Abstract:
Method for operating, a helium leak indicator ( 1 ) consisting of a helium detector ( 4 ) with evacuation devices comprising at least a high-vacuum pump ( 6 ), a backing pump ( 9 ) and a valve ( 16 ) whose level of flow conductance can be adjusted and serves to modulate the through-flow of test gas to be analyzed; in order to influence the sensitivity in a simple manner, the method provides that the modulation properties be changeable in such a manner that the effective pumping capacity at the inlet of the leak indicator is changed by the adjustment of the modulation properties.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for operating a helium leak indicator having the characteristics of patent claim 1. Moreover, the present invention relates to a helium leak indicator suited for the implementation of this method. 
     During helium leak searching, the detection limit may be improved by reducing the pumping capacity of the vacuum pumps. For the purpose of reducing the pumping capacity it is known to use constrictions which may be switched on, or to disconnect the backing pump. In both instances the pumping capacity is reduced by increments and with a likewise increase in sensitivity. The price to pay for such an increase in sensitivity are longer measurement times, since these increase when reducing the pumping capacity of the vacuum pumps. 
     SUMMARY OF THE INVENTION 
     It is the task of the present invention to simplify, within the scope of a method for operating a helium leak indicator of the kind affected here, the way in which the pumping capacity is influenced and thus the way in which the detection limit of the leak measurements is attained, in order to perform high sensitivity measurements within an optimally short time. Moreover, an infinitely variable modification of the detection limit shall become possible. 
     According to the present invention these targets are attained through the characteristic features of the patent claims. 
     The present invention allows, through a modification of the modulation properties of the gas flow in which the presence of the test gas is to be detected, to influence the effective pumping capacity of the vacuum pumps, and thus the sensitivity of the leak search. Thus it becomes possible to the user to adapt the detection limit to the particular application in each case by setting up certain modulation properties. This adaptation allows the use of optimally short measurement times in each case. Since according to the present invention the pumping capacity is influenced with the aid of an inlet valve which can be modulated, any special measures for modifying the pumping capacity at the vacuum pumps themselves are no longer required. 
     An expedient operating method which has become possible through the present invention is, that the detection limit may be modified automatically down to a lower limit while the leak measurement is in progress. As the manipulated variable the signal supplied by the mass spectrometer may be employed, the magnitude of which depends, in the case of a not yet registered leak, on the helium background in the gas flow which is to be analysed, and which reduces in magnitude with increasing measurement time. As long as the signal magnitude does not drop below a given level, the modulation properties are not modified, and a relatively high pumping capacity is maintained. As soon as the magnitude of the signal drops below a given threshold, the pumping capacity is reduced by modifying the modulation properties and thus sensitivity is increased. Each leak determined and measured in this manner may therefore be determined within an optimally short measurement time. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Further advantages and details of the present invention shall be explained with reference to drawing FIGS. 1 to  4 . Depicted in 
     drawing FIGS. 1 and 2 are schematically represented design examples for a helium leak indicator according to the present invention and 
     drawing FIGS. 3 and 4 are diagrams explaining the modulation properties. 
    
    
     DESCRIPTION OF THE INVENTION 
     In the helium leak indicator  1  according to drawing FIG. 1 the inlet is designated as  2 , the line connected to inlet  2  as  3 , and the test gas detector designed as a mass spectrometer as  4 . High vacuum pump  6  (preferably a turbomolecular pump) is connected via line  5  to the inlet of the mass spectrometer  4 . Connected to the discharge of the high vacuum pump  6  is line  7  with valve  8 , said line being connected to the inlet of the backing pump  9 . Connected to line  3  are line sections  11 ,  12  with valves  13 ,  14 . Via lines  11 ,  12  the gas entering through the inlet  2  passes either directly (line  11 ) or indirectly (line  12 ) into the mass spectrometer  4 . The path for the gas is selected with the aid of valves  13 ,  14 . If valves  8  and  13  are closed and when valve  14  is open the backing pump  9  may serve the purpose of evacuating the test sample or the test chamber (not shown), which is linked to inlet  2  before the leak search commences. 
