Device and method for distinguishing a test gas escaping from a leak from interfering gas

Abstract

A method for distinguishing a test gas escaping from a leak in a test object (21) from an interfering gas in the environment of the test object (21) during sniffing leak detection, having the steps: suctioning gas from the environment of the test object (21) in the region of the outer surface of the test object by means of a sniffing tip, which has a suction opening (14), which is connected, for gas conduction, to a sensor (18), which is designed to determine the test gas partial pressure of the test gas in the suctioned gas flow; varying, with periodic repetition, the flow intensity of the suctioned gas flow; setting a total pressure of the suctioned gas at the sensor (18) of at least 80 percent of the total pressure of the gas in the atmosphere (23) surrounding the test object (21); avoiding fluctuations of the total pressure of the suctioned gas at the sensor (18) of more than 10 percent; measuring the test gas partial pressure of test gas contained in the suctioned gas flow by means of the sensor (18); indicating that the test object (21) has a leak if the measured test gas partial pressure has a varying component, the average amplitude of which lies above a threshold value and which follows the variation of the suctioned gas flow.

Claims

1. A method for distinguishing a test gas escaping from a leak in a test object from an interfering gas in the environment of the test object during sniffing leak detection, having the steps: suctioning gas from the environment of the test object in the region of the outer surface of the test object by means of a sniffing tip, which sniffing tip has a suction opening, which suction opening is connected, for gas conduction, to a sensor, which sensor is designed to determine the test gas partial pressure of the test gas in the suctioned gas flow; varying, with periodic repetition, the flow intensity of the suctioned gas flow; setting a total pressure of the suctioned gas at the sensor of at least 80 percent of the total pressure of the gas in the atmosphere surrounding the test object; avoiding fluctuations of the total pressure of the suctioned gas at the sensor of more than 10 percent; measuring the test gas partial pressure of test gas contained in the suctioned gas flow by means of the sensor; and indicating that the test object has a leak if the measured test gas partial pressure has a varying component, the mean amplitude of which is greater than a threshold value and which follows the variation of the suctioned gas flow.

2. The method according to claim 1, characterized in that an indication is provided that there is no leak if the measured test gas component does not have a component exceeding the threshold value.

3. The method according to claim 1, characterized in that the varying, periodically repeated, of the flow intensity of the suctioned gas occurs in the form of a modulation with a modulation frequency in the range of 1 Hz to 20 Hz.

4. The method according to claim 1, characterized in that the total pressure of the suctioned gas flow at the sensor is set to a value in the range between 90 percent and 110 percent of the total pressure in the test object atmosphere.

5. The method according to claim 1, characterized in that modulated flow intensity signal of the suctioned gas flow is demodulated according to the principle of a lock-in amplifier with a defined frequency reference and phase reference for modulating the suctioned gas flow.

6. The method according to claim 1, characterized in that in addition there is a comparison measurement of the test gas partial pressure without varying the flow intensity of the suctioned gas flow for comparison purposes.

7. The method according to claim 1, characterized in that the total pressure in the atmosphere surrounding the test object in the region of the sniffer is atmospheric pressure in the range of approx. 900 mbar to approx. 1100 mbar.

8. A sniffing leak detector having: a sniffer having a suction opening; a gas pump; a sensor determining the test gas partial pressure of the test gas to be detected; a gas line path connecting the suction opening, the sensor, and the gas pump; a control device designed to repeatedly vary the flow intensity of the suctioned gas flow, to set the total pressure of the suctioned gas flow at the sensor to at least about 80 percent of the total pressure of the gas in the atmosphere surrounding the test object, and to avoid fluctuations in the total pressure of the gas at the sensor of more than 10 percent; and, an evaluation device designed to determine whether the test gas partial pressure of test gas contained in the suctioned gas flow has a varying component, the mean amplitude of which is greater than a threshold value and which follows the variation of the suctioned gas flow.

9. The sniffing leak detector according to claim 8, characterized in that the sensor is arranged downstream of the gas pump.

10. The sniffing leak detector according to claim 8, characterized in that the gas line path has a throttle between suction opening and sensor.

11. The sniffing leak detector according to claim 10, characterized in that the throttle is a capillary tube having a length in the range of approx. 2 cm to approx. 100 cm and having a diameter of a maximum of approx. 5 mm.

Description

BRIEF DESCRIPTION OF FIGURES

(1) Exemplary embodiments of the invention shall be explained in greater detail in the following using the figures.

