A pressure monitoring device includes a rigid base (2) which has secured thereto a lid (5) to define a hermetically sealed chamber (6). The lid (5) is of flexible metal material and the major surface (5A) of the lid forms a diaphragm which will deflect in response to changes in the pressure surrounding the device. A substrate (7) upon which a plurality of SAW devices are laid down is supported on a rigid frame (3A) of the base so that the central region thereof is unsupported between edges (11, 12). A dimple (10) is formed in the lid (5) and presses on a central region of the substrate (7). Changes in pressure will accordingly cause distortion of the central region of the substrate. The end regions (8, 9) of the substrate (7) carry further SAW devices so that, by analyzing the changes in characteristics of all the SAW devices a temperature compensated indication of pressure change, together with an indication of absolute temperature, may be provided by interrogating the SAW devices. The base (2) and the lid (5) are preferably both of metal alloy and may be secured together by soldering. Antennae associated with the SAW devices may be laid down as tracks on the surface of the base (2).