     In order to ensure operation of the helium leak indicator  1  in accordance with the present invention, a component (valve  16 ) allowing the gas flow passing through it to be modulated is located in line  3 . The cross section of the opening or the flow conductance of the valve  16  is variable. Details on modulations of this kind are basically known from DE-A-44 08 877. 
     A controller  21  is provided for actuating the valve  16 , this valve preferably being of the proportional action type. Part of this controller is a pulse generator. The flow conductance of the valve  16  changes in correspondence with the selected shape of the pulses. The shape of the pulses is adjustable in such a manner, so that on the one hand, the desired modulation of the gas passing through the valve and on the other hand the mean opening time or the mean conductance of the valve  16 , can be influenced. Design examples for this are explained with reference to drawings FIGS. 3 and 4. 
     Moreover, the signal supplied by the controller  21  is also supplied as the reference signal to an amplifier  23 . Amplifier  23  serves the purpose of processing the signals supplied by the mass spectrometer. After amplification and phase-sensitive rectification in accordance with the LOCK-IN principle, the signal is displayed. The unit displaying the signal is represented by block  24 . 
     The design example according to drawing FIG. 2 substantially corresponds to the design example according to drawing FIG.  1 . Differences or supplements are: 
     The high vacuum pump  6  is a two-stage friction vacuum pump having on its high vacuum side a turbomolecular pumping stage  6 ′ and a screw pumping stage  6 ″ on the forevacuum side. 
     Located between said pumping stages is an intermediate connection  31  to which line  11  is connected. 
     Arranged instead of valve  13  in line  11  is valve  16 . 
     The backing pump  9  is a diaphragm vacuum pump. 
     A pressure gauge  32 , the signals of which are supplied to the controller  21 , is provided for the purpose of driving the valve  16  during the pumpdown process and for indirect monitoring of the pressure in mass spectrometer  4 . 
     A further pressure gauge  33 , the signals of which are also supplied to controller  21 , is provided to monitor the pressure in the area of the inlet  2  of the helium leak indicator  1 . 
     Depicted in drawing FIGS. 3 and 4 are possibilities for the selection of modulations having such properties, that through their presence influence can be taken on the mean conductance of the valve  16 . Drawn into each of the drawing FIGS. 3 a  to  3   c  and  4   a  to  4   c  are two horizontal lines which correspond to the valve status “open” (line  35 ) and “closed” (line  36 ). 
     In the case of the modulations in accordance with drawing FIGS. 3 a  to  3   c , the pulse generator supplies a periodically varying signal of constant frequency and constant amplitude. The amplitude corresponds approximately to half of the distance between the lines  35  and  36 . The position of the curves  37  which are identical in shape differs in drawing FIGS. 3 a  to  3   c . In the case of the modulation properties according to drawing FIG. 3 a , the cross section of the opening of valve  16  changes periodically between half open and fully open. In the case of the modulation properties according to drawing FIG. 3 c , the cross section of the opening changes periodically between closed and half open. In the case of the modulation properties according to drawing FIG. 3 b , the cross section of the opening of valve  16  changes periodically between approximately one quarter open and three quarters open. It is apparent that in the states as depicted in drawing FIGS. 3 a  and  3   c  the mean cross section (averaged over time) of valve  16  differs. In the case of the modulation according to drawing FIG. 3 a  the mean cross section is relatively wide so that the effective pumping capacity of the vacuum pump (s) is relatively high. From this there results a relatively low sensitivity for the leak searching process. By modifying the modulation properties in accordance with drawing FIG. 3 b  (moderate pumping capacity) and  3   c  (low pumping capacity) the sensitivity of the leak searching process is increased. In the case of the modulation properties corresponding to the curves  38 ,  39 ,  40  depicted in drawing FIGS. 4 a  to  4   c , the cross section of the opening of valve  16  changes continuously between open and closed. In the case of the modulation according to drawing FIG. 4 b , the modulation is periodic so that a moderate pumping capacity and thus a moderate sensitivity results. In the case of the modulation according to drawing FIG. 4 a , the oscillations in the “open” direction are longer compared to the oscillations in the “closed” direction. The mean time during which the valve  16  is open, and thus the pumping capacity is relatively high, sensitivity is low. In the case of the modulation according to drawing FIG. 4 c , the timing characteristic is reversed, this means that the mean time during which the valve is open is relatively short and thus the pumping capacity is low, the sensitivity of the leak search using these modulation properties is high. 