(2) FIG. 1 is a schematic illustration of a first exemplary embodiment;

(3) FIG. 2 is a schematic illustration of a second exemplary embodiment;

(4) FIG. 3 is a schematic illustration of a third exemplary embodiment;

(5) FIG. 4 illustrates the curve of the suctioned gas flow over the pressure at the sensor for various diameters of the flow path;

(6) FIG. 5 is a detail from FIG. 4;

(7) FIG. 6 illustrates a fourth exemplary embodiment;

(8) FIG. 7 illustrates a fifth exemplary embodiment; and,

(9) FIG. 8 illustrates a sixth exemplary embodiment.

DETAILED DESCRIPTION OF THE INVENTION

(10) The sniffing leak detectors 10 in the three exemplary embodiments in FIGS. 1 through 3 are each connected in the conventional manner via a gas line path 20 to a sniffer 12 having a suction opening 14. Arranged along the gas line path 20 is a gas pump 16 that produces the gas pressure required for suctioning the gas from the atmosphere 23 surrounding the test object 21.

(11) The gas line path 20, in the form of a conventional gas line, furthermore, connects the pump 16 to a sensor 18 arranged immediately downstream of the pump 16. The sensor 18 is designed for measuring the partial pressure of the test gas in the suctioned gas flow. The sensor 18 may be an infrared absorption cuvette, for example. What is important is that the sensor 18 is designed to determine the test object partial pressure at nearly atmospheric pressure or at about 90-110 percent of atmospheric pressure. The test gas partial pressure is the component of the test gas in the gas mixture of the suctioned gas flow. The partial pressure of the test gas thus cannot be measured with a pressure sensor. A pressure sensor merely measures the total pressure of a gas mixture.

(12) Once the suctioned gas flow has flowed through the sensor 18, the gas line path 20 leads the gas flow out to the atmosphere via an outlet 36.

(13) The gas line path 20 can have a throttle 26. As illustrated in FIG. 1, the throttle 26 can be arranged upstream of the gas pump 16. A control device 22 is electronically connected thereto for controlling the gas pump 16. For example, the control device 22 can be designed for controlling the speed of the gas pump 16. FIG. 1 illustrates that the control device 22 can also be connected to the throttle 26 in order to change the admittance of the throttle 26. In addition, the control device can also be electronically connected to the sensor 18.

(14) An evaluation device 24 is electronically connected to the sensor 18 in order to process and evaluate the measurement signal. The evaluation device 24 is designed to determine whether the test gas partial pressure of the test gas contained in the suctioned gas flow has a varying component. The evaluation device 24 can in particular test whether the varying component of the test gas partial pressure has a mean amplitude that is greater than a threshold value. In addition, the evaluation device 24 can determine whether the varying component of the test gas partial pressure follows the variation of the suctioned gas flow. This is the case if the frequency of the varying test gas partial pressure component corresponds to the frequency of the varying gas flow or to a multiple of this frequency.

(15) To this end, the evaluation device 24 can be connected to the control device 22. The control device 22 varies, for example, the flow intensity of the suctioned gas flow in that the pump speed is varied. This can be accomplished in the form of a modulation, for example according to the principle of the lock-in amplifier. The evaluation device 24 can perform a comparison of the frequency of a varying test gas partial pressure to the modulation frequency of the suctioned gas flow.

(16) The evaluation device 24 is also designed to determine in the framework of a calibration, the leakage flow of a known leak with a known leak rate.

(17) The control device 22 in the first exemplary embodiment is also designed to set the total pressure of the suctioned gas flow in the region of the sensor 18 to at least about 90-110 percent of the total pressure of the gas in the atmosphere 23 surrounding the test object 21. As will be explained in the following with reference to FIG. 5, the relationship between gas flow and gas pressure in this pressure range is approximately linear. The total pressure of the suctioned gas flow at the sensor 18 can be set by controlling the speed of the gas pump 16 and/or by controlling the admittance of the throttle 26.

(18) The exemplary embodiments relate to sensors arranged directly downstream of the gas pump 16. With this arrangement, fluctuations in the total pressure of the gas at the sensor 18 are reduced. Alternatively, however, it is also possible to arrange the sensor 18 upstream of the gas pump 16, that, is between sniffer 12 and gas pump 16.

(19) The exemplary embodiment in FIG. 2 is distinguished from the exemplary embodiment in FIG. 1 in that a controllable valve 28 that can be controlled via the control device 22 is provided upstream of the gas pump 16 in order to change the cross-section of the line in the gas line path 20. The controllable valve 28 is preferably arranged between the throttle 26 and the gas pump 16. By changing the cross-section of the gas line path 20 using the controllable valve 28 it is possible to change, and in particular vary, the admittance of the gas line path 20. The flow intensity of the suctioned gas flow is varied repeatedly in this way in the second embodiment.

