Ohm symbol not showing in math mode using custom beamer styling

Question

So, I'm using a custom beamer styling required by my employer, and while trying to typeset some problem examples with the "ohm" symbol, I've noticed it doesn't always work.

At first, I noticed it didn't appear at all, using the following code:

\documentclass[9pt]{beamer}
\mode<presentation>
{
    \usetheme{JuanLesPins}
    \usefonttheme{serif}
    \usecolortheme{beaver}
    \setbeamercovered{invisible} \setbeamertemplate{blocks}[rounded][shadow=true] 
    \setbeamertemplate{navigation symbols}{} 
    \setbeamertemplate{footline}[frame number]
    \usecolortheme[RGB={122,4,24}]{structure}
}

\setcounter{tocdepth}{1}

\usepackage{lmodern}
\usepackage[T1]{fontenc}
\usepackage{polyglossia}
\setdefaultlanguage{brazil}

\usepackage{siunitx}
\sisetup{per-mode=symbol,output-decimal-marker={,},math-micro=\text{µ},text-micro=µ,exponent-product = \cdot}

\begin{document}

\begin{frame}
  \titlepage
\end{frame}

\section*{Sumário}

\begin{frame}{Sumário}
  \tableofcontents
\end{frame}

\begin{frame}{}
    You can see that this doesn't work: \SI{1}{\ohm}

    Nor does this: $ \SI{1}{\ohm} $

    Or this: $$ \SI{1}{\ohm} $$
    Or this: \[ \SI{1}{\ohm} \]
    Or \[ \text{\SI{1}{\ohm}} \]
\end{frame}

\end{document}

After adding detect-all=true to the \sisetup command I managed to get the \SI macros in text-mode to work, but none of the math ones, like so.

I noticed that if I remove the \usefonttheme{serif} line from the styling definition (idk how to call it, sorry), everything works as usual.

I thought of redefining the Omega characters used by the siunitx package, but to no avail.

\DeclareMathSymbol{\Omega}{\mathalpha}{letters}{"0A}
\DeclareMathSymbol{\varOmega}{\mathalpha}{operators}{"0A}

Also tried to change the font used by the package (math-rm=\mathrm, text-rm=\rmfamily in \sisetup), and to make it insensitive to outside family font (detect-family=false in \sisetup). All without any clear result.

Answer 1

It very much looks like the issue you've come across is a (bad) side-effect of using the polyglossia package, which seems to have a few issues when used with the brazil language option.

Unless you absolutely must use the polyglossia package, an easy remedy consists of (a) getting rid of (or, at least, commenting out) the instructions

\usepackage{polyglossia}
\setdefaultlanguage{brazil}

and (b) running, in their place, the instruction

\usepackage[portuguese]{babel}

A separate comment/observation: I would also get rid of the siunitx options math-micro=\text{µ} and text-micro=µ. For sure, when used in tandem with the babel package, these options generate incorrect output when writing, say, \si{\micro\ohm}.

\documentclass{beamer}
\mode<presentation>
{
    \usetheme{JuanLesPins}
    \usefonttheme{serif}
    \usecolortheme{beaver}
    \setbeamercovered{invisible} \setbeamertemplate{blocks}[rounded][shadow=true]
    \setbeamertemplate{navigation symbols}{}
    \setbeamertemplate{footline}[frame number]
    \usecolortheme[RGB={122,4,24}]{structure}
}

\usepackage{lmodern}
\usepackage[T1]{fontenc}

%\usepackage{polyglossia}
%\setdefaultlanguage{brazil}
\usepackage[portuguese]{babel}

\usepackage{siunitx}
\sisetup{per-mode=symbol,output-decimal-marker={,},exponent-product = \cdot}

\begin{document}
\begin{frame}{}
    This works now: \SI{1}{\micro\ohm}

    So does this: $\SI{1}{\micro\ohm}$

    And this: $$ \SI{1}{\micro\ohm} $$
    And this: \[ \SI{1}{\micro\ohm} \]
    And this: \[ \text{\SI{1}{\micro\ohm}} \]
\end{frame}
\end{document}

Answer 2

Just remove \usepackage{lmodern} and \usepackage[T1]{fontenc}.

\documentclass[9pt]{beamer}
\mode<presentation>
{
    \usetheme{JuanLesPins}
    \usefonttheme{serif}
    \usecolortheme{beaver}
    \setbeamercovered{invisible} \setbeamertemplate{blocks}[rounded][shadow=true] 
    \setbeamertemplate{navigation symbols}{} 
    \setbeamertemplate{footline}[frame number]
    \usecolortheme[RGB={122,4,24}]{structure}
}

\setcounter{tocdepth}{1}

\usepackage{polyglossia}
\setdefaultlanguage{brazil}

\usepackage{siunitx}
\sisetup{
  per-mode=symbol,
  output-decimal-marker={,},
  math-micro=\text{µ},
  text-micro=µ,
  exponent-product = \cdot
}

\begin{document}

\begin{frame}{}
    You can see that this doesn't work: \SI{1}{\ohm}

    Nor does this: $ \SI{1}{\ohm} $

    Or this: \[ \SI{1}{\ohm} \]
    Or \[ \text{\SI{1}{\ohm}} \]
\end{frame}

\end{document}