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Robin 2010-06-12 00:07:09 +01:00
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pt100/pt100.tex
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@ -22,6 +22,29 @@ Thus after the analysis the PT100 temperature sensing circuit, may be veiwed
from an FMEA persepective as a component itself, with a set of known failure modes.
\end{abstract}
}
{
\section{Overview}
The PT100, or platinum wire \ohms{100} sensor is
a widely used industrial temperature sensor that is
slowly replacing the use of thermocouples in many
industrial applications below 600\oc, due to high accuracy\cite{aoe}.
This chapter looks at the most common configuration, the
four wire circuit, and analyses it from an FMEA perspective twice.
Once considering single faults (cardinality constrained powerset of 1) and then again, considering the
possibility of double faults (cardinality constrained powerset of 2).
The analysis is performed using Propositional Logic
diagrams to assist the reasoning process.
This chapter describes taking
the failure modes of the components, analysing the circuit using FMEA
and producing a failure mode model for the circuit as a whole.
Thus after the analysis the PT100 temperature sensing circuit, may be veiwed
from an FMEA persepective as a component itself, with a set of known failure modes.
}
\begin{figure}[h]
@ -33,7 +56,7 @@ from an FMEA persepective as a component itself, with a set of known failure mod
\end{figure}
\section{Overview of PT100 four wire circuit}
\section{General Description of PT100 four wire circuit}
The PT100 four wire circuit uses two wires to supply small electrical current,
and returns two sense volages by the other two.
@ -277,7 +300,7 @@ Using the temperature ranges defined above we can compare the voltages
we would get from the resistor failures to prove that they are
`out of range'. There are six test cases and each will be examined in turn.
\subsubsection{ TC1 : Voltages $R_1$ SHORT }
\subsubsection{ TC 1 : Voltages $R_1$ SHORT }
With pt100 at 0\oc
$$ highreading = 5V $$
Since the highreading or sense+ is directly connected to the 5V rail,
@ -290,7 +313,7 @@ $$ lowreading = 5V.\frac{2k2}{2k2+212.02\Omega} = 4.56V$$
Thus with $R_1$ shorted both readingare outside the
proscribed range in table \ref{ptbounds}.
\subsubsection{ TC2 : Voltages $R_1$ OPEN }
\subsubsection{ TC 2 : Voltages $R_1$ OPEN }
In this case the 5V rail is disconnected. All voltages read are 0V, and
therefore both readings are outside the