Use Arial font in KOMA Class for section Headings and have siunitx in math font

Question

Follow up question for Use Arial font in KOMA Class for section Headings

• I need to the typical italic, math-font for equations using siunitx and normal Arial for headings and text.

• I would like to have normal Arial font where I use siunitx \SI{}{} in an table environment.

• Basically I want everything in Arial, only equations and SI units in them should have the italic math font.

Is this possible?

Here is my MWE which shows all possible situations in my text.

\documentclass[12pt, numbers=noenddot,parskip,headings=small,headsepline,listof=nochaptergap, listof=totoc,index=totoc]{scrreprt}
\usepackage[a4paper, includehead, left=3.5cm, right=1.7cm, top=2.5cm, bottom=2.0cm]{geometry}
\usepackage{textcomp}
\usepackage{fontspec}
\setsansfont{Arial}
\renewcommand\familydefault{\sfdefault}
\usepackage[ngerman]{babel}
\usepackage{array,booktabs,amsmath}

\usepackage[group-separator={.}]{siunitx}
\sisetup{load-configurations = abbreviations}
\sisetup{locale = DE}
\sisetup{per-mode=symbol}
\sisetup{detect-all, math-rm = \ensuremath}
\sisetup{detect-all}                                        


    \begin{document}

    \chapter{Heading}

    Test Test

    \begin{equation}
    \begin{aligned}
    t_a  = & \frac{v}{a} \\
    h_a  = & \frac{1}{2} \times a \times {t_a}^2 \\
    h_v  = & h_t - h_a - h_r \\
    t_v  = & \frac{h_v}{v} \\
    t_t  = & 2 \times (t_a + t_v + t_r + t_d)
    \end{aligned}
    \end{equation}  
    Equation 1.1 works fine. No SI in there.
    \begin{equation}
    \begin{aligned}
    h_a  = & \frac{1}{2} \times a \times {t_a}^2 \\
    h_v  = & \SI{120}{\metre\per\second\squared}\\
    \end{aligned}
    \end{equation}  
    Equation 1.2 does not look right, because of SI.

    \begin{align*}
    t_a &= \frac{\SI{6}{\metre\per\second}}{\SI{0,6}{\metre\per\second\squared}} \\
    &= \SI{10}{\second}
    \\
    h_a &= \frac{1}{2} \times \SI{6}{\metre\per\second\squared} \times {\SI{10}{\second}}^2 \\
    &= \SI{30}{\metre}
    \\
    h_v  &=  \SI{300}{\metre} - \SI{30}{\metre} - \SI{30}{\metre} \\
    &= \SI{240}{\metre}          
    \\          
    t_v  &=  \frac{\SI{240}{\metre}}{\SI{6}{\metre\per\second}} \\
    &= \SI{40}{\second}          
    \\          
    t_t  &= 2 \times (\SI{10}{\second} + \SI{40}{\second} + \SI{8}{\second}  + \SI{120}{\second}) \\
    &= \SI{356}{\second}                  
    \end{align*}
    Not working at all....

    \begin{table}[h!]
        \centering
        \begin{tabular}{r l r l}
            $t_a $:&Beschleunigungsdauer  &$v$:&Geschwindigkeit [\SI{}{\metre\per\second}] \\
    \end{tabular}\nonumber
    \end{table}
    This is fine.
    \end{document}

Answer 1

The problem is that siunitx detects the font family by comparing the current family with the \sffamily -- and if both are equal as in your case it assumes that you want math-sf/\mathsf.

One can demonstrate the effect by using a slightly different sans font. I also use the [no-math] option for fontspec to avoid that it changes the mathrm and mathsf fonts:

\documentclass[]{scrreprt}    
\usepackage[no-math]{fontspec}
\setmainfont{Arial}
\setsansfont{Arial}%[Scale=1.01] %activate to see the difference.
\usepackage[ngerman]{babel}

\usepackage[]{siunitx}
\sisetup{load-configurations = abbreviations}
\sisetup{locale = DE,group-separator={.}}
\sisetup{per-mode=symbol}
\sisetup{detect-all}

\begin{document}

\chapter{Heading}

Test  

\SI{10}{\meter} $\SI{10}{\meter}$  

$\mathrm{mathrm} \mathsf{mathsf}$

\end{document}

without the Scale option mathsf is used in math:

with the Scale option mathrm is used in math:

Imho it is better to keep the three standard families (rm, sf and tt) distinct. If you want your main font to be Arial, set it with \setmainfont. And if you want your headings to use the main font too, use the option that does it:

\documentclass[egregdoesnotlikesansseriftitles]{scrreprt}
\usepackage{textcomp}
\usepackage[no-math]{fontspec}
\setmainfont{Arial}

\usepackage[ngerman]{babel}
\usepackage{array,booktabs,amsmath}

\usepackage[]{siunitx}
\sisetup{load-configurations = abbreviations}
\sisetup{locale = DE,group-separator={.}}
\sisetup{per-mode=symbol}
\sisetup{detect-all}

\begin{document}

\chapter{Heading}

Test  

\SI{10}{\meter} $\SI{10}{\meter}$  

$\mathrm{mathrm} \mathsf{mathsf}$

\end{document}