with the reality rather well as in many applications when e g the filter or other predetermined flow subjected element degrades over time and the extra energy requiring compensation measures such as fan speed increase are needed the necessary tasks consumer more and more power energy and ultimately money due to increasing energy costs in any case a degraded element such as a contaminated filter causes severe losses to the fluid circulation system the curve 202 may be established utilizing the modeling techniques preferably regression explained herein line 204 represents the operating costs associated with traditional service routines following fixed schedules e g once in a year when the pressure loss is 200 pa etc thus defining 100 level or u2018 top level u2019 for the costs relative to which cost savings may be sought after energy cost bar 204 represents the dynamic energy costs that increase often exponentially throughout the measurement period between the maintenance operations maintenance cost bar 206 represents the typically fixed maintenance costs known to be realized upon each maintenance operation such as filter cleaning or replacement preliminary cost gain 310 represents a concept or a parameter that reflects cost arising from estimated energy consumption during a predetermined amount of operation time line 312 indicates the start of the preparation period and thus the instant for triggering the maintenance preparatory signal indicative of the forthcoming estimated optimum maintenance instant the preliminary cost gain may be considered to turn into energy cost when the optimum maintenance instant approaches in fig4 the preliminary cost gain 310 has turned into energy cost that.