Make up air controller for use with fume hood systems

Apparatus for controlling the volume of make-up air supplied to an area in which is located a fume hood having an exhaust volume which varies as the fume hood sash is opened and closed comprising:

• transducer means, responsive to the fume hood sash position, for providing an output signal representative of the area of the sash opening;
• make-up air control means, connected to an air supply and responsive to a make-up air command signal, for providing a make-up air flow the volume of which is determined by the make-up air command signal, and
• a make-up air controller for providing the make-up air command signal, including:
• means for mu1tiplying the transducer means output signal by a scale factor to provide a scaled output signal;
• offset means, responsive to the scaled output signal, for subtracting therefrom a signal representative of a desired air flow volume differential between the fume hood exhaust volume and the make-up air volume to provide an output signal representative of a resulting make-up air volume;
• and
• means, responsive to the offset means output signal, for applying a make-up air command signal to the make-up air control means so as to produce a make-up air flow the volume of which equals said resulting make-up air volume.