in operation a fully reliable picture of the blocking of the wire cannot be obtained by means of this method now the permeability of the wire can be determined in the following manner the magnitude of the gap s as well as the width l of the measurement beam or of a part of the same are known quantities when the dynamic pressure p dyn is measured the quantity of air ejected through the wire can be calculated by means of the following formula equ1 wherein v u quantity of air ejected m 3 s when these values are placed in the above formula the result is on the other hand it is possible to measure from underneath the measurement beam 2 the total negative pressure which may be e g of the order of 50 pa thus this value now corresponds to the 34 suction flow 34 which is the above 367 m 3 h if the height h of the measurement beam is e g 0 8 m then the suction flow has taken place through the area 0 8 u00d7 0 5 0 4 m 2 the suction flow per area there is the permeability can now be calculated in the normal manner as follows equ2 if the method described above cannot be made to operate reliably then the permeability of the wire w can be studied by following the outgoing quantity of air v out for example when the drying wire is blocked i e when the permeability is reduced the outgoing air quantity is reduced from the side of positive pressure the measurement can be carried out in principle.