\subsection{  Tube layer pitch}

 Tube layers  already glued to the 
spacer are only held at the ends and in the middle and thus will sag
in between,  touching the tubes of the next layer to be glued
which are  held in a plane by the 11 combs. 
As the outer tube diameter is 65 \mum smaller than the wire pitch
the layer distance between touching tubes will not be the same as the layer
distance between wires and moreover the effect  is
cummulative when the third layer is added.. 

 To reduce this effect   strips of 50 \mum thick scotch 
tape are  put
on the (next) layer of tubes lying in the combs  . 
 Since this 
requires non-trivial adjustments to the automatic glueing machine 
to avoid interference of glue ropes and strips 
and  additonal labour for each glueing step,
we tried to minimize the extra work.


 In fig [multidik.ps] we show the results of measurements 
 of the total thickness of the multilayers 
 measured at several positions between the crossplates. 
 For this we used a schuifmaat and only the sides of the
 two multilayers can be measured.
  The data are plotted as average deviations from the nominal value
  ( 61.922 mm) at approximately 25cm intervals along the length of chamber
  starting from the RO side. 

  The first trial was on the BOL0 prototype with 2 strips applied 
 over the full width of the chamber between the Middle and HV
 cross plates and no strips between Middle and RO. 
 As can be seen in the plot ( black line) , the strip
 side deviates by 50 \mum and the no strip side by 120 \mum,
 Although the strips clearly help in reducing the effect,
 applying strips over the full width of the chamber
 cannot be done on the assembly table. The spacer
 has to be lifted in a vertical position next to the table
 to put  the tape on  the layers already glued to it.
 This is potentially hazardous and time consuming.
 Consequently the choice was made  to put strips in regions spanning 10 tubes
 near the sides and near the long beams
 at 1/3 and 2/3 of the distance between 
 middle and outer cross plates. This can be done quickly on the table
 after the glue ropes are laid. The corresponding measured average deviation is
 about 80 \mum ( green line in fig).

