GB2143949A - Filament support - Google Patents

Abstract

A support arrangement for an electrically heated filament 4 connected between substantially rigid posts 6,8 which act as electrodes for passing current through the filament, includes spring means 10 for tensioning the filament together with a damping member 12 which is arranged to engage the filament 4 so as to be effective to inhibit filament vibration. The embodiment illustrated is part of a Pirani vacuum gauge; the invention is also applicable to a thermionic valve, barreter or electric lamp. <IMAGE>

Description

SPECIFICATION Pirani gauge filament support This invention relates to filament supports that is to say supports for filaments adapted to be electrically heated in an inert or reduced pressure ambient. The invention is particularly, though not exclusively directed to a support arrangement for the filament embodied in a Pirani gauge for the measurement of low gas pressure in an evacuated enclosure.

Pirnai gauges for low pressure measurement are well known. Such gauges comprise a thin filament of inert metal generally of tungsten and typically of some 50 mm to 100 mm in length which is suitably supported within an envelope adapted to communicate with the enclosure being pumped or maintained at low pressure.

The filament in such a gauge is electrically heated in conventional manner from a suitable current source, to maantain the filament at a constant resistance irrespective of variation in heat loss arising from changes in the pressure of gas in the gauge envelope. Constant resistance is maintained by embodying the filament into one arm of an electrical bridge kept in balance by controlling the voltage which is applied across the bridge and which is indictive of heat loss and thereby gas pressure being measured. The bridge may include ambient temperature compensation in the manner disclosed and claimed in our co-pending United Kingdom patent appliation number 8224544.

In conventional Pirani gauges, the filament is mechanically supported by semi-rigid metal posts; the ends of the filament are welded to the posts which sealingly pass through the base of the gauge envelope and which act as electrodes for passing current through the filament.

The generally small diameter of the gauge filament typically of the order of 0.1 mm coupled with the thermal cycling which it undergoes renders the filament susceptible to mechanically and electromagnetically induced vibration. Gauges embodying such a typical filament support have accordingly been found prone to filament failure particularly during high filament operating temperatures.

It is accordingly one object of the present invention to provide a filament support effective to reduce the onset of failure.

The present invention according to its broadest aspect provides a support arrangement for an electrically heated filament connected between substantially rigid posts, which act as electrodes for passing current through the filament, the filament being tensioned by spring means and being arranged to engage a damping member effective to inhibit filament vibration.

In one embodiment, the spring means, which conveniently is a metal spring, forms part of the filament series circuit by being secured between one end of the filament and the adjacent electrode post. In this case, which is particularly applicable to a substantially rectilinear filament, the filament is wholly or partially circumscribed by a damping member which suitably is in the form of a wireloop supported by the electrode post. The wire loop suitably is spaced from the filament to permit longitudinal movement produced for example by heating but to engage the filament to produce damping in the event that transverse movement or vibration exceeds a threshold determined by the spacing. The damping member may alternatively be in the form of an apertured disc also conventiently disposed adjacent the spring.In this case the damping member may have an electrically insulating or conducting surface where it can have no shunting effect on the filament.

In an alternative embodiment of the invention which is particulaly applicable to a convoluted filament, the metal spring may merely engage the filament at a point intermediate its ends to produce the required degree of tensioning. Suitably the spring engages the filament at its mid point to produce a filament of V configuration.

In this embodiment the damping member preferably is of cylindrical form adapted to engage the filament over an arcuate portion of its periphery. Conveniently the damping member also is disposed adjacent the spring tensioner and is secured to the support for the spring. In this case the damping member will be of electrically insulating material such as ceramic or glass but may be of metal provided with a suitable electrically insulating coating or layer.

The present invention is based on the discovery that resiliently supporting an electrically heated filament increases rather than decreases the susceptibility to mechanical vibration and accordingly increases rather than decreases the incidence of failure.

Embodiments of the invention will now be particularly described by way of example with reference to the accompanying drawing in which Figure 1 is a side view of one arrangement for supporting a filament in a Pirani gauge, Figure 2 is an end-on view of an alternative arrangement for supporting a filament in a Pirani gauge and, Figure 3 is a sideview of the arrangement of Fig. 2.

