Patent Application
20050066708
The present invention relates to a test gas leakage detector having the characteristics of patent claim 1.
Leakage detectors of this kind are also termed counterflow leakage detectors since the test gas entering into the high vacuum pump system (one or several series connected high vacuum pumping stage(s), preferably friction pumping stages) through the intermediate inlet, flows upstream against the pumping action of the pump system towards the test gas detector. Leakage detection is performed such that the inlet of the instrument is connected to a unit under test, for example, being sprayed from the outside with test gas. In the instance of a leakage being present, the test gas penetrates into the unit under test and is detected with the aid of the test gas detector. To the inlet of the leakage detector also a test chamber may be connected in which one or several test gas containing units under test are accommodated. During the leakage detection process and in the instance of one or of several objects under test being leaky, test gas enters into the test chamber and is detected with the aid of the test gas detector. The vacuum leakage detection process detailed, may be performed qualitatively or quantitatively. As the test gas, helium has been found well-proven.
From U.S. Pat. No. 4,472,962 a leakage detector of the here affected kind is known. A test chamber is connected to its inlet. Evacuation of the test chamber is performed through the fore-vacuum pump which is necessary for operating the high vacuum pump system being implemented by way of a turbomolecular vacuum pump. During the leakage detection process, the gas which is to be analysed for the presence of the test gas is supplied to an intermediate inlet at the turbomolecular pump.
It is the task of the present invention to improve a leakage detector according to the state-of-the-art regarding speed and sensitivity of leakage detection.
This task is solved through the present invention through the characterising features of the patent claims. Through the utilisation of a further high vacuum pump (booster pump) it is achieved that the test chamber connected to the inlet of the leakage detector, respectively the unit under test connected to the inlet, can be tested at shorter response times, since the response time is dependent on the time constant τ=V/SHe. V=volume of the unit under test, SHe=pumping capacity of the booster pump for He. In spite of the high pumping capacity of the booster pump (11), detection sensitivity is maintained at a high level since it is only dependent on the high vacuum pump system and the design of its counterflow stages. Through the utilisation of a relatively large fore-vacuum pump the evacuation time is short, which in addition is achieved through a relatively high switchover pressure upon switching over to the booster pump.
Surprisingly it has been demonstrated that not only the sensitive, but also the prior gross and fine leakage detection steps can be performed with an open connection between the outlet of a further high vacuum pump and the intermediate inlet at the high vacuum pump system. For this reason this connection can be held open permanently. The installation of valves can be dispensed with. Continuous measurement operations ranging from gross to high sensitivity leakage detection are possible.
Further advantages and details of the present invention shall be explained with reference to an example of an embodiment for a leakage detector according to the present invention depicted in the drawing figure.
The drawing figure schematically depicts the components of a leakage detection instrument 1, the inlet of which is designated as 2. Connected to the inlet are two line sections 3 and 4. Line section 3 is equipped with a valve 5 and is linked through the line section 6 to the inlet 7 of the fore-vacuum pump 8. Located in the line 4 is a valve 9 and a high vacuum pump 11 (booster pump), the outlet 12 of which is connected through a constantly open line section 13 to an intermediate inlet 14 at a second high vacuum pump 15. The outlet 12 of the high vacuum pump 11 is, moreover, connected via line sections 16—with valve 17- and 6 to the inlet 7 of the fore-vacuum pump 8.
The high vacuum pump 15 is the leakage detection pump into which the test gas flows—at least section-wise—against the direction of its pumping action. Connected at its inlet 21 is the test gas detector 22—when employing helium as the test gas, preferably a mass spectrometer. The outlet of said high vacuum pump 23 is connected via the line section 24 to the inlet 7 of the fore-vacuum pump 8. In line section 24 there is present—in the direction of the flow—a throttle 25 and a valve 26, which may also be designed by way of a single component (valve of a small nominal width, for example).
Two pressure gauges 27 and 28 supply information on the inlet pressure (gauge 27) and the pressure at the intermediate inlet 14 (gauge 28). With the aid of the supplied pressure signals, leakage detection operation is controlled. A controller and the corresponding lines are not depicted.
