Abstract: An improved fluid parameter sensing apparatus particularly adapted for comparing one value of at least one air parameter as sensed at a first location in an air treatment system with at least one other value of the air parameter as sensed at at least a second location in the air treatment system includes a housing defining a pneumatic chamber; first means for feeding a stream of air from the first location to the housing; second means for feeding a stream of air from the second location to the housing; means for alternately admitting air from the first and second air stream feeding means to the chamber; sensing means disposed in the chamber for providing an output signal proportional to the at least one air parameter; means connected to the output of the sensing means for comparing a first output signal from the sensing means with a second output signal from the sensing means, the second signal being spaced in time relative to the first signal; and means operatively connected to the air admitting means for co