Automatically spacing the tilde (~) symbol as a prefix number using the `siunitx` package

Question

I'm trying to work out if there is a way to get siunitx to natively handle the ~ symbol (\sim) in the same way that it can handle the < and > operators before a number, as I would like to be able to use ~ as short hand for approximately. For example:

\documentclass[12pt,a4paper]{report}
\usepackage{siunitx}
\begin{document}

% Values output the same, with spacing after the < and before the % symbol
\SI{< 10}{\percent} \\
\SI{<10}{\percent} \\

% When writing in math mode a space in placed after the ~ symbol, but not before the % symbol
$\sim10\%$ \\

\end{document}

Ideally I'd like to be able to write something like \SI{~ 10}{\percent} or \SI{\tilde}{10}{\percent}, where {\tilde} is a custom value defined using the DeclareSIUnit\tilde{~} command. But I can't seem to find anything like this for prefix symbols in the documentation. Has anyone else come across a solution to this?

Answer 1

Using \SI{\sim 10}{\percent} should be OK; maybe you want \approx, instead.

\documentclass[12pt,a4paper]{report}
\usepackage{siunitx}
\begin{document}

\SI{< 10}{\percent}   

\SI{<10}{\percent}   

\SI{\sim 10}{\percent}

\SI{\approx 10}{\percent}

\end{document}

Answer 2

I haven't seen an interface for new "prefixes" either, thus the following example patches internals of siunitx. Also I had trouble using ~ as prefix, probably because of rescans, where the catcode of ~ is set to 9, that ignores the characters. Therefore the example uses \~ instead. <> is used a symbol for \approx:

\documentclass[12pt,a4paper]{report}
\usepackage{siunitx}

\ExplSyntaxOn
\exp_args:NNo \cs_set:Nn \__siunitx_number_in_sign_replace:N
 {
  \__siunitx_number_in_sign_replace:N { #1 }
  \tl_replace_all:Nnn #1 { \~ } { \sim }
  \tl_replace_all:Nnn #1 { <> } { \approx }
 }
\tl_put_right:Nn \l__siunitx_input_protect_tl    { \~ }
\tl_put_right:Nn \l__siunitx_input_comparator_tl { \~ }
\ExplSyntaxOff

\begin{document}
  \noindent
  \SI{<10}{\percent}\\
  \SI{\~10}{\percent}\\
  \SI{<>10}{\percent}
\end{document}