Abstract:
The leak detection device comprises a plurality of measuring cells ( 10 ) in whose interior the absorption of a laser beam ( 17 ) is influenced by the presence of tracer gas. All of said measuring cells ( 10 ) are connected to a host unit ( 25 ) via light-conducting fibers ( 28,34 ). In the host unit ( 25 ), a laser ( 26 ) designed for modulation and a photodetector ( 37 ) are arranged. Modulation of the laser radiation is preferably performed by two-tone frequency modulation. This has the effect that the fiber length cannot significantly skew the result of the measurement.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a device for performing leak detection at a plurality of test sites. 
         [0003]    2. Description of the Prior Art 
         [0004]    For leak detection, use is made of a tracer gas, particularly helium, argon or another inert gas, which will pass through a possibly existing leak and be detected on the opposite side. For detecting the tracer gas, there is normally used a mass spectrometer. Since mass spectrometers are very complex and expensive, also other approaches have been developed for detection of a tracer gas. One such approach consists in using a gas-tight measuring cell which is closed by a membrane that is selectively permeable to the tracer gas. In the interior of the measuring cell, a total pressure is generated which corresponds to the partial pressure of the tracer gas outside the measuring cell. By measurement of the total pressure in the measuring cell, the presence and the concentration of the tracer gas can be determined. 
         [0005]    Detection of the gas is also possible by absorption spectroscopy. By excitation, tracer gas existing in the measuring cell can be brought to a higher energetic level which is referred to as a metastable state. Metastable gas atoms have a characteristic absorption spectrum and thus are optically detectable by spectroscopy. In spectroscopy, it is required to modulate a laser beam passing through the metastable gas, and to evaluate the signals of the respective radiation receiver. The apparatus involved therein are very complex. In cases where a plurality of test sites exist, the respective technical investment required will be multiplied accordingly. 
         [0006]    It is an object of the invention to provide a device for performing leak detection at a plurality of test sites, said device comprising a host unit which is to be used for all of the measurement sites and which can be situated remote from the measuring sites. 
       SUMMARY OF THE INVENTION 
       [0007]    The device according to the invention is defined by claim  1 . Said device comprises a plurality of measuring cells for optical detection of a tracer gas, which measuring cells are connected to a host unit via light-conducting optical fibers. The host unit comprises a frequency-variable laser and a photodetector. The host unit is configured for selective cooperation with each of the measuring cells. Thus, there is required only one host unit in which the laser beam will be generated and modulated and which further includes the evaluation unit for evaluating the received laser radiation. This makes it possible, in a technical installation, to distribute a large number of measuring cells onto different sites where leak detection is required. Evaluation of all leak tests is carried out in the host unit. 
         [0008]    The means for transforming the tracer gas into a metastable state can comprise electrodes for generating a plasma using a buffer gas. Said plasma excites the tracer gas into a higher energetic state which is optically detectable. A further option for excitation into the metastable state is bombardment with electrons from an electron source. In this case, no buffer gas will be required. 
         [0009]    In the device of the invention, signal transmission is performed via light conductors bridging the distance between the individual measuring cells and the host unit and establishing a connection between the measuring cells and the host unit. In the host unit, the laser beam will be modulated, and in the measuring cell, the modulation spectrum will be changed corresponding to the characteristic absorption line of the tracer gas. The resulting signal will be returned to the host unit via light-conducting fibers. 
         [0010]    Preferably, the laser is operated with two-tone modulation (TTM). In TTM spectroscopy, two relatively closely adjacent modulation frequencies are generated whose frequency spacing (intermediate frequency) is relatively small, preferably under 1 MHz. Such an intermediate frequency Ω is not affected by chromatic dispersion in optical fibers. Thus, for applications with high modulation frequencies and long optical transmission paths, TTM spectroscopy is particularly useful. TTM spectroscopy is described in greater detail in Avetisov V. G. and Kauranen P., Appl. Opt. 35, 4705 (1996). A further limiting factor is the absorption in the optical fiber. For wavelengths of 1083 nm, this absorption will be about 1.3 dB/km; for higher wavelengths, however, it will be lower. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    A full and enabling disclosure of the present invention, including the best mode thereof, enabling one of ordinary skill in the art to carry out the invention, is set forth in greater detail in the following description, including reference to the accompanying drawing in which 
           [0012]      FIG. 1  is a schematic view of a measuring cell, 
           [0013]      FIG. 2  is a schematic view of a network comprising a plurality of measuring cells connected to a host unit, 
           [0014]      FIG. 3  is a diagram representing the fiber transmission versus the frequency, with the characteristic chromatic dispersion line of the fiber superimposed, 
           [0015]      FIG. 4  is a diagram representing the frequency spectra at the input and the output of a light-conducting fiber, for the cases of single-frequency modulation of the input beam and resulting amplitude modulation of the output beam, 
           [0016]      FIG. 5  is a diagram representing the signal spectrum for two-tone frequency modulation, with the characteristic chromatic dispersion line of the fiber, respectively, and 
           [0017]      FIG. 6  is a diagram representing the spectra at the input and the output of the light-conducting fiber, for TTFM modulation. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0018]      FIG. 1  illustrates a measuring cell  10  for optical detection of a tracer gas, particularly helium. Measuring cell  10  is made of a gas-impermeable material, particularly of glass. The interior  11  of measuring cell  10  is closed by a test-gas inlet  12 . Said test-gas inlet  12  includes a membrane  13  which is exclusively or preferably permeable to the tracer gas but is impermeable to other gases. Membrane  13  is permeable in both directions, resulting in a pressure in the interior  11  of cell  10  that is equal to the partial pressure of the tracer gas externally of the cell. Cell  10  has been evacuated beforehand so that it cannot contain any other gas except for the tracer gas. Within cell  10 , an excitation means  18  is arranged for transforming the tracer gas into a higher excitation state. Said excitation means  18  can be an electron source adapted to bombard the tracer gas with electrons, thus transforming it into a higher excitation state. In another cell type, a gas discharge path can be provided wherein a buffer gas will be ionized for bringing the tracer gas into a higher excitation state. The inducing of the metastable state can also be effected by X-rays, by multiphoton excitation, by Raman type population and by collision with neutral atoms/molecules, e.g. by an ultrasonic beam. 
