\hhline double line imperfection

Question

Using the \hhline command i found some imperfection in the double line. How can i fix it?

Imperfection

Table

\documentclass[11pt]{amsart}
\usepackage{amssymb}
\usepackage{hhline}                 
\usepackage{siunitx}
\sisetup{output-decimal-marker={.}, group-separator={\,}}
\usepackage{eurosym}
\usepackage{dcolumn}                
\usepackage{multirow}



\begin{document}

\begin{table}[htbp]
\begin{center}
\begin{tabular}{|lr|lr l|} \hline


H       & 5             & $W_N$ & 250       & \si{\gram}        \\
L       & 26            & $W_L$ & 241       & \si{\gram}        \\
P       & \num{3000}    & $C_u$ & 3E{-4}    & \euro/\si{\gram}  \\
$n_C$   & \num{142506}  & $c_0$ & 2E-01     & \euro/pack        \\
$\delta$& 1             & $c_1$ & 5E-01     & \euro/h           \\
\hhline{=====}


&   \multicolumn{2}{c}{$r.s_{std}$} &   \multicolumn{2}{c|}{$r.s_{iter}$} \\

\hhline{=====}

Media(W)    &   \multicolumn{2}{D{.}{.}{3.4}}{245.9463}                                                 &   \multicolumn{2}{D{.}{.}{3.4}|}{245.9459}\\
DevStd(W)   &   \multicolumn{2}{D{.}{.}{3.4}}{3.9932}                                                   &   \multicolumn{2}{D{.}{.}{3.4}|}{3.9939}\\
\emph{f.o}  &   \multicolumn{2}{D{.}{.}{3.4}}{0.27395}                                                  &   \multicolumn{2}{D{.}{.}{3.4}|}{0.27395}\\
$n_B$       &   \multicolumn{2}{D{.}{.}{3.4}}{3000}                                                     &   \multicolumn{2}{D{.}{.}{3.4}|}{3000}\\
$t_{tot}$   &   \multicolumn{2}{D{.}{.}{3.4}}{2612.22}                                                  &   \multicolumn{2}{D{.}{.}{3.4}|}{6906.10}\\

\hhline{=====}

Config.     &   \multicolumn{2}{c}{10 20 40 50 200}                                                     &   \multicolumn{2}{c|}{40 130 130 220 220}\\ \hline

\end{tabular}
\begin{footnotesize}
\caption{Confronto $r.s_{std}$ Vs $r.s_{iter}$ [$C_u > 0$]}
\label{tab:Confronto_rs_C_mag_0}
\end{footnotesize} 
\end{center}
\end{table}

\end{document} 

Answer 1

I've never seen that command, but I'd replace that with two \hline commands, like this:

\hline
\hline

I've changed the style of the table a little bit, in case you don't mind a different solution. The changes I did are:

1. Inserted booktabs and removed all of the vertical lines except one in the upper table.
2. Fixed the caption using the suggestion from this answer (FIY, now it's actually footnotesize, earlier the command wasn't working) and I have shifted it a little bit below.
3. Changed the caption according to egreg's suggestion (I agree with him on that one).

Doubt: are they two distinct tables? If the r.s std is referring to the lower part, then you should consider adding some whitespace or trying a different solution, as in splitting the tables. Let me know if you want something of that sort.

Here is the code (I have written notes for my additions in the code itself):

\documentclass[11pt]{amsart}
\usepackage{amssymb}
\usepackage{hhline}                 
\usepackage{siunitx}
\sisetup{output-decimal-marker={.}, group-separator={\,}}
\usepackage{eurosym}
\usepackage{dcolumn}                
\usepackage{multirow}
\usepackage{booktabs}
\usepackage{caption} % Allows to set the caption font size

\begin{document}

\begin{table}[htbp]
    \captionsetup{font=footnotesize} % Set the caption font size
\begin{center}
\begin{tabular}{lr| lr l}

\toprule

H       & 5             & $W_N$ & 250       & \si{\gram}        \\
L       & 26            & $W_L$ & 241       & \si{\gram}        \\
P       & \num{3000}    & $C_u$ & 3E{-4}    & \euro/\si{\gram}  \\
$n_C$   & \num{142506}  & $c_0$ & 2E-01     & \euro/pack        \\
$\delta$& 1             & $c_1$ & 5E-01     & \euro/h           \\

\bottomrule

&   \multicolumn{2}{c}{$r.s_{std}$} &   \multicolumn{2}{c}{$r.s_{iter}$} \\

\midrule

Media(W)    &   \multicolumn{2}{D{.}{.}{3.4}}{245.9463}                                                 &   \multicolumn{2}{D{.}{.}{3.4}}{245.9459}\\
DevStd(W)   &   \multicolumn{2}{D{.}{.}{3.4}}{3.9932}                                                   &   \multicolumn{2}{D{.}{.}{3.4}}{3.9939}\\
\emph{f.o}  &   \multicolumn{2}{D{.}{.}{3.4}}{0.27395}                                                  &   \multicolumn{2}{D{.}{.}{3.4}}{0.27395}\\
$n_B$       &   \multicolumn{2}{D{.}{.}{3.4}}{3000}                                                     &   \multicolumn{2}{D{.}{.}{3.4}}{3000}\\
$t_{tot}$   &   \multicolumn{2}{D{.}{.}{3.4}}{2612.22}                                                  &   \multicolumn{2}{D{.}{.}{3.4}}{6906.10}\\

\midrule % automatically provides a little space before and after it (up and down)

Config.     &   \multicolumn{2}{c}{10 20 40 50 200}                                                     &   \multicolumn{2}{c}{40 130 130 220 220}\\

\bottomrule

\end{tabular}
\vspace{10pt} % a little space to shift the caption down
\caption{Confronto tra $r.s_{std}$ e $r.s_{iter}$ [$C_u > 0$]}
\label{tab:Confronto_rs_C_mag_0}
\end{center}
\end{table}

\end{document}