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Hello, Well for nature photography it's cheaper, simpler and safer to put a diffuser on the exhaust of an OC unit ;) Besides your not fooling no one, aquatic animals have eyes and can plainly see you. With the diffuser your exhaust is re-directed behind your head out of your field of vision, the bubbles/noise is seriously reduced. Ed -----Original Message----- From: Don Burke [mailto:donburke56@ne*.ne*] Sent: Sunday, July 08, 2001 3:04 PM To: techdiver@aquanaut.com Subject: Re: What Are the Circumstances where a rebreather is appropriate? Was RE: This is inspiration diving and people are supposed to die Hi Joe, Comments scattered within. From: Joe W <arizonajeep@ho*.co*> > Could someone or a few someone's list the circumstances where using a > rebreather is appropriate? I only have two: 1. Dives where gas volume requirements can't be met by open circuit or surface supplied rigs. 2. Dives where stealth is a go/no-go issue such as some nature photography, military missions, law enforcement, or criminal activity. > Let me also add, as an electrical engineer who worked for NASA for 6 years: > > It is entirely possible to build an electronic rebreather who's combined > component failure rate is less than that of any currently existing > open-circuit SCUBA assembly. For the electronic box itself, no doubt. However a valve assembly as reliable as the best open circuit regulator is a pretty tall order. It can probably be done, but not by much. The O2 sensors are going to be the deal breaker. Under the best conditions, they are unreliable. When you start varying the pressure and temperature, look out. I don't think you can get there from here. You need a technology breakthrough for this. > In fact; it will most likely look very similar to the life support systems > worn by astronauts when going EVA. At one point, NASA took advantage of a property of space that divers do not have access to, the lack of pressure. The early suits ran pure oxygen at a reduced total pressure. This meant the wearer was getting the right amount of gas, too much, or too little. A fault was pretty easy to diagnose. They might still do it that way. I know I would. > I'd be happy to entertain a public discussion with anyone who believes > otherwise. The problem goes beyond the raw fault rate. In open circuit systems, the diver is getting the right amount of gas, too much, or too little. The failure modes are easily identified with no instruments (although a pressure guage will help you work out the details) and the responses are easily drilled. In a rebreather, the unit can provide the wrong gas mix. That isn't always easily picked up without instruments. Someone on the list said you've got three breaths to figure it out and do something about it. My opinion of that statement is meaningless, but I'll take it at face value. A catastrophic scrubber failure or failure (open or shut) of the valve that adds oxygen will make life complicated in a hurry. If the failure rate of a super rebreather was two per billion and the failure rate of open circuit was ten per billion, there is a pretty good chance that the death rate for open circuit would be lower than the rate for the rebreather since the detection and response to the faults would come into play. For many missions, the US Navy uses rebreathers which use a variation of the NASA trick in that they work on pure oxygen and add only oxygen for depth changes. Those units are very reliable since they have a limited number of failure modes. The price of this is a shallow depth limit and a time limit based on oxtox. IMNHO, the best idea I have seen is the semi-closed unit where the gas used to keep the oxygen level up is actually trimix or nitrox with a PPO2 that can be breathed undiluted on the bottom. Since there is "inert" gas added along with the oxygen, the stealth of these units suffers since bubbles are released. That probably isn't an issue for most of us. v/r Don Burke NetZero Platinum No Banner Ads and Unlimited Access Sign Up Today - Only $9.95 per month! http://www.netzero.net -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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