A method of manufacturing a pressure monitoring package comprises the steps of mounting a plurality of resonators onto a substrate 1, mounting the substrate 1 in a package base 11, mechanically constraining the package base 11 within a mechanical preloading jig, and adjusting the preload using the jig whilst measuring the frequency of the SAWs until the required frequency is obtained. A lid 7 is then welded onto the package base 11 using a laser welder, after which the preload frequency of the package 10 is fine tuned by using the laser welder to produce laser marks 21-28, 31-38 on the outside of the package 10.