\section{Control of chamber assembly}
The strict quality control is essential to keep the precision and quality
of the chambers at a constant and high level. Various checks are performed
by the assembly crew. The vital checks use automated montoring systems which were introduced in Section~\ref{monitoring}.
Our experiences  with these systems  will be presented in this section.


\input{guido.tex}

\input{layer.tex}


\subsection{Final measurements}
For each chamber we perform a final measurement 'in the air'. We take measurements in the 'up' and 'down' orientations of the airborne spacer. 

The gravitational sag of the chamber amounts about $\mp$1.8~mm in the 'up' and 'down' position  respectively. An assymmetry in the 'up'/'down' sag points to 
a problem in the sagcompensation procedure. Sofar we found no deviations beyond the level of 10\%, which is the accuracy of sagcompensation system.

From the change of the in-plane readings and the reasonable 
assumption that we introduce a pure sag in the $Y$ direction, the mask angles
can be simply obtained from $dX/dY$. This provides a check on the calibration
of the in-plane mask angle with the calibration tower described above.  


 