After connecting a unit under test to the inlet 2, the unit under test is initially pre-evacuated. During this phase all valves—with the exception of valve 5—are closed. Already after a short period of time (inlet pressure a few mbar, depending on the properties of the pump 15) the valve 26 may be opened. Should be unit under test exhibit a gross leak, test gas passes through valve 26, throttle 25 and high vacuum pump 15 (in a counterflow) to the detector 22 and is recorded.
The transition to fine leakage detection is performed in that valve 5 is closed and the valves 9 and 17 are opened. This step may already be performed at that point of time when the inlet pressure attains approximately the level which is necessary at the intermediate inlet (14) so as to just maintain the operating pressure in the test gas detector (mass spectrometer). In this state, the pumping capacity of the booster pump 11 is low, thus not effecting any detrimental pressure increase at the intermediate inlet. Moreover, during this phase the pumping capacity of the fore-vacuum pump 8 is still higher than the pumping capacity of the booster pump 11.
Should the unit under test exhibit smaller leaks, then these are detected (fine leakage detection).
With a progressing pressure decrease at inlet 2, the pumping capacity of the booster pump 11 increases, the unit under test is evacuated relatively rapidly to lower pressures. When the inlet pressure attains a level of 10−2 mbar, for example, high sensitivity leakage detection commences in that the valve 17 is closed. Test gas escaping through a possibly still present very small leakage in the unit under test enters entirely into the intermediate inlet 14 and is recorded by the mass spectrometer 22.
As a fore-vacuum pump 8, oil-sealed or dry (oil-free) pumps may be employed, expediently at pumping capacities≧16 m3/h (i.e., relatively “large” pumps), due to the relatively large volumes of unit under test or test gas chamber. Throttle 25 has the effect of ensuring that the sensitivity for the leakage detection, dependent on KTMP×SVVP (KTMP=compression of the leakage detection pump, SVVP=pumping capacity of the fore-pump) does not diminish to a level being too low, although the fore-vacuum pump is “large”.
The booster turbomolecular pump 11 should be selected such that it offers already at a high inlet pressure a significant pumping capacity. For the leakage detection pump 15 it is required, that it offers within the lower range a relatively high compression so as to isolate the unstable fore-vacuum pump. So-called compound pumps, i.e. friction vacuum pumps with a turbomolecular pumping stage on the intake side and a molecular pumping stage on the delivery side offer these properties.
An embodiment according to the drawing figure may be implemented with special advantage through a speed controlled turbo pump 11 at the inlet 2 of the leakage detector 1. Whereas in the instance of the solution involving an uncontrolled rotational speed for the pump, the line 16 with the valve 17 is required so as to divert the occurring increased gas flow during the so-called “cross over” (transition to fine leakage detection, opening of valve 9) into the fore-vacuum pump 8, line 16 and valve 17 can even be eliminated provided the speed of the booster TMP 11 is controlled at all times such that the maximum permissible gas flow is not exceeded. This means that the condition not to exceed the pumping capacity of the pump 11 at the beginning of the “cross over” is fulfilled through the speed control.
This possibility exists only in the instance of a booster pump 11 located upstream of the inlet 141) of the counterflow detection system. In this instance, the pumping capacity at leakage detection inlet 2 (in contrast to a main flow leakage detector or a simple counterflow leakage detector) is not linked to the sensitivity defining pumping capacity of the high vacuum or fore-vacuum pump. The leakage gas flow must entirely flow through the pump 11 at every pumping capacity (every rotational speed).
Translator's note: The German text states “2” here whereas “14” would be more in line with the drawing figure and the remainder of the text. Therefore “14” has been assumed for the translation.
Typically the booster pump 11 will operate at a low “standby” speed as long as inlet valve 9 is closed. As soon as the inlet pressure drops below the level permissible for the valve-less intermediate inlet 14 of the counterflow pump, valve 5 is closed and valve 9 is opened. At this moment the effective pumping capacity of the booster pump 11 must be approximately equal to the at this time existent pumping capacity of the fore-pump 8, so that no pressure rise will occur affecting the fore-vacuum.
| Number | Date | Country | Kind |
|---|---|---|---|
| 101-56-205.5 | Nov 2001 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP02/12601 | 11/12/2002 | WO |
