Abstract:
The invention relates to a leak detector comprising a first sensor for detecting a gas component (helium) in a gas taken in. Because the sensor is susceptible to saturation or contamination, a second sensor is provided. The sensor is a thermal conductivity sensor. The thermal conductivity sensor has a lower detection sensitivity, yet, at a high concentration of the gas component, it does not risk being contaminated. The two sensors together allow for a large detection range, from extremely sensitive measurements to instances with high concentrations of the gas components as those which can occur with gross leaks.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a national phase entry under 35 U.S.C. §371 of International Application No. PCT/EP2011/065110, filed on Sep. 1, 2011, which claims priority from German Patent Application No. 10 2010 048 982.4 filed Oct. 20, 2010 and German Patent Application No. 10 2010 044 222.4 filed Sep. 3, 2010, all of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention refers to a leak detector for detecting at least one gas component in an aspirated gas. 
     2. Description of the Related Art 
     For a leak test using helium as the tracer gas, commercial apparatus generally detect the presence of helium by means of a mass spectrometer whose operation requires the generation of a high vacuum. Another detection method uses a membrane selectively permeable to helium, the membrane closing off a cavity in which a pressure sensor is located. Inside the cavity, a pressure is built that corresponds to the partial pressure of helium in the surrounding atmosphere. The membrane usually needs heating. Such a sensor is referred to as a partial pressure sensor in Wise Technology. It is manufactured by INFICON GmbH. Leak detectors with Wise Technology are described in DE 10 2005 021 909 A1 and in DE 10 2005 047 856 A1. Such sensors in the form of mass spectrometers or partial pressure sensors have a high sensitivity, but are susceptible to excessive concentrations of the tracer gas. The detection of helium by means of Wise Technology is limited to a partial pressure of 0.5 mbar. With gross leaks, the sensor must be protected against high concentrations. After a gross contamination, the device is blind for many seconds so that a user cannot continue the leak test. In particular, it is not possible to localize a gross leak site. When the sniffer probe reaches the vicinity of the gross leak, the system switches the sensor to the blind mode for protection. In a similar manner other sensor systems are susceptible to contamination or saturation. Also when a saturation limit is reached, concentration measurements are no longer possible. 
     Another sensor of the saturable gas-selective sensor type is the mass spectrometer. The mass spectrometer has a very high sensitivity and gas selectivity, but its operation requires a high vacuum, i.e. a very low value of the total pressure at its measuring input. If the total pressure rises above the allowable limit value, the mass spectrometer reaches the saturation range. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the invention to develop a leak detector of high sensitivity such that the measuring range is widened towards higher concentrations and higher total pressures, respectively. 
     A first variant of the leak detector according to the invention is defined in claim  1 . 
     According to the invention, a first sensor of the saturable gas-selective sensor type is provided. In addition, a second sensor of the thermal conductivity sensor type is provided. The first sensor serves to measure low concentrations and the second sensor takes over the measurement at higher concentrations at which the first sensor is no longer functional. 
     Leak detectors including a thermal conductivity sensor are known. Examples for such sensors are described in U.S. Pat. No. 3,020,746, U.S. Pat. No. 3,786,675 and JP 09292302 A. The sensors have a temperature-dependant resistor included in a measuring bridge and arranged in a gas or a gas flow. The gas flow or the gas dissipates heat from the resistor. The higher the density of the gas is, the greater the heat dissipation from the resistor. Generally, the heating power is measured that is necessary to maintain a constant resistor temperature. The magnitude of the thermal conductivity of the resistor is indicated thereby. When the gas flowing is air including helium, the density of the overall gas decreases as the helium portion increases. Thereby, also the thermal conductivity decreases. Thermal conductivity sensors have a low gas selectivity. However, they are not dependant on saturation or contamination levels and function even at high concentrations of a gas component in an ambient gas. However, the measuring sensitivity is limited. 
     The definition that the first sensor is a sensor of the saturable gas-selective sensor type should be understood such that the sensor does not provide a useful quantitative measuring result above a certain concentration or a certain partial pressure of the gas component. This includes a case of contamination. Contamination occurs with high concentration values. After a gross contamination, the detector system is blind for many seconds so that the user cannot continue the leak test. In particular it is not possible to localize a gross leak site. When the sniffer probe reaches the vicinity of the gross leak, the system switches the sensor to a blind mode for protection. Among the sensors of the saturable gas-selective sensor type are the following: mass spectrometers, a wise-sensor, metal-hydride-based sensors for the detection of H 2 , metal-oxide-based for the detection of H 2  (for the analysis of refrigerants or alcohol), sensors with dispersive or non-dispersive absorption. 
     Since the second sensor, being a thermal conductivity sensor, is not susceptible to excessive concentration values, it may be permanently active. With low concentrations, both sensor types are active, while the first sensor is switched to a blind mode when concentrations are elevated. Switching to the blind mode may be controlled both as a function of measured values of the first sensor and measured values of the second sensor. 
