Low pressure leak detector

A leak detector for detecting the presence of a leak in a vacuum vessel exposed on its exterior to a test gas, comprising, a turbo vacuum pump having an inlet connected to a gas sensor for sensing the presence of a test gas and an outlet, a forepump connected to the outlet for receiving gas from the outlet and an intermediate inlet in the turbo vacuum pump connected to the test vessel. When test gas is supplied to the intermediate inlet from the test vessel, indicative of a leak, the test gas flows in counter-current flow to the operating direction of the turbo vacuum pump, to the gas detector, where the test gas is detected.

FIELD AND BACKGROUND OF THE INVENTION 
The present invention relates to leak detectors in general and, in 
particular, to a new and useful low pressure leak detector which utilizes 
a turbo vacuum pump, plus a forepump. The turbo vacuum pump is equipped 
with a gas detector which detects gas flowing in a direction opposite to 
the operating direction of the turbo vacuum pump, to sense the presence of 
a leak. 
Such leak detectors are known, for example, from German Pat. No. 1,648,648 
and Swiss Pat. No. 519,137. They operate on the so-called "couter-current 
principle". The test gas, having penetrated the vacuum vessel, passes 
through a supply line to the outlet side of a turbo vacuum pump and 
against the displacement direction of the pump, to a detector which is 
connected to the inlet side of the pump and serves to detect the presence 
of the test gas. An arrangement using a diffusion pump instead of a turbo 
vacuum pump is known from U.S. Pat. No. 3,690,151. 
The prerequisite for a successful application of the counter-current 
principle is to have a counter-current pump which permits the adjustment 
of a stable test gas compression factor which is smaller than the 
compression factor for other gases evacuated from the tested vessel which 
other gases interfere with the detection of the test gas by the detector. 
The selected compression factor for the test gas is to be small enough to 
allow the test gas to penetrate against the pumping direction to the 
detector and to build up a measurable partial pressure therein. The 
compression factor for the other gases, on the contrary, should be as high 
as possible, to minimize their contribution to the interfering background. 
Turbo vacuum pumps are particularly suitable as counter-current pumps 
because their compression factor depends on the molecular weight of the 
kind of gas being pumped. 
In prior art arrangements, a disadvantage of systems based on the 
counter-current principle is that they are relatively expensive since a 
separate counter-current pump is needed, so that the application of the 
principle is economically prohibitive in many instances. Still more 
serious is another disadvantage, namely, that after each peak of the test 
gas partial pressure in the detector, a relatively long time period is 
necessary to regain a full detecting sensitivity for following weaker test 
gas signals. This limitation is mostly due to the fact that the 
determining factor for evacuating the test gas from the detector and/or 
the container is substantially only the pumping speed of a forepump used 
in the system. 
SUMMARY OF THE INVENTION 
The present invention is directed to a leak detector arrangement of the 
above-mentioned kind which operates with a single turbo vacuum pump and a 
forepump, so that it is less expensive than prior art arrangements 
requiring a turbo vacuum pump and, in addition, another high vacuum pump 
with a forepump. 
Accordingly, an object of the present invention is to provide a leak 
detector for disconvering leaks in a vacuum vessel by means of testing for 
the presence of a test gas which has penetrated into the vessel from the 
outside, comprising, a turbo vacuum pump having its inlet connected to a 
gas detector for detecting the test gas and its outlet connected to a 
forepump, with a connecting line provided for supplying the test gas 
evacuated from the vessel to be tested to the turbo pump, the connecting 
line terminating at an intermediate stage of the turbo vacuum pump, 
between its inlet and outlet. 
