from the memory 33 the divider 37 then divides the nominal value at the second input 38 by the average at the input 36 and sends the result by way of a line 40 to a multiplier 41 disposed between the controller 27 and the fan motor 28 the multiplier 41 multiplies the output of the controller 27 by the signal value obtained by the divider 37 and thus corrects the fan feed pressure p1 the reason for the divider 37 dividing the reference value by the average instead of the other way round is that the fan pressure p1 has to be reduced for an increase in absolute air humidity and vice versa increased for a reduction in absolute air humidity however the average could of course be divided by the reference value and the multiplier 41 replaced by a divider also of course if a microprocessor is used for the closed loop control as is now conventional the values go directly from the facility for measuring air humidity to the microprocessor where they can be processed in accordance with the programming thereof in this case the nominal value can be regarded as a special input to the microprocessor however it can be contained therein in the form of a read only memory the important consideration is that in working out the.