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\path [line] (dec4) -- node[near start] {Nein} (dec4|-expr1);

\path [line] (dec3)  -- node[near start] {Nein} (dec3|-expr1) -- (expr1);

\tikzset{
  decision/.style = {diamond, draw, fill=yellow!10, text width=7.5em, 
    text badly centered, inner sep=0pt, aspect=1.5}, 
  block/.style = {rectangle, draw, fill=yellow!20, text width=15em, 
    text centered, rounded corners, minimum height=3em},
  cloud/.style = {ellipse, draw, fill=yellow!20, minimum height=2em}, 
  line/.style = {draw, -latex'}
}

\documentclass[ngerman,fontsize=12pt]{scrartcl}
\usepackage[
	rm=txfonts:new,
	sf=txfonts:new:scale:lowercase,
	tt=txfonts:new:scale:lowercase,
	math=txfonts:new:libertine,
	sym=amssymb]{typeface}
\usepackage{babel,microtype,tikz}
\usetikzlibrary{shapes,arrows}

\begin{document}
% Define block styles
\tikzstyle{decision} = [diamond, draw, fill=yellow!10, text width=7.5em, 
	text badly centered, inner sep=0pt, aspect=1.5]
\tikzstyle{block} = [rectangle, draw, fill=yellow!20, text width=15em,
	text centered, rounded corners, minimum height=3em]
\tikzstyle{cloud} = [draw, ellipse,fill=yellow!20, minimum height=2em]
\tikzstyle{line} = [draw, -latex']

\begin{tikzpicture}[auto,line width=1pt,scale=0.8, 
	every node/.style={transform shape}]
	% Place nodes
	\node [cloud] (start) {Messwert $i$ in mA};
	\node [decision, below of=start,node distance=3cm] (dec1) 
		{\small $\mathrm{MPX_3} > \mathrm{MPX_2}$};
	\node [decision, right of=dec1, node distance=4.5cm] (dec2) 
		{\small $i \ge \mathrm{MPX_3}$};
	\node [block, right of=dec2, node distance=6.5cm] (expr3) 
		{\small $\mathrm{Wert} = 10^{\left(i\cdot \mathrm{MPA_3}+\mathrm{MPB_3}\right)} + 
		\mathrm{MPC_3}$};
	\node [decision, below of=dec1, node distance=4.5cm] (dec3) 
		{\small $\mathrm{MPX_2} > \mathrm{MPX_1}$};
	\node [decision, right of=dec3, node distance=4.5cm] (dec4) 
		{\small $i \ge \mathrm{MPX_2}$};
	\node [block, right of=dec4, node distance=6.5cm] (expr2) 
		{\small $\mathrm{Wert} = 10^{\left(i\cdot \mathrm{MPA_2}+\mathrm{MPB_2}\right)} + 
		\mathrm{MPC_2}$};
	\node [block, below of=expr2, node distance=3.0cm] (expr1) 
		{\small $\mathrm{Wert} = 10^{\left(i\cdot \mathrm{MPA_1}+\mathrm{MPB_1}\right)} + 
		\mathrm{MPC_1}$};
	% Draw edges
	\path [line] (start) -- (dec1);
	\path [line] (dec1)  -- node[near start] {Ja}   (dec2);
	\path [line] (dec2)  -- node[near start] {Ja}   (expr3);
	\path [line] (dec1)  -- node[near start] {Nein} (dec3);
	\path [line] (dec2)  |- node[near start] {Nein} ([yshift=10mm]dec3.north);
	\path [line] (dec3)  -- node[near start] {Ja}   (dec4);
	\path [line] (dec4)  -- node[near start] {Ja}   (expr2);
	\path [line] (dec3)  |- node[near start] {Nein} (expr1);
	\path [line] (dec4.south) -- node[near start] {Nein} ([xshift=-30.5mm]expr1.west);
\end{tikzpicture}
\end{document}