(20) The third exemplary embodiment is distinguished from the second exemplary embodiment in that a bypass 30 bridges the gas line path 20 between the sniffer 12 and the gas pump 16 and in particular the throttle 26. The bypass 30 is provided with a throttle 34, the admittance of which is much greater than the admittance of the throttle 26. The bypass line 30 has a controllable valve 32 that, for controlling the latter, is electronically connected to the control device 22. When the admittance of the valve 32 is increased, the gas flow in the bridged gas line path 20 is reduced. When the admittance of the valve 32 is reduced, the gas flow in the bridged gas line path 20 is increased. In this way the flow intensity of the suctioned gas flow may be varied using the control device 22 and the controlled valve 32 in the bypass line 30.

(21) The throttle 26 may be a capillary tube having a length in the range of approx. 2 cm to approx. 10 cm and a diameter of a maximum of about 5 mm. In FIGS. 4 and 5, the resulting gas flow is plotted in sccn (standard cubic centimeters per minute, cm.sup.3/min) on the vertical axis (ordinate) over the pressure in mbar (millibars) on the horizontal axis (abscissa) for various diameters of the throttle 26 embodied as a capillary tube. In the case of pressure P.sub.2, plotted on the horizontal axis, the pressure P.sub.2 is inside the gas line path 20 downstream of the gas pump 16 in the region of the sensor 18. The environmental pressure in the environment 23 of the test object 21 is 1013 mbar (atmospheric pressure). Atmospheric pressure shall be understood in this case to be a pressure that can be in the range of approx. 900 mbar to approx. 1100.

(22) FIG. 4 illustrates the curve for self-setting gas flows for various diameters d of the capillary tube for the throttle 26 in the range between 0 mbar and 1000 mbar. The length of the capillary tube is 5 cm. FIG. 5 illustrates the curves according to FIG. 4 in the pressure range between 950 and 1015 mbar. It may be seen from FIG. 5 that the relationship between gas flow and gas pressure is approximately linear when the pressure is at least 950 mbar. It is therefore advantageous according to the invention when the total pressure of the suctioned gas flow at the sensor 18 is set to a value in the range between about 90% and 110% of the total pressure in the environment of the test object 21. It is basically particularly important that the total change in pressure is negligible and thus causes a major change in flow.

(23) Due to a slight change in the low pressure at the sensor, for instance 985 mbar to 1000 mbar of a capillary length of 5 cm and a diameter of 3 mm, the flow changes by a factor 2 of 100 sccn to 50 sccn. This aspect is distinguished from the applications in the vacuum region as are described, for example, in DE 4408877 A/EP 7050738 B1. If the pressure P.sub.2 at the site of the sensor 18 is very low, as is the case with vacuum leak detectors, for example, a change in pressure of, for example, 0.1 mbar to 50 mbar has only a minor effect on the gas flow.

(24) A typical leak in the test object 21 can cause a leakage gas flow of 1.Math.10.sup.4 mbar.Math.1/s. The flow or flow intensity of the suctioned gas flow in the range between 120 sccm and 12 sccm is modulated with a modulation frequency of 6 Hz. With the modulation frequency, the total pressure fluctuates between 1000 mbar and 950 mbar. The environmental concentration c.sub.0 can be 400 ppm. The total pressure fluctuation of 50 mbar is relatively high. Nevertheless, the partial pressure fluctuation caused by the total pressure fluctuation is low and thus is negligible in comparison to the varying component of the partial pressure that results from the flow modulation. In practice, the fluctuation in the total pressure is even much lower than 50 mbar.

(25) The exemplary embodiment according to FIG. 6 is distinguished from the exemplary embodiment according to FIG. 1 in that the gas pump 16 is not arranged between the throttle 26 and the sensor 18 in the gas line path 20, but instead is arranged in the gas line path 20 between the sniffer 12 and the throttle 26, that is, upstream of the throttle 26.

(26) The exemplary embodiment in FIG. 7 is distinguished from the exemplary embodiment in FIG. 2 in that the gas pump 16 is not arranged between the valve 28 and the sensor 18, but rather, is upstream of the throttle 26, as in the exemplary embodiment in FIG. 6.

(27) The same is true of the exemplary embodiment according to FIG. 8, in which the gas pump 16 is not arranged between the parallel circuit of throttle 26 and valve 32 and the sensor 18, but rather in the gas line path 20 upstream of the parallel circuit of throttle 26 and 34.