and combined for example by summer 168 c with the estimated smoke particle transmittance to produce an output estimating the combined particle transmittance in practice the values used for predetermining the weighting operation applied to the integrated smoke hours value stem from the physical properties of a given filter and the environmental conditions in which the filter is operating these values may be stored in the look up table 168 b the output of estimator 168 or the output of dust counter 156 is applied via selector 162 to comparator 158 which may compare the outputs of estimator 168 or the outputs of counter 156 to given preset thresholds 170 the preset thresholds may be preset counts in the case of a dust count alone and or preset particle transmittances in the case of either the estimated smoke particle transmittance alone or a combined estimated particle transmittance the output 160 of comparator 158 may be used to set warning indicators based on the result of comparisons with the given preset threshold values 170 fig4 illustrates the relationship between particle transmittance t x and time t in graphical form for three cases it would be understood by the person skilled in the art that for the purposes of this description the actual magnitude of the particle transmittance t x is arbitrary rather it is comparative values of transmittance t x that are noteworthy for this discussion thus the vertical axis in fig4 indicating transmittance t x is shown as a discontinuous axis dashed line s represents particle transmittance t x in a smoke detector system with no filter trapping smoke or dust particles also shown are two curves a and b representing estimated filter particle transmittances determined in accordance with the present invention curve a.