Device for measuring gas density

A method of making a device for measuring density of a gas, said device comprising two resonators having resonant frequencies located within a specified frequency tolerance, each of said resonators including a piezoelectric crystal and at least two electrodes applied to each piezoelectric crystal, a first of the two resonators being mounted in a first chamber wherein the gas to be measured is input, and a second of the two resonators being mounted in a second chamber which is sealed to prevent input of the gas, said method comprising, prior to the mounting of the two resonators, the steps of:

• machining a multiplicity of piezoelectric crystals from a single crystal wafer or from a batch of wafers obtained from a crystal block, said multiplicity of piezoelectric crystals having physical properties and geometrical dimensions which are similar to one another;
• applying electrodes to each piezoelectric crystal of the multiplicity of crystals for generating a multiplicity of resonators having a statistical spread of resonance frequencies which is greater than said frequency tolerance;
• determining the resonance frequencies for plural resonators of the multiplicity of resonators using similar measuring conditions;
• selecting a pair of resonators from said multiplicity of resonators for use as said first and second resonators;
• and
• machining one of said selected first and second resonators to match its resonance frequency with the resonance frequency of the other of said first and second resonators within said frequency tolerance.

A device for measuring the density of gas, in particular of the insulating gas of a gas-insulated electrical installation, comprising:

• two resonators each containing a piezoelectric crystal and each containing at least two electrodes applied to the crystal, of which two resonators a first is mounted in a first chamber containing gas to be measured and a second is mounted in a second chamber sealed with respect to the gas, wherein the two resonators are drawn from a multiplicity of similar resonators manufactured by mass production and not subjected to a frequency alignment, wherein the first resonator is unaltered with respect to the manufacture in mass production, and wherein the second resonator is likewise either 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 within a specified frequency tolerance or, alternatively, under vacuum and with equality of temperature, it has the same resonance frequency as the first resonator within the specified frequency tolerance after its electrodes have been altered, said first resonator being disposed in a first sleeve manufactured during mass production and forming the boundary of the first chamber, and the second resonator being disposed in a second sleeve manufactured during mass production and forming the boundary of the second chamber, the first chamber being connected via an opening in the wall of the first sleeve to a third chamber which contains the gas to be measured and which receives the first and the second sleeve, the first chamber being constructed such that acoustic interactions with the surroundings of the first resonator are largely avoided, and being large compared with the wavelength of a sound wave radiated by the first resonator into the gas to be measured at resonance frequency and/or wherein the first chamber has walls which absorb ultrasound.

A device for measuring the density of gas, in particular of the insulating gas of a gas-insulated electrical installation, comprising two resonators each containing a piezoelectric crystal and each containing at least two electrodes applied to the crystal, of which two resonators a first is mounted in a first chamber containing gas to be measured and a second is mounted in a second chamber sealed with respect to the gas, wherein the two resonators are drawn from a multiplicity of similar resonators manufactured by mass production and not subjected to a frequency alignment, wherein the first resonator is unaltered with respect to the manufacture in mass production, and wherein the second resonator is likewise either 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 within a specified frequency tolerance or, alternatively, under vacuum and with equality of temperature, it has the same resonance frequency as the first resonator within the specified frequency tolerance after its electrodes have been altered, said first resonator being disposed in a first sleeve manufactured during mass production and forming the boundary of the first chamber, and the second resonator being disposed in a second sleeve manufactured during mass production and forming the boundary of the second chamber, the first chamber being connected via an opening in the wall of the first sleeve to a third chamber which contains the gas to be measured and which receives the first and the second sleeve, the first chamber being constructed such that acoustic interactions with the surroundings of the first resonator are largely avoided, and being of axially symmetrical, in particular cylindrical, construction, wherein the diameter of the first chamber is less than half the wavelength of a sound wave radiated by the first resonator into the gas to be measured at resonance frequency.