     Realizing the blind state of the first sensor can be achieved in various ways. One possibility is to cut off the gas flow to the first sensor. Another possibility is to switch the first sensor to a deactivated mode. With a wise sensor this is achieved by interrupting the heating current passing through the gas-selective membrane so that the membrane cools down and becomes less permeable. 
     Among other applications, the invention is useful in sniffer leak detectors where gas is aspirated into a handheld sniffer probe. The second sensor may be arranged at the sniffer probe or in a base apparatus with which the sniffer probe is connected via a flexible conduit. 
     Useful thermal conductivity sensors are, for example, the sensor AWM 2300 by Hamamatsu or the sensor TCS208F3 from the company Gerhard Wagner. 
     Using the leak detector of the present invention, it is also possible to detect refrigerants as they are used in air condition systems or refrigerators. Refrigerants have a lower conductivity than air and can therefore be differentiated from helium or hydrogen by the sign of the signal of a thermal conductivity sensor, which sign is the opposite of that of air. 
     A second variant of the leak detector according to the invention is defined in claim  7 . Here, a switching from the first sensor to the second sensor is controlled as a function of the total pressure. Such a leak detector is useful in connection with a first sensor that is total-pressure-sensitive. Here, the second sensor covers the range above a critical total pressure, while the first sensor is used for fine measurement. The second sensor may additionally be configured such that it simultaneously measures the total pressure. In this case, the second sensor can cause the switching of the first sensor to the blind mode. As an alternative, it is also possible to provide a total pressure gauge that is independent of both sensors and controls the same. 
     It is also possible to operate with both sensors at the same time, with the quantity measurement being effected by means of the thermal conductivity sensor and the quality measurement (evaluation of the gas type) is effected by means of a partial pressure sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following is a detailed description of embodiments of the invention made with reference to the drawings. 
       In the Figures: 
         FIG. 1  is a schematic illustration of a sniffer leak detector according to a first variant of the invention, 
         FIG. 2  is a schematic illustration of a leak detector with a test chamber, 
         FIG. 3  is a diagram showing the measuring ranges of the two sensor types, and 
         FIG. 4  is a schematic illustration of an embodiment according to a second variant of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The sniffer leak detector of  FIG. 1  is provided with a base apparatus  10  connected with a sniffer probe  12  via a valve V 2 . The sniffer probe  12  may be guided by hand in order to check the object under test for leaks from which gas escapes. 
     The base apparatus  10  comprises a vacuum pump  13  which, in the present example, is a two-stage pump with the pump stages  13   a  and  13   b  that are configured as membrane pumps. The vacuum pump generates a final pressure of about 3 mbar. 
     A vacuum conduit  14  leads from the vacuum pump  13  to the suction chamber  15 . The suction chamber  15  is formed upstream of the tracer gas sensor  16 . The walls of the suction chamber  15  adjoin the housing of the tracer gas sensor  16 . The sensor surface  17  of the tracer gas sensor  16  is enclosed by the suction chamber  15 . Within the suction chamber  15 , a gas guiding plate  18  is arranged facing the sensor surface  17  with a distance therebetween and being arrange in parallel with the same. The sniffer conduit  11  opens into the gas guiding chamber  19 . The same is provided with lateral openings  20  in opposite ends, through which gas can enter the suction chamber  15 . The gas guiding chamber  19  effects a distribution of the gas in front of the sensor surface  17 . 
     The tracer gas sensor  16  is configured in the same manner as the sensor described in DE 100 31 882 A1. The sensor surface  17  is formed by a membrane selectively permeable to helium and adapted to be heated electrically or by heat radiation. For the rest, the tracer gas sensor  16  includes a Penning pressure sensor or another pressure sensor generating an electric signal that indicates the pressure in the housing closed off by a quartz membrane. From this pressure, the signal for the detected quantity of tracer gas is derived. 
     The vacuum conduit  14  includes a first throttle D 1  between the vacuum pump  13  and the suction chamber  15 , which throttle determines the suction power for the normal mode of operation. The first throttle D 1  is bridged by means of a bypass conduit  26  that includes a valve V 1 . 
     A throttle D 2  is provided in an air inlet conduit. The valve V 3  connects either the inlet E 1  or the inlet E 2  with the outlet A. The inlet E 1  is connected with the first flow divider  30  which is connected with the inlet of the tracer gas sensor  16  via a conduit  31 . The conduit  31  includes a throttle D 4 . 
     Another path extends from the flow divider  30  via a throttle D 2  and a valve V 4  to the vacuum conduit  14 . The throttles D 2  and D 4  are adjusted to each other such that the flow through D 2  is much larger than the flow through D 4 . The flow through D 2  is at least 10 times the flow through D 4  and in particular at least 50 times. Preferably, the flow through D 2  is about one hundred times the flow through D 4 . 
