A flow sensor of the disc type for a compressed air lubricator comprises a flexible elastomeric disc (15) having a pair of radial channels (19, 19' and 20, 20') formed in each of its major surfaces. The channels afford lines of weakness which enable the disc (15) readily and progressively to deform into a fluted cone-like configuration as the air flow rate through the lubricator increases, thereby increasing the potential air flow capacity of the lubricator compared to those including known designs of disc type flow sensor (Fig 5).