Improved getter pump, particularly for a portable chemical analysis instrument

Abstract: An improved getter pump, particularly suitable for use in a vacuum device wherein it is necessary to minimize the volume taken up by the on-site pump and the heating power it requires, includes, around a known assembly of getter elements (1) mounted on a sheath (20) which houses a heating member (2), a plurality of tubular metallic shields (7, 8, 9) coaxial with one another and with said heating member, preferably in the number of three. Said tubular shields are secured at one closed end thereof to a base support (6), and at the other end they communicate with the space to be evacuated through two metallic nets (17, 18) at the two innermost shields (7) and (8), whereas the outermost shields (if they are more than two) of larger diameter are free at said end. The outermost net (18) is preferably more close-meshed and formed by thinner metallic wire with respect to the inner net (17).

Description: The present invention relates to an improved getter pump, particularly suitable for use in a vacuum device wherein it is necessary to minimize the volume taken up by the on-site pump and the heating power it requires, while maximizing the heating speed and capability. In particular, the pump according to the invention is suitable for use in portable instruments requiring the employment of vacuum. Getter pumps for maintaining the vacuum in larger or smaller spaces have been known for some thirty years now. Their working is based on the chemisorption of all gases, excluding only noble gases, with the advantage that they do not have moving parts, whereby there is no need to use lubricants which could pollute the chamber to be evacuated. Furthermore, these pumps do not transmit vibrations, often undesired, to the system to which they are connected. It is also known that the non-evaporable getter materials, which are the active element of said pumps, require a heating device which must be supplied with a fairly high power. This implies a structure, and therefore a weight, of the getter pump which is not a problem for fixed plants, whereas it usually makes the assembly scarcely suitable for use in small-volume vacuum devices, particularly in portable apparatuses. Therefore the object of the present invention is to provide a getter pump sufficiently light and requiring a low power for its working, so that it can be used in apparatuses, e.g. chemical analysis instruments, which are portable. The description will make reference, though not necessarily, to getter pumps whose heating assembly is of the type disclosed and claimed in the patent application n.MI95A000954 (equivalent to the non pre-published application EP-A-0 742 370) in the name of the same applicant, wherein the support of the getter elements also houses the heating member. These objects are achieved by means of the getter pump according to the present invention having the features of claim 1. These and other objects, advantages and characteristics of the getter pump according to the invention will be clearer from the following detailed description, reported as a non-limiting example, with reference to the annexed drawings wherein: Figure 1 shows a sectional view of a getter pump according to the invention; and Figures 1a and 1b show, on an enlarged scale, two details of fig.1. Referring to the drawings, the illustrated embodiment of the getter pump according to the present invention includes, as known, several getter elements 1 formed in this case by disks of non-evaporable getter material coaxially mounted at a short distance from one another around a central heating member 2, as described for example in the published patent application JP-A-04/45480, but preferably according to the Italian patent application MI95A000954 in the name of the same applicant, which discloses the heating member 2 housed within the sheath 20 formed by a portion of the wall of the pump which defines a space, indicated by numeral 10, in communication with the chamber to be evacuated (not shown). According to this embodiment, the heating member 2 is mounted on a first flange 3, whereas the portion of wall forming sheath 20 is mounted on a second flange 4. According to the present invention, a plurality of thermal shields (three in the illustrated embodiment) 7, 8 and 9, of cylindrical shape and coaxial with heater 2, laterally define volume 10 closed at one end by a support 6, on which the thermal shields are fixed, the latter being made of metal, preferably steel or nickel in case of high-temperature applications, and having a small thickness, in the order of 0.1-1 mm. The metallic shields are polished so as to increase their reflectivity and thus reduce the heat dissipation outwards, and they are preferably spaced evenly at a distance varying between about 1 and 3 mm, depending in any case on the size of the apparatus. Said metallic shields, of tubular shape, are secured to a foot 6a preferably by spot welding; foot 6a is then secured by spot welding to support 6, which is in turn connected to flange 4 by a number of supporting brackets 5 through screws or bolts 15. As shown in fig.1, the supporting brackets are preferably three, spaced 120° apart. It has been found that the tubular shields 7, 8 and 9 will preferably be three, in that if they were only two the heat dissipation would still be excessive and the limited power which this kind of pump is required to work on would not be sufficient to maintain the getter elements at the operating temperature, which can be considered to be between 150° and 400°C. However, for operating temperatures at the lower limit of the above-mentioned range of values, i.e. 150-200°C, two thermal shields only may be adopted, whereas above about 300°C there may be four shields, compatibly with the size problems. At the end opposite to that of securing to flange 6, near a deflector disk 11 which closes at the top the stack of getter elements 1, the inner tubular shield 7 communicates with the volume to be evacuated through a metallic net 17, while the intermediate tube 8, slightly higher than the preceding one, also communicates with the chamber to be evacuated through a second net 18 which is outside net 17 and has a slightly larger diameter, corresponding to the diameter of the intermediate tube 8. On the contrary, the third outermost tube 9 is open at the end opposite the securing end. Obviously, in case of a number of metallic shields different from three, always the two innermost shields will be provided with the net at their free end, and in case of two shields only, they will both end with said net. It should be noted that the innermost metallic net 17, having a diameter equal to the diameter of tube 7, is preferably formed by a comparatively thick metallic wire, with a mesh size corresponding to 30-40 mesh according to the ASA standard (about 180-210 meshes/cm 2 ). On the contrary, the outermost net 18 is formed by a thinner metallic wire and considerably more close-meshed (up to 10 times) with a mesh size equal to about 320-400 mesh. This is done to allow the innermost net to retain the largest and hottest particles possibly released by the getter elements, whereas a thinner wire would risk to melt. Therefore, the outermost net 18 can retain even the smaller particles which pass through the less close-meshed innermost net 17, without risking that the thin wire melts, thanks to the smaller amount of heat which can be transferred by particles having such a small mass. Furthermore, still according to the invention, a third net on the outermost tube 9 is preferably not provided in order to prevent an excessive reduction of the conductance outwards, i.e. the possibility of maintaining a sufficient passage cross-section area between the chamber to be evacuated and the getter pump. It should be noted that the protection given by the present invention is not intended to be limited to the getter pump embodiment as above described and illustrated, but it is to be considered as extended to all the mechanical equivalents achieving the same utility. In particular, the arrangement of the getter elements 1 within volume 10 may be different and possibly different may also be the means for mounting the thermal shields.

