Filter apparatus and method with filter evaluation by sample solutions

A method for filtering a thixotropic fluid using a filter apparatus having a housing with an interior volume V, a filter provided within the interior of the housing in such a way that the filter divides the interior into a primary region and a secondary region having a dead space, said filter apparatus having an index .THETA./H>0.95, which is determined by:

• providing a first solution;
• providing a second solution which differs from the first solution in one characteristic so that the first solution can be distinguished from the second solution;
• placing said first solution in every part of said housing;
• displacing said first solution in every part of said housing with said second solution;
• recovering a filtered solution from said secondary region;
• measuring a duration of time, .THETA., required for said filtered solution to change from consisting of only said first solution to consisting of only said second solution;
• calculating a mean residence time H, where H=V/v and where v is a flow velocity in the filter apparatus for said first and second solutions;
• calculating the index .THETA./H;
• supplying said thixotropic fluid to said filter apparatus;
• and
• passing said thixotropic fluid from said primary region through said filter into said secondary region.