         [0019]    For optical detection of the metastable tracer gas, use is made of a measuring path  14  comprising a radiation source  15  and a radiation receiver  16  receiving the laser beam  17  of said radiation source. The wavelength of the laser beam emitted by radiation source  15  is set e.g. to 1083.034 nm, so that, with helium being used as the tracer gas, starting from the metastable condition 2 3 S 1 , a higher energy level of 2 3 P 2  can be reached. With a laser wavelength of 1083.025 nm, the energy level of 2 3 P 1  would be reached, and with a wavelength of 1082.908 nm, the energy level of 2 3 P 0  would be reached. When tracer gas being in its metastable state is hit by a laser beam of the designated wavelength, the radiation of this wavelength will be absorbed. Details can be gathered from DE 198 53 049 C2. 
         [0020]    Instead of the above described type of measurement cell, one can use similar measuring cells in modified versions, e.g. a measuring cell connected to a pump device for removal of tracer gas from the interior of the measuring cell. 
         [0021]    Radiation source  15  is a part of a light-conducting fiber  20 , and radiation receiver  16  is a part of a light-conducting fiber  21 . 
         [0022]    As shown in  FIG. 2 , a host unit  25  is provided for generating therein the laser beams for all measuring cells  10  and for evaluating therein the laser radiation exiting from the measuring cells. Said host unit  25  includes a laser  26  with controllable laser wavelength. With the aid of a coupling-in device  28   a , the laser beam  27  generated by the laser is coupled into a light-conducting fiber  28  leading to a beam splitter  29 . Said beam splitter  29  will distribute the laser beam simultaneously or sequentially onto the light-conducting fibers  20  of the individual measuring cells  10 , which cells are arranged at different test sites  30  so as to be able to detect tracer gas independently from each other at different test sites. The laser beam  17  will be coupled out from said light-conducting fiber  20  and, after passing through the metastable tracer gas, the laser beam will be incoupled into said light-conducting fiber  21 . All of said light-conducting fibers  21  are connected to a beam selector  33  which will selectively couple the light-conducting fibers  21  to a light-conducting fiber  34  connected to host unit  25 . There, an output coupling device  35  is arranged which is operative to feed the emergent light beam to a photodetector  37  connected to a processor  38 . 
         [0023]    Said laser  26  is controlled by a controller  40  which also has the function of a modulator for modulating the laser beam with the two frequencies of a frequency generator  41 . 
         [0024]      FIGS. 3 and 4  show representations of the amplitudes A of the spectra at the input and at the output of the entire light-conducting fiber path  50  including the measuring cell  10 . The input spectrum  51  includes a center frequency ω 0  which is the radiation frequency of the laser, and two sidebands which are defined by the modulation frequency. Also represented in  FIG. 3  is the nonlinear characteristic chromatic dispersion line  54  of the light-conducting fiber. In the resulting output spectrum  52 , the sidebands have different amplitudes. 
         [0025]    The representations in  FIGS. 3 and 4  are based on the cases where a frequency modulation FM of the laser radiation is performed. If the modulation frequency ω 1  is smaller than 1 MHz (in wavelength modulation), the chromatic dispersion is negligibly small for a length of several tens of kilometers. For high modulation frequencies (in the GHz range and above), the dispersion will lead to a FM/AM conversion which is not distinguishable from an absorption by metastable gas in the measuring cell. 
         [0026]    A solution is offered by the two-tone modulation (TTFM) as represented by  FIGS. 5 and 6 . In addition to a first modulation with the frequency ω 1  (e.g. 1 GHz), a second modulation is performed with a much lower second frequency Ω≦10 MHz. The first frequency ω 1  is situated in the GHz range, and the second frequency Ω in the MHz range. In  FIG. 5 , the input spectrum  51  is represented. The two modulation frequencies ω 1 ,Ω are generated and processed simultaneously, thus generating the side bands (ω 0 −w 1 )±½Ω and (ω 0 +ω 1 )±½Ω. 
         [0027]    In  FIG. 4 , the spectrum of the frequencies of the single-frequency-modulated laser (FM) with the amplitude A, is represented on the input side of the fiber path  50 , and the same frequencies with amplitude A a  effected by the fiber chromatic dispersion are represented on the output side. In two-tone frequency modulation according to  FIG. 6 , by contrast, the amplitude of the frequency Ω will not be significantly influenced along the length of the light conductor path, neither due to chromatic dispersion nor due to absorption. 
         [0028]    Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the true scope of the invention as defined by the claims that follow. It is therefore intended to include within the invention all such variations and modifications as fall within the scope of the appended claims and equivalents thereof.