[Tex/LaTex] Tilde in math mode using helvetica

Question

I am forced to use Arial for my thesis. My configuration as shown in the minimal working example below works fine for me exept for the tilde. It does not blend in well and has too much space above and below it (compare it to the tilde in text-mode).

Does anybody have a hint?

\documentclass{scrbook}

\usepackage[scaled=0.92]{helvet}
\usepackage[ngerman]{babel} 
\usepackage{mathtools}
\usepackage[helvet]{sfmath}  

\renewcommand{\familydefault}{\sfdefault} 


\begin{document}
     \~{l} = \~{a}
    \begin{align*}
        \overset{\sim}{l}= \sqrt{\overset{\sim}{[a]}}\\
    \end{align*}
\end{document}

Answer 1

I would seriously consider ignoring (or getting changed) a rule that says that you should use a sans serif font for a mathematical thesis.

However the markup for a tilde accent in math is \tilde not \overset{\sim}

\documentclass{scrbook}

\usepackage[scaled=0.92]{helvet}
\usepackage[ngerman]{babel} 
\usepackage{mathtools}
\usepackage[helvet]{sfmath}  

\renewcommand{\familydefault}{\sfdefault} 


\begin{document}
\begin{center}     \~{l} = \~{a}\end{center}
    \begin{align*}
        \overset{\sim}{l}= \sqrt{\overset{\sim}{[a]}}\\
    \end{align*}
    \begin{align*}
        \tilde{l}= \sqrt{\widetilde{[a]}}\\
    \end{align*}

\end{document}