A relative humidity or water activity sensor includes a body defining a cavity with at least one resonant humidity sensing element comprising a crystal plate coated in part with an adsorbent material and at least one resonant temperature sensing element disposed inside the cavity. A membrane suitable for passing water vapor acts as a barrier to liquids and is situated between the cavity and the medium whose relative humidity and/or water activity is to be measured. The value of the relative humidity or of the water activity of the medium, and the temperature are determined from the resonant frequencies of the sensing elements. The invention also provides an installation including a plurality of such sensors.

A dual monitor comprises a piezoelectric sensor in the form of a quartz crystal and an electrode system thereon. An exposure device in the form of a dust discriminator is provided to expose the piezoelectric sensor at least periodically to a stream of a gas which may contain dust or other organic or inorganic particulate material whose presence in or absence from the gas is to be monitored. The gas of said stream consists primarily of air. An electronic monitoring device is provided coupled to the electrode system of said piezoelectric sensor to monitor variations in the resonant frequency of said piezoelectric sensor induced by the aggregation of dust or other particulate material on said piezoelectric sensor.

A device for measuring thickness and/or rate of thickness increase of a film comprises at least one piezoelectric element, and first and second electrodes in contact with the piezoelectric element. A method of measuring thickness and/or rate of thickness increase of a film comprises applying a voltage across a piezoelectric element from a first electrode to a second electrode, thereby causing the piezoelectric element to vibrate, and measuring the rate of vibration of the piezoelectric element. Heat may be applied to the piezoelectric element. The piezoelectric element may be formed of quartz crystal, e.g., IT-cut.

The device serves to measure the density of a gas, in particular of the insulating gas of a gas-insulated electrical installation. It contains two resonators (11, 12) each comprising a piezoelectric crystal (14, 15) and each comprising two electrodes applied to the crystal. A first (11) of the two resonators (11, 12) is mounted in a first chamber (5) containing the gas to be measured, whereas a second (12) is mounted in a second chamber (6) sealed with respect to the gas. Despite small dimensions and despite simple and robust construction, this device is intended to have a high measurement precision over a long period of time. This is achieved by drawing the two resonators (11, 12) from a multiplicity of similar resonators manufactured by mass production and not subjected to a frequency alignment. The first resonator (11) is unaltered with respect to the manufacture in mass production. The second resonator (12) is likewise unaltered with respect to the manufacture in mass production and, under vacuum and with equality of temperature, it has the same resonance frequency as the first resonator (11) within a specified frequency tolerance. The second resonator (12) may, however, also be modified, by altering its electrodes (18, 19), in such a way that, under vacuum and with equality of temperature, it has the same resonance frequency as the first resonator (11) within the specified frequency tolerance.

The present invention provides a method of mounting a piezoelectric device having a plurality of mounting pads directly to a printed circuit board comprising the steps of providing a piezoelectric device, miniature conductive springs and a printed circuit board, attaching the springs to mounting pads of the piezoelectric transformer, and attaching the springs to the printed circuit board. Also disclosed is an apparatus made according to the method.