described in u s pat no 6 050 260 the contents of which are incorporated herein by reference it will be appreciated that in such applications it may be possible to connect the controllers of the various devices together in an arrangement such that data may be readily exchanged in such cases the rate of flow of the gases may be estimated directly from information available either from the fan or where provided a flow sensor in this embodiment of the present invention the flow is estimated based on the loading of the fan generally the fan will be controlled to run at a specified speed and therefore deliver a constant pressure output the flow rate of the gases will depend on the restrictions in the flow path in turn in order to maintain the specified speed a certain power input will be required for the fan therefore an algebraic relationship between the actual gas flow rate and the power input to the fan can be developed for a fan of known characteristics this relationship may either be determined empirically by experimentation or theoretically calculated using specified motor characteristics a number of methods are known in the art for determining the loading on a motor from the supply it draws the simplest such method would be to firstly meter the current drawn 148 from the fan 150 as indicated in fig3 the current 148 is the input to the conduit heater element controller 102 where either an algebraic relationship or a look up table is used to determine the flow rate of the gases for example in u s.