guaranties a long moulding time. Even in case the flow rate of the sample gas is increased, there is only a small difference pressure on the filter matrix between clean gas and crude gas side. Therefore, the solid particles do not get into the filter’s pore structure. They are deposited as filter cake on the filter’s surface and cause a prolonged moulding time in its property as prefilter. The M&C-prefilters SP2000 / V20.. /V12.. /20SS are selected according to the specific application. The basis for selection are the process parameters, i.e. the gas compounds, dust loading, grain sizes, water vapure saturation, temperature, pressure and gas velocity. The M&C prefilters /V20.. are equipped with a welded thread connector for mountage into the mounting flange of the M&C gas sample probe. For an optimal sampling position in the process, the prefilters /V20.. can be mounted to the sample probe via an extension tube /Vm... In case of a high velocity of the process gas, shape protection plates /AB.. are used in order to protect the prefilters /V20.. against abrasion. The prefilters /V12-... have got a mounting flange adapter of stainless steel, executed with 4 threaded bolts for easy mounting. The prefilter versions /V12-.. /SS, /V12-.. /IC are equipped with a welded protection tube of stainless steel or Incoloy on the mounting flange adaptor. The feeding of process gas is made via the three longitudinal slots distributed on 180° tube periphery. The special double-ply construction of the prefilter /V12-1, /V12-3 is granting a particularly high back purging efficiency. Depending on fabrication, the prefilters /V20.., /V12.. are partially made of different material combinations. See table. For reduction of the delay time in case of a small sample gas flow, prefilters with volume displacer have to be used. The M&C tube prefilter /V20-T of PTFE cloth with interweaved metal filaments for reduction of the surface resistance (in order to avoid frictional electricity) disposes over an excellent back purging quality. It is used for example in case of gas samplingout of coal bunkers with a high content of superfinedust. The prefilter /20SS 150 is used for direct mounting on the M&C sample tubes SP2000/ SS. It is supplied with a shape protection plate and a tube fitting with double cutting ring. If there are problems eg. in case the depth for assembling is too small or the flow velocity is too high, or the dust content or the temperature are too high, then the electrical heated adapter tube /AR should be used. It is equipped with a flange connection at both sides and is to be mounted on the plant’s connection flange.