Probably stupid question from a non-RB person

[TSandM]

I did a dive yesterday with a fellow on a rebreather. I know very little about them, but I did ask him some questions about his unit before we got in the water. I wanted to know where his O2 add was, whether he had a HUD (which he didn't), what was in his tanks, if he had a BOV (he did) and how to do a dil flush on his unit . . . and that last point baffled me, because apparently there is no way to do that while diving. He told me the unit automatically adds dil, so if he needs to do a flush, he just exhales through his nose and lets the unit add dil by itself.

My question is, if you had an O2 spike, would that flush the system fast enough?

The unit in question is one of the KISS rebreathers. Apparently there is a button you can push to add dil, but it's up behind the diver's head, and apparently not accessible while you're diving.

Please excuse (and educate me out of) any misconceptions in the post. As I said at the beginning, I know little about these things, and up until recently, I was too uncomfortable to dive with someone using one. (It still makes me kind of nervous.)

 

[rjack321]

Vlad dives a Kiss sport.

 

[Scott L]

Debersole as well...

 

[down4fun]

I'm not 100% sure, but I believe the only manual add on a stock KISS is the O2, but I could be wrong. Dil is added via the BOV or the ADV.

The KISS being a manual CCR, it doesn't have a solenoid, rather it has a mass flow orifice and a manual addition button to keep the PO2. If the flow rate is set correctly (to the diver's resting metabolism rate,) barring some catastrophic failure, it's unlikely the O2 would "spike" during the dive. There are situations where the the PO2 could be higher than desired, which can be addressed by doing what your friend described - going below minimum loop volume and letting some dil flow in through the ADV to lower the PO2. This is more than adequate to lower a slightly high PO2.

Now if your friend was a jackass, and wasn't paying attention at all, and somehow let the PO2 climb to dangerous levels, the BOV could always be flipped, and go to a safe OC breathing mix.

Truth be told, I'd be far more worried about hypoxia than hyperoxia. There are plenty of divers who have stupidly gone to insanely high PO2's for short periods of time. (2.5 and above.) The US Navy divers sometimes have working PO2's of 2.0 and higher, and during a table 6 treatment, one is subject to high PO2's. (Yes I know, dry warm, resting environment. My point is divers can survive a "high" PO2 for a short period of time, but not one diver I know can survive a hypoxic event.

And not diving with a HUD is not a terribly bright idea IMO.

 

[formula1mb@aol.com]

I don't dive a rebreather but am curious and have read the KISS manual before. You can also turn the BOV only partway to add dil so in essence it acts like a semi-closed rebreather or I suppose you could even hit the purge than on the BOV to add a lot of dil.

If it became a critical situation though, easiest way is to flip the BOV all the way and go open circuit.

 

[limeyx]

Lynne, those are excellent questions to ask a CCR diver.

I would suggest from your perspective as an OC buddy, attempting a dil flush for your CCR buddy is likely to lead to more issues than it solves.

Given that he had a BOV, I think as his buddy, this should be your primary method of intervening if you believe you need to rescue.

Reasons for this are

- good, known gas instantly delivered (at a good PO2 as long as he dives sensible bailout)
- enough known gas to surface (again as long as good planning is done)
- you can easily tell if he is breathing or not
- a simple dil flush might solve the immediate problem but if your buddy is incapacitated or distracted, it might be only a temporary solution.

One other thing. If you do flip the BOV, and need to rescue your buddy, definitely make sure you know how to dump their counterlungs before ascending them as otherwise they act like a second (very unwanted) BC...

 

[Plumb bob]

My question is, if you had an O2 spike, would that flush the system fast enough?

The unit in question is one of the KISS rebreathers. Apparently there is a button you can push to add dil, but it's up behind the diver's head, and apparently not accessible while you're diving.

Please excuse (and educate me out of) any misconceptions in the post. As I said at the beginning, I know little about these things, and up until recently, I was too uncomfortable to dive with someone using one. (It still makes me kind of nervous.)

It comes down to how high the spike is as to how you would react.

For me if it was just a small spike I would dump gas out of my nose or around my lips then inhale until the Auto Diluent Valve (ADV, a very high cracking pressure second stage in the head of the RB) was activated. This would bring some gas with a lower PO2 into the loop.

If it was a large spike I would bail out, whether I stay bailed out and that's the end of the dive depends on the reason for the spike (if I could determine it).

So the unit will not add dil by "itself" as it requires the diver to have drawn all of the gas in the loop into their lungs to create a negative pressure in the loop to activate the ADV.

 

[TSandM]

Well, I think as an OC buddy, I had already decided that if I have ANY questions about what's going on with an RB diver, off loop is the answer. I just thought it was odd that there wasn't any way to do a dil flush; if that constant flow O2 thingy malfunctions and oversupplies, I guess you'd just close the BOV and go off the loop.

Thanks for the hint about asking how to dump the counterlungs. I didn't ask that one.

 

[down4fun]

Well, I think as an OC buddy, I had already decided that if I have ANY questions about what's going on with an RB diver, off loop is the answer. I just thought it was odd that there wasn't any way to do a dil flush; if that constant flow O2 thingy malfunctions and oversupplies, I guess you'd just close the BOV and go off the loop.

Thanks for the hint about asking how to dump the counterlungs. I didn't ask that one.

As they say "when in doubt bailout" Something else to be aware of if diving with a RB diver, hypercapnia is very insidious and a common symptom is denial (just like dcs) so if your buddy is acting weird be insistent that they bailout.

 

[limeyx]

Well, I think as an OC buddy, I had already decided that if I have ANY questions about what's going on with an RB diver, off loop is the answer. I just thought it was odd that there wasn't any way to do a dil flush; if that constant flow O2 thingy malfunctions and oversupplies, I guess you'd just close the BOV and go off the loop.

Thanks for the hint about asking how to dump the counterlungs. I didn't ask that one.

Lynne,

You can read more about the "leaky valve" or MAV or whatever, but in theory it's impossible to oversupply O2 as long as the diver is on the loop because the O2 flow is going through a fixed size orifice I think, and at a rate that is somewhat less than that required to sustain life (usually via a modified first stage without the ambient-pressure block so a constant flow is achieved)

Maybe it's possible for IP creep in the 1st stage to generate an oversupply but according to the gurus on the "other board" a malfunction is virtually impossible.