A low-profile dryness indicator for point-of-use operation in an air/gas line delivering compressed air or gas to an air/gas-driven tool. The apparatus includes a vessel having a first end, a second end, and a central portion formed by a light transmissive wall that defines a containment chamber containing a dryness-indicating material, such as a color-changing desiccant or moisture sensitive paper. A rigid protective housing is disposed around the vessel wall and has one or more view ports for viewing the dryness-indicating desiccant in the containment chamber. The protective housing is closed by first and second end cap fittings that are each configured for connection to a compressed air/gas line, or to an air tool, and include a passage for transferring air or gas to or from an adjacent one of the plenum-defining cavities.