SCR vs. CCR

[VTWarrenG]

On an SCR monitoring really isn't a big issue.. If you chose the right orafice [sic] for the gas and depths you will be diving you mix will be fine..

YIKES.

This isn't supported at ALL by the evidence. How many divers have died now, diving the Inspiration? Please don't even try to play it off. You don't just select an orifice and jump off the boat.

An SCR without adequate monitors (and a user who understands how to use them) is a death trap. Monitoring your SCR is CRITICAL.

- Warren

 

[Aviatrr]

One thing that I have wondered is how safe CCR's are on long penetration dives, such as caves. I have only seen a couple divers using rebreathers in caves, but I have wondered what their bailout situation would be like. Lets say you have 2 cave divers on CCRs. One has a failure of the 02 sensors or computers. Now, I understand that there are 3 sensors and 2 computers normally - so how likely(or possible) is it for such a thing to happen? If it does, isn't the only integrated bailout gas your dilutent gas? If you are 1000' back in a cave, a 19cf bottle sure won't get you out. Is there any way to "share air" with a CCR without losing your dilutent gas?

The little bit that I do know about rebreathers I have learned from reading bits and pieces online and by talking to people that dive rebreathers - no training on the subject(and probably many years from even considering a rebreather) - so I'm just trying to learn a bit more.

Mike

 

[padiscubapro]

Originally posted by VTWarrenG


YIKES.

This isn't supported at ALL by the evidence. How many divers have died now, diving the Inspiration? Please don't even try to play it off. You don't just select an orifice and jump off the boat.

An SCR without adequate monitors (and a user who understands how to use them) is a death trap. Monitoring your SCR is CRITICAL.

- Warren

First ..an inspiration is a CCR not an SCR.. it needs full monitoring..

an SCR you PLAN for depth and predicted work loads... small variations are expected, if the you are working outside of you planned parameters you have to adjust.. if you are working excessively hard you want to get as much new gas in the loop as possible to the point were its near a 100% flush so yo know your FO2 level.. it is especially critical at shallow depths.. SCRs can be run without monitoring safely but monitoring is preferred... I'm not brushing it off.. carefully pplanning is required... its always better to err on the side of caution when no monitoring is present...

I can fly my inspiration totally manually and without any electronics maintain a stable mix.. I have practiced it over and over with my electronics as a guide to see my progress..

HYPOXIC mixes are more likely on a CCR than an scr since most CCRS are electronically controlled and failure to monitor or IMPROPER calibration.. With an SCR as long as you mix is planned for the shallowest depths (if you plan the wrong mix you can go hypoxic) with the highest workload you plan its very unlikely..
MOst divers with normal swimming have a oxygen metabolic rate of around 1.0 lpm, most divers plan with a rate of 2.0 so there is usually more oxygen in the mix than planned than was planned.. as long as the MOD of the mix is not exceded if you have gas with an SCR it should be breathable if the scrubber is working.. even if you went all out and for a short period of time you were using 3.0 lpm your breathing rate would increase, infusing more fresh gas.. again this is not the case for a CCR..
Most SCRs don't come standard with any PO2 monitoring even the newest AZIMUTH this is the case.. with a CCR you better know your PO2 at all times.
Both types of rebreathers have quite different failure modes..

 

[padiscubapro]

Originally posted by Aviatrr
One thing that I have wondered is how safe CCR's are on long penetration dives, such as caves. I have only seen a couple divers using rebreathers in caves, but I have wondered what their bailout situation would be like. Lets say you have 2 cave divers on CCRs. One has a failure of the 02 sensors or computers. Now, I understand that there are 3 sensors and 2 computers normally - so how likely(or possible) is it for such a thing to happen? If it does, isn't the only integrated bailout gas your dilutent gas? If you are 1000' back in a cave, a 19cf bottle sure won't get you out. Is there any way to "share air" with a CCR without losing your dilutent gas?

The little bit that I do know about rebreathers I have learned from reading bits and pieces online and by talking to people that dive rebreathers - no training on the subject(and probably many years from even considering a rebreather) - so I'm just trying to learn a bit more.

Mike

as with any technical diving proper dive planning is a must...
In many cases its not realistic for a single CCR diver to 100% of the gas needed to exit, so its pretty common practice for a BUDDY TEAM of CCR divers to carry 1.5 times the gas required to get 1 diver back to safety on oc.. A skilled CCR diver has many options before being forced to open circuit... unless I have total scrubber or total loop failure I have no reason to jump to OC.. Total electronics failure is no reason to abandon the loop.. its quite easy to run the unit as a manual SCR without any electronics and to also manually control the loop content for someone who knows their normal metabolic consumptions...
In my case, I take 1 breath for every 10 meters before I inject any additional o2 and my loop PO2 stays extremely stable.. so at 30 meters I take 3 breaths then inject..if I had no idea what my loop was at I'd do a flush so I have a know starting point... I would also do a flush whenever I change depths so I get another good know starting point...

AT worst case I can run the RB at an average of 5 breaths to one flush and my 3l cylinder (about 20 cuft) is now like I'm carrying a 100 cu ft tank.. once I get to 20ft I'll flush the loop with 100% o2 and run it as an o2 rebreather.

All the reports I have seen about the inspiration deaths have been operator error, some have been major violations of training..

1 CCR death I am aware of the operator never analyzed his oxygen cylinder(You must always analyze your gasses), it had AIR not oxygen so when he told the unit during calibration it was 100% his loop was really off...

at a setpoint of .7 the loop would at most contain 17.5% (on the surface), the po2 at any depth would at best case be 25% of its target if the unit was able to inject enough gas... most rebreathers have a mandatory delay between injections (and limits max injection time)to prevent large o2 spikes... the delay in the inspiration for example is 6 seconds.. using 3 seconds of constant o2 (in reality AIR) injection followed by 6 seconds of no injection would never be able to keep the .7 setpoint.

 

[deepblueh2o]

plus side - less gas, more bottom time, less gas loading,
minus side - more things to go wrong