A gravity feed shelving system is provided in which specially configured tiers contain necked-down channels for the receiving, storing and dispensing of series of flexible packages such as flex bags. The dc-based uninterruptible power system includes a multiplexer, a battery unit, a linear regulator, a switch-transistor, a voltage comparator and micro controller. The insulating material is, for example, a resin. The calculated impulse response may be used to improve signal demodulation or to calculate the coefficients for a digital filter applied before demodulation to compensate for pulse-shape distortion. The door panel is selectively moveable between a closed position, wherein the door panel restricts airflow through the inlet port, and an open position, wherein the door panel permits airflow through the inlet port. The tubular body has a through hole formed inside.