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Hi Rich,

Another response.--- Dave

11 Feb 1996

To:  Richard Pyle
From: Jack Kellon
Subject: Electric and electronic failures in rebreathers.

Apparently we are becoming bogged down in semantics. When I refer to
electrical devices I am including galvanic oxygen sensors. In Bill Stone's
otherwise excellent recently published article about failure probabilities
and  nonlinear redundancy, the sensors weren't mentioned. The reason that
electronically controlled rebreathers have three sensors and sensor
averaging circuits in the first place is because of the historical
unreliability of oxygen sensors in marine environments. I can't speak from
detailed knowledge of the Cis-Lunar, but on the CCR-1000's and decendants,
there are five places on the secondary or passive display that water
intrusion can occur and short out all three sensor outputs (only
millivolts) simultaneously: two seals on the wire penetrator entering the
display case, two seals on the rotary selector switch shaft penetrator
entering the case and the analog meter seal. The analog meter that is
directly driven by the sensor outputs selected by the switch is also a
single point failure source without water intrusion. If it fails (keep in
mind that it's on the end of a multiconductor wire outside the rebreather
case and subject to many shocks), NO sensor outputs can be read. In
addition the elastomeric cover on the wire bundle is a source of water
intrusion shorts to all three sensor circuits if abraded, cut or punctured.

If any of these things occur, a life supporting diluent (which is extremely
limited) is your only onboard bailout, and cannot be monitored with the
secondary display. Even if you are still getting SOME reading from one or
more sensors after a partial flood, that reading cannot be trusted.

The reason for using a rebreather instead of open circuit at depth is
because it reduces the bulk of the primary breathing supply by a
substantial margin. The bailout supplies would be required regardless of
the primary breathing source. There are other advantages that you should
ask George Irvine about.

Best regards,
Jack


                                xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                                                Dave Schubert

                              Instructor Trainer: PADI, IANTD, DAN
                                     Photo...Video...Rebreathers
                                         1201 N. Ocean Drive
                                       Singer Island, FL. 33404
                                407/844-5100, FAX: 407/848-0627
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