer than most people want to be in the water on one dive.

Thanks - so practically speaking, how much longer bottom time does that translate to at a given depth (as a percentage, on average)?

 

[PfcAJ]

Thanks - so practically speaking, how much longer bottom time does that translate to at a given depth (as a percentage, on average)?

Depends on the depth and the size of the tanks you’re carrying.

I guess for a 150’ dive with 50% in an al40 for deco, youd have about 30mins of total available time on that al40. On OC you’d need that reserve, so you’d be limited to a 20min deco. On the breather you have that full 30mins available. It’s roughly 1:1 deco time to bottom time at 150’ so you’d snag an extra 10mins of bottom time.

Roughly. Just off the top of my head.

 

[broncobowsher]

To stir the pot a little more, there is the BOB (Bail Out Breather). Instead of taking open circuit for a bailout you take a second rebreather.

 

[ccrprospect]

Depends on the depth and the size of the tanks you’re carrying.

I guess for a 150’ dive with 50% in an al40 for deco, youd have about 30mins of total available time on that al40. On OC you’d need that reserve, so you’d be limited to a 20min deco. On the breather you have that full 30mins available. It’s roughly 1:1 deco time to bottom time at 150’ so you’d snag an extra 10mins of bottom time.

Roughly. Just off the top of my head.

Hardly worth the trouble and expense of a CCR and associated training - no? Unless kensuf's criteria are used

 

[PfcAJ]

Hardly worth the trouble and expense of a CCR and associated training - no? Unless kensuf's criteria are used

Well, it depends.

if you’re doing diving in remote areas, now you don’t need fills. If helium is expensive, you don’t have to pay for it (break even takes a long time to reach though). If you’re doing deep penetration dives, the breather gives you time to sort out problems. Long range cave diving only requires “one way” gas instead of gas for in, out, and reserve.

for some folks, some or all of those reasons can be compelling. For others, those things might not apply.

 

[ChuckP]

Hardly worth the trouble and expense of a CCR and associated training - no? Unless kensuf's criteria are used

Cost of gas is a huge factor for people diving a lot.