Claims: A getter pump particularly suitable for maintaining vacuum in a portable apparatus, including an assembly of getter elements (1) mounted on a sheath (20) which houses a heating member (2) mounted in turn on a first flange (3) of the pump, characterized in that it includes, coaxially with said heating member (2), a plurality of coaxial tubular metallic shields (7, 8, 9) secured at one closed end to a support (6) of the pump, two of said tubular shields (7, 8) being in communication, at the end opposite the end secured to the support (6), with the apparatus to be evacuated through respective nets (17, 18) of metallic mesh. A getter pump according to claim 1, characterized in that said tubular metallic shields (7, 8, 9) are evenly spaced and have a same thickness, in the order of 0.1-1 mm. A getter pump according to claim 1 or 2, characterized in that said tubular shields are three. A getter pump according to claim 3, characterized in that the tubular shields provided with the net at their free end are the innermost shield (7) and the intermediate shield (8), the relevant net (18) of the latter being more close-meshed and formed by a thinner metallic wire with respect to the inner net (17) of smaller diameter.

Inventors: BELLONI FORTUNATO, CARELLA SERGIO, CONTE ANDREA, BELLONI, FORTUNATO, CARELLA, SERGIO, CONTE, ANDREA
Applicants: GETTERS SPA, SAES GETTERS S.P.A.
Application year: 1996
Main classification: F04B39
IPC codes: F04B37