     The sniffer conduit  11  leading from the sniffer probe  12  to the base apparatus  10  includes a measuring conduit  35  connecting the sniffer probe with the valve V 2  and an intake conduit  36  connected with the inlet of the vacuum pump  13  via a valve V 5 . The intake conduit  36  has a much greater suction capacity than the measuring conduit  35 . For example, the flow rate of the gas taken in through the measuring conduit is 300 sccm, whereas the flow arte through the intake conduit  36  is 2700 sccm. The intake conduit  36  serves to increase the distance sensitivity of the sniffer leak detector by taking in much more gas as in the case of the measuring conduit. The measuring sensitivity is increased by deactivating the intake conduit. 
     According to the invention, a second sensor  38  is provided in addition to the first sensor  16  which is configured as a Wise Technology sensor, the second sensor being a thermal conductivity sensor. The second sensor  38  is preferably arranged in the sniffer probe  12  and in particular in the intake conduit  36  thereof. It may also be situated in the base apparatus  10  at a position  38   a . In any case, it is advantageous to arrange the second sensor at a site where a high total pressure prevails, since this is where the partial pressure of the gas component of interest is the highest and thus the detection limit is most favorable. Another possibility for the positioning of the second sensor exists at the outlet of the vacuum pump  13 . Here, it would be unfavorable, however that the signal from the second sensor would occur later in time than the signal from the first sensor. Preferably, the signal from the thermal conductivity sensor should be available earlier than the signal from the first sensor. 
     The signals from the first sensor  16  and from the second sensor  38  are supplied to a control means  40  switching the first sensor  16  to the blind mode via a control line  41 , when the first sensor or the second sensor measures a concentration above a limit value. Thereby, it is prevented that the first sensor becomes saturated or exceeds the contamination limit, respectively. 
     The embodiment shown in  FIG. 2  is a leak detector which is provided with a vacuum-tight test chamber  50  in which a test object  51  is placed. The test object  51  is filled with a tracer gas  52 . The test chamber  50  is evacuated so that in case of a leak in the test object  51 , tracer gas leaks from the test object. A vacuum pump  53  is connected to the test chamber  50  via an intake conduit  57 . The intake conduit  57  includes a first sensor  16  and a second sensor  38 . The first sensor may be a Wise Technology sensor, for example, while the second sensor is a thermal conductivity sensor. In the flow path of the gas taken in, the second sensor  38  is arranged upstream of the first sensor  16 . The first sensor  16  is bridged by a bypass conduit  54  that can be opened and closed by the valves  56 ,  56   a.    
     The valve  56  is controlled by a control device as a function of the signal from the second sensor  38 . If the tracer gas concentration measured by the second sensor exceeds a limit value, the valves  56  and  56   a  are switched over such that the first sensor  16  is bridged by means of the bypass conduit  54 . Thereby, the first sensor is protected from contamination. 
       FIG. 3  illustrates an example for the measurement ranges of the first sensor and the second sensor using the gas component helium as an example. The helium concentration is plotted along the abscissa. It can be seen that the measurement range MB 1  of the first sensor ranges from less than 1E-05% (=10 −4  mbar) to slightly above 1E-01% (=1 mbar), whereas the measurement range MB 2  of the thermal conductivity sensor covers the entire range above 1E-02%. Thus, both sensors complement each other. 
       FIG. 4  illustrates an embodiment according to the second variant of the invention, in which a first sensor  16   a  is provided whose function depends on the total pressure at its measuring inlet  60 , e.g. a mass spectrometer. The measuring inlet  60  is connected to a vacuum pump  13  including, arranged in succession, a high vacuum pump  13   a , for example a turbomolecular pump, and a rough vacuum pump  13   b . An intake  62  of the high vacuum pump  13   a  is connected with an inlet conduit  64  via a valve V 2 , the conduit comprising a connection  65  for connecting a test object  66 . The test object  66  is a hollow body to be tested for tightness. In the present embodiment, an atmosphere of a tracer gas  68  is created outside the test object using a spraying apparatus  67 . The tracer gas can be identified by the two sensors included in the leak detector. When tracer gas is identified, the test object  66  has a leak through which tracer gas  68  has entered. 
     The inlet conduit  64  is further connected to a connecting conduit  70  that connects the two vacuum pumps  13   a  and  13   b.    
     A valve V 2  is connected between the intake  62  and the inlet conduit  64 , the valve being controlled in dependence on the total pressure and being switched to a closing mode when the total pressure rises above a limit value. When the valve V 2  is closed, the first sensor  16   a  is separated from the test object  66  so that the sensor is switched to the blind mode. 
     The second sensor  38  is connected to the inlet conduit  64 , the sensor being a thermal conductivity sensor. This thermal conductivity sensor is designed such that, at higher pressures, it operates independent of the total pressure. With lower total pressures prevailing at the measuring inlet  60  both sensor types are active, whereas at higher total pressures, the first sensor  16   a  is switched to the blind mode by closing the valve V 2 . In the present embodiment, the total pressure is measured at the inlet conduit  64  using the second sensor  38 . Alternatively, it could also be effected at the measuring inlet  60  of the first sensor. It is also possible to use a separate apparatus for the measurement of the total pressure.