This invention relates to a device for calibrating the detector of a leak searching apparatus. The device has a leak simulator and a valve situated between the leak simulator and the detector.
Contemporary leak detectors which work with helium as the test gas are capable of detecting helium leakage rates of less than 10.sup.-10 mbar 1/sec. With the aid of the leak simulator--which preferably has its own gas reservoir--there is effected a verification and/or alignment (adjustment) of the leak detector. Leak simulators simulate a leak with a known leakage rate. It is therefore generally required of leak simulators that they deliver stable test gas flow over long periods. Further, the test gas flow should be available as soon as possible upon the beginning of a calibrating process without any affections by unwanted gas sources.
The calibrating of a helium leak detector is performed by comparing the known leakage rate of the leak simulator with the indications of the leak detector. From the two values either a correctional factor is formed or the indication is set to the correct value by means of an appropriate setting element.
Leak simulators may be generally divided into two groups. To the first group there belong leak simulators with leakage rates that are greater than 10.sup.-6 mbar 1/sec. These have a relatively high test gas consumption which--in case of continuously leaking test gas--would lead relatively early to pressure changes which, in turn, alter the leakage rate of the leak simulator, so that it may no longer be used for calibrating purposes. It is therefore known to equip leak simulators of this type (that is, for leakage rates greater than 10.sup.-6 mbar 1/sec) with a shutoff valve as described in German Offenlegungsschrift (non-examined published application) No. 3,243,752. When the leak simulator is not operating, the shutoff valve is closed.
To the second group there belong leak simulators which deliver leakage rates of less than 10.sup.-6 mbar 1/sec, and thus are adapted for calibrating highly sensitive leak detectors. Because of the very small leakage rates, such a leak simulator is capable of continuously delivering an approximately constant gas flow for years from a relatively small gas reservoir. Leak simulators of this type therefore have no shutoff valves. If, however, a need to interrupt the test gas flow is anticipated, a leak simulator of this type also has to be equipped with a shutoff valve. Such a need exists, for example, in case of a leak simulator incorporated in a highly sensitive leak detector, in order to initiate a calibrating process a any time even when the specimen to be tested for leaks is attached or during the performance of automatic processes.
When a shutoff valve is used with a leak simulator, the following disadvantages are unavoidable:
Since the shutoff valve often has to remain closed for very long periods between two calibrating processes, test gas accumulates in the space (dead space) in front of the closed shutoff element. Due to such an accumulation, after opening the shutoff valve, first a "test gas surge" occurs which is of such a magnitude and whose concentration is so high that there is a danger of a contamination and/or over-regulation of the recording apparatus, such as a test gas-sensitive mass spectrometer-detector. Further, the shutoff element can be in the closed position of the valve, exposed to the full test gas pressure (which amounts to a few bars). As a result, the test gas is enriched in the closing element, particularly in the sealing materials. If then the valve is opened for initiating a calibrating process, the embedded gas particles escape (drift) from the closing mechanism and falsify the leakage flow proper of the leak simulator for a relatively long period. In case the closing element is a rubber or synthetic diaphragm, the mentioned drift phenomena appear in a particularly pronounced manner. Further, in the closed position of the valve--thus, during the leak detecting operation--a permeation through the diaphragm is unavoidable. If the leak simulator is installed in a leak detector, the gas particles penetrating through the diaphragm enter in the test gas-sensitive detector.
In the leak simulator which is described in German Offenlegungsschrift No. 3,243,752 and which belongs to the first group of leak simulators, that is, it delivers a leak rate of greater than 10.sup.-6 mbar 1/sec, a dead space in front of the shutoff mechanism of the shutoff valve is almost nonexistent. The disadvantageous phenomena such as test gas surge, drift and permeation are therefore suppressed to such an extent that they are negligible as compared to the desired leak rate and consequently they do not distort the calibration.
A shutoff valve according to German Offenlegungsschrift No. 3,243,752 is, however, not usable with a leak simulator belonging to the second group, that is, which has leak rates of less than 10.sup.-6 mbar 1/sec, because gas streams which appear by virtue of the disadvantageous phenomena, such as test gas surge, drift and permeation, are not negligible as compared to the desired leak rates. In order to avoid distortions of this nature, it is known to utilize a "pumped" leak simulator as a closable leak simulator for small leak rates. The outlet of such a leak simulator is, by means of a branch conduit and a valve disposed therein, continuously connected with the inlet of the leak detector in which it is incorporated. The branch conduit merges into the main conduit which is connected with the specimen to be tested for leaks and is equipped with a valve as well. During normal operation the valve in the branch conduit is closed whereas the valve in the main conduit is open. In a device of this type it is necessary to attach, with the aid o a further valve, a separate vacuum pump to the chamber between the outlet of the leak simulator and the first-noted valve for continuously removing the test gas emanating from the leak simulator. Without evacuation the space would fill with test gas in high concentration and thus could not be connected immediately with the detector to be calibrated. With the high test gas concentration which leads to an undesired "test gas surge" there is coupled the danger of a contamination and/or over-regulation of the mass spectrometer detector. Also, the above-described drift phenomena would appear as well. For the calibrating process the valve associated with the separate vacuum pump is closed and the valve to the leak detector is opened.
A "pumped" leak simulator involves substantial expense: in the first place, as noted, a separate vacuum pump is needed. Conventionally, for this purpose an auxiliary pump serving for tough-evacuation is being used. The vacuum pumps utilized for providing a vacuum for the mass spectrometer cannot be utilized because the removed helium diffuses back through these pumps and would thus falsify the indications of the leak detector. Portable leak detectors, because of the weight of a separate (additional) vacuum pump cannot be equipped with a "pumped" leak simulator. Such "pumped" leak simulators therefore can find application only in large installations where either costs and weight problems are not primary considerations or which have further vacuum pumps adapted to assume the pumping function for the leak simulator.