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Subject:RE>>Draeger Atlantis Sent on: 20/7/95 1:49 am
Rich Wrote --->>
Take a scuba tank. Now, add a Y-valve and a second regulator, either one
of which can provide you with breathing gas from your tank. The regulator
complexity has just increased by a factor of 2, so the probability of a
regulator failure has doubled. However, since simultaneous regulator
failures are required to bring the whole system down, the probability of a
system failure is MUCH less than the probability of a single regulator
failure (assuming the two regulators are truly independant entities, which
means they can be isolated in the event of a blown hose).*
*Note: I've removed the math because it's incorrect....
<<<<----
As Rich says its all to do with the probability of both components of the
redundant system failing at the same time : mathmatically the failure of both
regs is deemed to be a parallel failur. In electronics these type of redundany
systems are calcultaed by assuming that the possibility of two parallel
components is based on a poisson (usually) distribution of failure rate and
that the window of outage (ie the time for which both components are non
functional) is equivalent to the MTTR (mean time to repair) for the the first
system.
To calculate the MTBF (Mean Time before failure) of a parellel component the
following eqaution is used :-
Reliability of a System (Rsys) =
exp(-8760/MTBF)+((exp(-MTTR/MTBF))*(1-exp(-8760/MTBF)))
FITS (failures in million hours) = 1/(-1/LN(Rsys))*8756.56
MTBF Redundant system = 1/(FITS)* 1000000
As we are talking regulators instead of electronics, the MTTR is in reality the
time from first failure to surfacing as its irrelevant if the backup reg fails
once we are on the surface - anyway what it boils down to is that
1) I'm a sad person for knowing the above rubbish
2) Redundancy works as Rich says the occurance of a system failure is MUCH less
than the occurance of a single regulator failure
G
Mail Address : graeme_davison@ne*.co* _
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