Microwave cavity resonator for continuous spectroscopic gas analysis

• US 5,949,237 A
• Filed: 07/15/1998
• Issued: 09/07/1999
• Est. Priority Date: 02/03/1996
• Status: Expired due to Term

• Current Assignee: Forschungszentrum Karlsruhe Gmbh
• Original Assignee: Forschungszentrum Karlsruhe Gmbh
• Inventors: Schmitt, Gerhard, Metzger, Hans-Dieter, Berger, Lutz, Koniger, Franz
• Primary Examiner(s): DO, DIEP N
• Application Number:
  US09/115,729
• Time in Patent Office: 419 Days
• Field of Search: 324/634, 324/636, 73/23.2, 333/227, 333/230, 333/231, 333/232
• US Class Current: 324/636
• CPC Class Codes: G01N 22/00 Investigating or analysing materials by the use of microwaves or radio waves, i.e. electromagnetic waves with a wavelength of one millimetre or more G01N3/00 - G01N17/00, G01N24/00 take precedence
• CPC Class Codes: H01P 7/04 Coaxial resonators

Abstract

In a microwave resonator for continuous spectroscopic gas analysis with a microwave cavity having dimensions depending on the operating frequency and a setting for a low quality Q of its base mode and microwave reflectors disposed at its opposite ends and including a round hollow conductor part and a co-axial conductor part separated by a pressure-tight dielectric window, an internal conductor part is axially movably supported in the coaxial conductor part by the microwave reflector at one end of the co-axial conductor part, the other reflector being axially movable for tuning the resonator to a maximal absorption frequency and a by-pass line extends past the co-axial conductor part in the area of the dielectric window and has a short branch connected to the co-axial conductor part and including a restrictor for supplying only a relatively small flow of gas from the bypass line to the conductor part of the microwave resonator.

Claims

1. A microwave cavity resonator for continuous spectroscopic gas analysis to determine the concentration of a given compound in a process medium flow, said microwave resonator having a cavity with a diameter which depends on the operating frequency of the microwave resonator and other dimensions and a setting for a low Q-factor of the base mode of the microwave resonator with the highest ratio of cut-off wave length to the diameter of the resonator and further having a resonator performance line with a half value width set to a value which is greater than the half value width of an absorption line to be measured by a factor in the lower single digit range, said resonator being a hollow cylindrical body having microwave reflectors disposed at opposite ends of the microwave resonator and including a round hollow conductor part and a coaxial conductor part which are separated by a gas and pressure-tight dielectric window, an internal conductor disposed in said coaxial conductor part so as to be axially movable therein and forming an electrode for the application of a Stark modulation, said internal conductor extending through, and being supported by, the microwave reflector at one end of said coaxial conductor part, two hollow conductors attached to said round hollow conductor symmetrically with respect to the axis of said resonator, one of said microwave reflectors being stationary and the other being axially movable for tuning said resonator to a predetermined maximal absorption frequency and for a setting providing for optimal in- and out-coupling of the microwave, a bypass line extending past said coaxial conductor part adjacent said dielectric window and having a branch of short length extending to said coaxial conductor part and including a restrictor generating a pressure drop for supplying only a restricted continuous gas flow to said coaxial conductor part and means for removing gas from said coaxial conductor part through openings formed in the microwave reflector at the end of said coaxial conductor part.

2. A microwave cavity resonator according to claim 1, wherein said base mode is the H₁₁ mode.

3. A microwave cavity resonator according to claim 1, wherein said resonator has thick walls to provide for good mechanical stability and a high heat capacity.

4. A microwave cavity resonator according to claim 1, wherein the inner diameter of the hollow cylindrical resonator body is at least of a size as determined by the excitation of the predetermined mode.

5. A microwave cavity resonator according to claim 4, wherein said microwave reflector, which is stationary in said coaxial conductor part, acts as a mode filter and supports the Stark electrode and further includes openings through which the gas to be measured can be uniformly sucked out of the coaxial conductor part and wherein said microwave reflector in said round hollow conductor part acts as a λ

/4 transformer and is separated from the resonator wall in an electrically conductive way by a thin-walled dielectric hollow cylinder.

6. A microwave cavity resonator according to claim 5, wherein said Stark electrode has a tip formed at its free end to provide for a transition between the round hollow conductor part and the coaxial conductor part, which transition is microwave shock-free, said electrode being supported coaxially by way of a dielectric star-like support member which does not essentially affect the gas flow.

7. A microwave cavity resonator according to claim 6, wherein said restriction is exchangeably and includes a nozzle-like projection extending into an area of said by-pass line with fast gas flow.