A spray system detects the pressure differential across a plurality of spray section filters and determines whether the resulting values meet predetermined criteria. The elastic tension structure comprises bellows formed by an axial pail of the wall of the parts. A playground structure including a number of such posts is also provided. Each set of the functional region and its corresponding substrate functions as an independent unit, such that the substrates are not affected by each other, thereby providing good compatibility, improved reliability and reduced packaging area.