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On Mon, 18 Mar 1996, rfarb wrote:
> Would you please translate the coronor's report or whatever it is into
> something that makes sense about how the rebreather killed him. Thanks. Rod
First: a very simplified description of a rebreather:
(My apologies to all who feel insulted by the basic level of explanation)
A mouthpiece connected to a hose connected to a bag full of oxygen.
This would be fine if the body did not produce carbon dioxide (CO2).
CO2 would rapidly build up to unbearable concentrations in the bag.
To remedy this we introduce a "scrubber" in the system, a device that
will remove the CO2 from the system. This is usually a canister with a
granular substance.
Now, this rebreather would work fine except for the fact that after a
while the oxygen would be used up. To fix this we take along a bottle of
oxygen (O2) and connect it to the system.
How fast should the oxygen be fed to the bag?
There are several ways to decide this:
1.A constant mass flow valve (cmfv) will feed oxygen into the bag at a
constant rate. If the flow is to great the bag will get bigger and bigger
and the diver will have to adjust the filling rate. If the flow is too low
the bag will eventually be drained of gas. Before this happens, however,
the diver will note an increased resistance at the end of a deep breath
and increase the flow. Keep this in mind in the following discussion!
2. A mechanical device sensing the bag volume will fill it when volume
has decreased below a certain threshold.
Now for a short inert gas digression:
At sea level, a human will have approxemately 1.5 liters of Nitrogen
(N2) dissolved in the body tissues. Why? Because air is mostly made up of
N2.
What happens if we start breathing pure oxygen?
The body is saturated with N2 and there is no N2 in the gas we breathe.
N2 will start to diffuse out of the tissues into the gas. If this gas is
the O2 in the rebreather bag the O2 will be diluted by N2 because the
scrubber will only remove CO2 and nothing else. Over time, the O2 will
be diluted by N2. If the flow of O2 in the bag is impeded for some
reason, (corroded valve, empty O2 bottle etc) the volume of the bag will
*NOT* decrease, there will be no increase in resistance to signal the
diver. Instead, the concentration of O2 will decrease until the diver
becomes unconcious. Remember that lack of oxygen (hypoxia) is very
insidious, the victim will hardly notice anything before (s)he passes out.
(The same mechanism works in rebreathers that automatically sense the bag
volume)
The way to avoid this is to empty the bag of gas completely. When should
this be done? After a while when N2 has begun accumulating in the bag.
The procedure will have to be repeated at intervals.
If we use more complex equipment for diving, we need more training,
knowledge and awareness to dive safely.
/RS
(o o)
+------------------------------oOO--(_)--OOo---------------------------+
| Richard Soderberg, MD, PADI DM | The Karolinska Institute |
| Systems analyst | MIC-KIBIC |
| Voice#: +46 8 728 80 00 | Library and |
| Fax# : +46 8 33 04 81 | Medical Information Center |
| Snail : PO Box 200 | Doktorsringen 21 C, |
| S-171 77 Stockholm | S-104 01 Stockholm |
| Email :richard.soderberg@mi*.ki*.se* | SWEDEN |
+------------------------------------+---------------------------------+
| Diving Physician at Dykhuset/Scubahouse, Stockholm, Sweden |
| URL:http://www.dykhuset.se |
+----------------------------------------------------------------------+
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