Pipe Storage Box

A sensor assembly supported on a horizontal directional drilling machine adjacent a magazine. The magazine includes a plurality of vertical columns configured to store a plurality of pipe sections. The sensor assembly comprises an elongate tower that suspends a rigid support structure above the plurality of columns. The rigid support structure includes a sensor housing that carries a plurality of proximity sensors. The proximity sensors correspond with the columns in one-to-one relationship. The proximity sensors measure values indicative of the number of pipe sections contained within each column and transmit the measured values to a processor included in the drilling machine.