Since, in accordance with the invention, only a part of the turbo vacuum 
pump is used as a counter-current pump, the risk was run that as compared 
to the prior art arrangements, the pressure ratio of test gas pressure to 
total pressure in the detector will be greater by orders of magnitude and, 
consequently, that the interference background caused by the total 
pressure and, particularly, by the pressure variations in the test vessel, 
will be much stronger. Surprisingly, however, it has been found that this 
loss of detecting sensitivity, to be expected theoretically, does not 
occur. Presumably, in the prior art arrangements, a temporary hight test 
gas peak causes a gas absorption in the oil present in the bearings on the 
pressure side of the turbo vacuum pump and of the forepump, which absorbed 
gas is then only slowly released again. In the inventive arrangement, 
however, the partial pressure portion thereof is suppressed or made 
ineffective by the interconnected intermediate stages of the turbo vacuum 
pump which is not used as a counter-current pump. The inventive solution 
has, therefore, substantially the following two advantages: 
1. The time constant for regaining full sensitivity after a test gas peak 
is reduced in proportion to the pumping speeds of the residual stages of 
the turbo vacuum pump used for evacuating the tested vessel, and of the 
forepump. 
2. The high pressure side of the pump section evacuating the detector to 
the turbo vacuum pump remains securely free from hydrocarbons having a 
capacity of absorbing the test gas, i.e., the so-called memory effect of 
the forepump remains ineffective, so that, with the inventive arrangement, 
the detecting sensitivity of the prior art devices may even be surpassed. 
Another object of the present invention is to provide a leak detector which 
is simple in design, rugged in construction and economical to manufacture. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its uses, reference 
is made to the accompanying drawing and descriptive matter in which a 
preferred embodiment of the invention is illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the following, one embodiment of the invention is exlained in more 
detail with reference to the accompanying single diagrammatical figure 
showing a turbo vacuum pump, generally designated 1, a detector 2 (for 
example, a quadrupole mass spectrometer) connected to the inlet thereof 
for detecting the test gas, the outlet side 3 of the pump, and a forepump 
7 connected thereto through a line 6, a valve 5, and a connection 4. 
Further shown is a connecting line 8 for conducting the test gas from a 
vessel 9 to be tested, which line, in accordance with the invention, 
terminates in an intermediate stage between the inlet and the outlet of 
the turbo vacuum pump. Vessel 9 is exposed at its outside to the test gas. 
Most advantageously, line 6 terminates at a location where the compression 
ratio, relative to the inlet of the pump, for the interfering gases (air) 
is just sufficient to maintain an optimum operating vacuum in the 
detector, for example, if a mass spectrometer is used as the detector, a 
vacuum of the order of magnitude of 10.sup.-5 millibar. This requirement 
at the same time determines the minimum compression ratio for the 
interfering gases in a specific instance. It is further advantageous to 
join the connecting line 8 so as to open into an annular channel 10 
provided in the housing of pump 1 and surrounding the respective stage 
thereof, in order to best utilize the suction capacity of this stage. 
The drawing further shows a valve 11 with which line 8 may be shut off, for 
example, to be able to exchange the vessel 9 to be tested without the 
necessity of venting the entire pump. Also shown is a line 12 with a 
shutoff valve 13, permitting the establishment of a direct communication 
between the tested vessel and the forepump. This is to make possible a 
direct, thus faster, preliminary evacuation of the test vessel by means of 
forepump 7, for creating the operating condition. 
Within the context of this specification, a turbo vacuum pump means a 
vacuum pump which a motion impulse is transmitted to the gas to be 
displaced in the displacement direction by rapidly rotating elements such 
as blades or rotary discs. Thus, in this specification, the mentioned term 
covers both turbo vacuum pumps operating in the viscous flow range and 
such displacing the gases at low pressures within the range of molecular 
flow. In this latter case, the impulses are transmitted by each rotary 
element to every individually displaced molecule and such pumps are then 
frequently termed turbo molecular pumps. 
By pumping stages, the individual blade rings or discs of a turbo vacuum 
pump are meant which, when in rotary motion, displace the gas in the 
pumping direction, while compressing it by a definite factor. It may be 
advantageous to design the individual pumping stages with different 
compression capacities, e.g., those pumping stages which operate as a 
counter-current pump with a lower compression capacity and the other 
stages with a higher compression capacity, for better separation relative 
to the forepump. The two different sets of discs are then mounted on a 
common shaft and in a common pump housing. Such a turbo vacuum pump is 
particularly suitable for the application of the invention. Blades or 
discs 15 are thus upstream of the annular channel 10 and discs 16 (of the 
second set of discs) are downstream thereof in the direction of arrow 17 
which is the operating flow direction of pump 1. 
While a specific embodiment of the invention has been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.