     Depicted in drawing FIGS. 3 and 4 are only two types of pluses of many through which the desired targets may be attained. It is important that the modulation properties be selected through electronic means, so that on the one hand the inflowing gas which is to be analysed with respect to the test gas is modulated, and that on the other hand the mean cross section of the opening of valve  16  be variable infinitely or in steps (and thus the pumping capacity of the vacuum pumps effective at the inlet). In order to operate a helium leak indicator of the kind according to the present invention, a calibration curve is determined. It gives the relative sensitivity as a function of effective pumping capacity. The calibration curve is employed for establishing the correct measurement signal. This is necessary, since at a certain leak rate there result differing measured values which depend on the modulation properties. 
     For implementing a leak detection method with a leak indicator according to drawing FIG. 1, the test sample which is to be inspected for the presence of leaks is first connected at inlet  2 , and then evacuated by the backing pump  9  with valves  14  and  16  open and with valves  8 ,  13  closed. In a manner which is basically known, this may be followed by the detection of gross leaks which becomes possible as soon as the pressure has reached such a value that the valve  8  may be opened. If the test sample does not exhibit a gross leak, valve  14  is closed, valve  13  is opened and the inflowing gas is modulated with the aid of valve  16 . The modulation properties are selected either once so that they correspond to the desired sensitivity. Immediately after valves  13 ,  14  have been switched over, the leak search then commences at the given sensitivity. 
     An other possibility exists by continuously increasing the sensitivity of the leak search with increasing measurement time. This is performed by initially selecting the modulation properties in such a manner that a high pumping capacity and a low sensitivity are employed. Thereafter the modulation properties are varied infinitely, for example, in such a manner that a transition is performed from a relatively high pumping capacity with low sensitivity, to a low pumping capacity and with a high sensitivity. If at some time between gross leak detection, fine leak detection with low sensitivity and fine leak detection with high sensitivity, the presence of the test gas is recorded, then the test sample has a leak. The leak search is interrupted. The searching process for the leak has taken place within an optimally short period of time. 
     In case the gas flow passing through the valve  16  exhibits a helium background, the signal supplied by the mass spectrometer  4  may be employed as the manipulated variable for infinitely varying the modulation properties. In the case of a leak-tight test sample, the signals provided by the mass spectrometer  4  correspond to the helium background in the gas flow to be analysed. With increasing measurement time the magnitude of the signals decreases. As long as the signal does not drop below a given signal level, the modulation properties are not changed so that an initially high effective pumping capacity is maintained. Only after the signal has dropped below a given level, are the modulation properties changed in such a manner that the effective pumping capacity is reduced thereby increasing sensitivity. In the case of a leak-tight test sample, the leak search is terminated after the desired sensitivity has been attained. If in the course of the leak searching process it is determined that the test sample has a leak, this leak search is always performed within an optimally short time. 
     In the design example according to drawing FIG. 2, the valve  16  is located in the line  11  which is linked to intermediate connection  31 . This arrangement allows for early throttled opening of valve  16  in order to support, with the aid of the screw pump  6 ″ the relatively weak pumping capacity of the diaphragm pump  9  at the ultimate pressure level. Moreover, the line  3  does not contain the valve  16  so that the valve will not impair the evacuation of the test sample via line  3  at the beginning of the leak search. 
     A pressure gauge  32  is connected to line  7 . It delivers its measurement signals to the controller  21  which controls the valve  16  among others. Thus too early or too wide opening of valve  16  may be prevented, which would cause the pressure in the mass spectrometer to rise above its operating pressure. A pressure gauge fulfilling the same purpose may also be connected to intermediate connection  31 . 
     The pressure in the area of inlet  2  of the helium leak indicator  1  is also monitored by the controller  21 . The pressure gauge  33  arranged at this point supplies its signals to the controller  21 . The inlet pressure must not rise above a certain threshold. This could happen, if the pumping capacity were reduced too quickly with the aid of valve  16  for the purpose of increasing the sensitivity.