Error with siunitx package, \pi as a number, and detect-all

Question

I have a problem when using the siunitx package, specifically when using the detect-all option and trying to use the \pi symbol as a number.

Minimum working example (MWE) below:

\documentclass{article}
\usepackage{XCharter}
\usepackage{siunitx}

\begin{document}

This works happily: \SI{2\pi e15}{\radian\per\second}

\sisetup{detect-all,}
But this gives an error : \SI{2\pi e15}{\radian\per\second}.

\end{document}

Answer 1

If you run \sisetup{detect-all}, siunitx requires the entire \SI directive to be in math mode in order to process \pi correctly, i.e., it requires you to run

$\SI{2\pi e15}{\radian\per\second}$

On p. 5 of the package's user guide, one finds:

By default, all text is typeset in the current upright math font. This can be changed by setting the appropriate options: \sisetup{detect-all} will use the current font for typesetting.

What this means is that if detect-all is in force, siunitx does not automatically use a math font to generate the output of \SI and \si. Thus the need for the $ symbols.

---

\documentclass{article}
\usepackage{XCharter,siunitx}
\begin{document}
\SI{2\pi e15}{\radian\per\second}

\sisetup{detect-all}
$\SI{2\pi e15}{\radian\per\second}$
\end{document}

Answer 2

The siunitx documentation suggests the following approach to add more valid symbols like pi in the section "Symbolic 'digits'":

\sisetup{input-digits = 0123456789\pi}

9.15 Symbolic ‘digits’

In some cases you may want to use ‘digits’ which do not fall within the usual set 0123456789. This can be done by setting the input-digits option, but bearing in 68mind that this will affect (prevent) for example rounding.

\sisetup{input-digits = 0123456789\pi}%
\num{4\pi e-7}
4π × 10−7

Each extra entry should be a single token, and should either have a definition which is safe in both math and text mode, or should only be used when the output mode is known. It may be necessary to make some tokens robust using etoolbox for this to work, for example

\robustify\dots
\sisetup{input-digits = 0123456789\dots}%
\qty{0,4066\dots}{\metre\squared}
0.406 6 ... m^2