siunitx + fouriernc = Size substitutions with differences? [duplicate]

Question

Possible Duplicate:
xfrac + siunitx gives me a font warning

This document (New Century Schoolbook font in \SI)

\documentclass{scrreprt}

\usepackage[utf8]{inputenc}
\usepackage{fouriernc}
\usepackage[T1]{fontenc}
\usepackage{siunitx}

\begin{document}

\SI{1}{\metre\per\second}

\end{document}

produces this warning

LaTeX Font Warning: Size substitutions with differences
(Font)              up to 2.01195pt have occurred.

Additional information: This does only happen with the NC font in \SI with \per ("power to the -1"). Outside of \SI this does not happen (for example `$a^{-1}$ does not produce the warning). This does only happen on the units, not on magnitudes (10^-1).

• Why does this happen?
• Does siunitx require another font size for powers? Why?
• How to fix this warning?

Answer 1

The siunitx package jumps through a lot of 'hoops' to give the correct appearance of output as far as possible. That means quite a bit of font detection and math/text mode switching. The warning can be generated by an example such as

\documentclass{article}
\usepackage{fouriernc}
\usepackage{siunitx}
\begin{document}
\ensuremath{^{\text{{\unboldmath$-1$}}}}
\end{document}

where you'll note that there is no change of font at all (without \unboldmath the warning is slightly different). That I know of, there should be no change in the fonts actually used in the output: a quick check shows that everything looks OK.