..... midnight oil...

pt100/pt100.tex

@@ -468,7 +468,9 @@ give the following failures in ${10}^6$ hours:
 
 While MIL-HDBK-217F gives MTTF for a wide range of common components,
 it does not specify how the components will fail (in this case OPEN or SHORT). {Some standards, notably EN298 only consider resistors failing in OPEN mode}.
-FMD-97 gives 27\% OPEN and 3\% SHORTED, for resistors under certain electrical and environmental stresses. This example 
+%FMD-97 gives 27\% OPEN and 3\% SHORTED, for resistors under certain electrical and environmental stresses. 
+% FMD-91 gives parameter change as a third failure mode, luvvverly 08FEB2011
+This example 
 compromises and uses a 90:10 ratio, for resistor failure. 
 Thus for this example resistors are expected to fail OPEN in 90\% of cases and SHORTED 
 in the other 10\%.

survey/survey.tex

@@ -292,18 +292,41 @@ assigned a probability $\beta$ factor by the design engineer. The use of  a $\be
 is often justified using Bayes theorem \cite{probstat}.
 %Also, it can miss combinations of failure modes that will cause SYSTEM level errors.
 %
+\paragraph{FMECA `t' Value}
+The time that a system will be operating for, or the working life time of the product is
+represented by the variable $t$. for probability of failure on demand studies,
+this can be the number of  operating cycles or demands expected.
+
+\paragraph{Severity `s' value}
+Component failure modes can cause failures that have levels of severity or seriousness.
+Typical classifications are as follows:~\cite{FMD-91}
+\begin{itemize}
+ \item Category I - Catastrophic
+\item Category II - Critical
+\item Category III - Marginal
+\item Category IV - Minor.
+\end{itemize}
+Thus a component, because it may fail in different ways, may cause
+different severity SYSTEM level errors on failing.
+
+%I AM TOO TIRED
+%this is fucking torture
+
 \paragraph{Results of FMECA}
 The results of FMECA are similar to FMEA, in that component errors are
 listed according to importance, based on 
 probability of occurrence and criticallity.
 % to prevent the SYSTEM fault of given criticallity.
 Again this essentially produces a prioritised `to~do~list'
-sorted by severity and liklihood.
+sorted by severity and likelihood.
-Each component failure mode has a criticallity number $C_m$, (where t is the operating time or product life time in hours), which can be calculated thus:
+A criticality number $C_m$, 
+%(where t is the operating time or product life time in hours), 
+which can be calculated for a given component failure mode $cfm$ for a given severity
+$s$ thus:
 
 
 \begin{equation}
- C_m = \beta \alpha {\lambda}_p t  
+ C_m(s) = cfm_{\beta} cfm_{\alpha} cfm_{{\lambda}_p} cfm_t  \; where \; cfm\rightarrow severity = s \;.
 \end{equation}
 
 %%-WIKI- Failure mode, effects, and criticality analysis (FMECA) is an extension of failure mode and effects analysis (FMEA).
@@ -314,20 +337,22 @@ Each component failure mode has a criticallity number $C_m$, (where t is the ope
 %%-WIKI- FMECA tends to be preferred over FMEA in space and North Atlantic Treaty Organization (NATO) military applications, 
 %%-WIKI- while various forms of FMEA predominate in other industries.
 
-A second result, representing the overall reliability and safety of the product $P$, 
+A second result, representing the overall reliability and safety of a component or item\cite{FMD-91}[2-17] $C$, 
-, termed a criticallity number $C_r$ 
+termed a criticallity number $C_r$ for the component.
-(where we can consider $P$ to be a flat set of component failure modes
+We can consider $C$ to be a flat set of component failure modes, using $cfm$ as a variable to represent them.
-which we can use the variable  $c_f$  to represent
 % where $f \in F$)
-can calculated thus
+The $C_r$ value, for a given serverity $s$ is calculated thus
 \begin{equation}
- C_r = \sum_{c_f \in P} {\beta \alpha {\lambda}_p t} c_f
+ C_r(s) = \sum_{cfm \in C}  cfm_{\beta} cfm_{\alpha} cfm_{{\lambda}_p} cfm_t  \; where \; cfm\rightarrow severity = s \;.
 \end{equation}
 
 
+
+
 \subsubsection{ FMECA weaknesses }
 \begin{itemize}
 \item Possibility to miss the effects of failure modes at SYSTEM level.
+\item Component failure modes are tied to one SYSTEM level error.
 \item The $\beta$ factor is based on heuristics and does not reflect any rigourous calculations.
 \item The $\alpha$ factor is based on heuristics or general data, and may not to specific to the environmental or operational conditions
 under which the equipment is operating.