Abstract: Energy limiting, bidirectional signal passage, bandpass filtering, and DC isolation is provided in an intrinsically safe manner, for electrical signals coupled between various electrical equipment located both outside and within a storage tank containing highly combustible matter, for example, via apparatus including a ferrite core transformer potted within a cavity of an explosion proof housing mounted upon a wall of the tank, whereby one winding of the transformer is connected to the outside equipment, and another winding of the transformer is connected to equipment located within the tank.

Abstract: An audio dosimeter for determining the exposure to noise as a function of both time and noise level. Input sound energy is appropriately weighted so as to account for the effect thereof on the human ear. The weighted sound signal is then utilized to gate pulses of selected discrete frequency levels to a counter, such that as the sound level increases, correspondingly higher frequency pulses are coupled to the counter. The counter stores the count therein until read out by an appropriate display device.