a noise immune detection system includes a plurality of detectors that generate respective indicia representative of adjacent ambient conditions . a communications link extends between the detectors . a control element is coupled to the link to receive and process the indicia and to adjust an alarm threshold level in response to noise levels in the system . respective indicia are filtered twice by the control element . in the presence of noise , as reflected in relative values of the filtered values of the indicia , the threshold value is automatically increased .