Referring to Fig. 1 of the accompanying drawing, the Pirani gauge comprises a glass, ceramic or PTFE base 2 which forms part of the gauge envelope (not shown). The envelope has an opening (also not shown) for suitably communicating with an evacuated enclosure whose internal gas pressure is to be measured by measurement of heat loss from the gauge filament 4 when electrically heated.

Heat loss as herein recited is detected by embodying the filament into one arm of an electrical bridge circuit and maintaining the bridge in balance and filament resistance constant by varying the voltage applied across the bridge. The applied voltage will be indicative of heat loss and thereby pressure in the gauge being measured.

The gauge is connected into the measuring circuit by way of external electrodes 6, 8 which extend into the envelope to form substantially rigid support posts for the filament 4. As shown in Fig. 1 one end of the filament is secured to post 6; the other end of the filament is secured to one end of a thin helically wound spring 10 the other end of which is secured to an extended and angled post 8 by way of an electrically conducting bridging member 16. Spring 10 is of metal and together with bridging member 1 6 forms part of the series circuit including the filament 4.

Also secured to post 8 is a relatively rigid wire 12, which forms a loop circumscribing but spaced from, the bridging member 16.

The loop is adapted to permit longitudinal movement of the filament 4 but to engage the bridging member and produce damping in the event that transverse filament movement exceeds a threshold determined by the loop/bridging member spacing. In this way filament failure arising from excessive transverse movement or vibration which may be mechanically induced or induced by interaction between filament current and external magnetic fields, is inhibited.

In the arrangement shown in Fig. 1, the low resistance of the spring and the bridging member is such that the surface of the wire loop may be either electrically insulating or conducting. In the case where the loop is disposed intermediate the ends of the filament, electrical insulation to avoid shunting a portion of the filament must be provided.

In an alternative embodiment of the invention shown in Figs. 2 and 3, in which like parts have like numerals, the ends of a Vshaped filament 4 are secured for example by welding to short posts 6 and 8 effective to carry current to the filament. A further independent support post 14 also sealed into the envelope base 2 supports one end of the spring 10, the other end of which engages the mid-point of the filament to produce tensioning. In this embodiment spring 10 tensions two opposed regions of the filament into contact with a damping member 12 in the form of a ceramicor other electrically insulating cylinder also supported by post 14. As shown more clearly in Fig. 3 the filament engages the cylinder over an arc of the cylinder periphery to produce mechanical damping without introducing localised areas of stress to any point of the filament. As shown also in Fig. 3, the damping member 12 engages two opposed regions of the filament to produce the same degree of damping for each filament span.

The invention provides an improved gauge life unattainable by either rigidly supporting the filament or by resiliently supporting the filament by spring means alone. Having regard to the extensive cost and need for accurate calibration of Pirani gauges, technical as well as cost advantages unexpectedly arise from use of the present invention.

It will be appreciated that while the invention has been described with reference to the filament of a Pirani gauge it is equally applicable to the support of filaments in other devices such as valves, barreters, electric lamps and the like.

Claims (14)

1. A support arrangement for an electrically heated filament connected between substantially rigid posts which act as electrodes for passing current through the filament, the arrangement including spring means for tensioning the filament together with a damping member which is arranged to engage the filament so as to be effective to inhibit filament vibration.

2. A support arrangement as claimed in Claim 1 in which the spring means comprise a metal spring connected in series between one end of the filament and the adjacent electrode post.

3. . A support arrangement as claimed in Claim 1 in which the spring means comprise a metal spring having one end secured to the filament intermediate its ends.

4. A support arrangement as claimed in Claim 3 in which the metal spring is effective to tension the filament into a substantially Vshaped configuration.

5. A support arrangement as claimed in Claim 3 or Claim 4 in which the other end of the spring is secured to a post electrically distinct from the electrode posts.

6. A support arrangement as claimed in any preceding Claim in which the damping member is arranged to partly or fully circumscribe the filament in spaced relationship whereby to engage the filament and produce damping in the event of excessive vibration.

7. A support arrangement as claimed in Claim 6 in which the damping member comprises an apertured disc.

8. A support arrangement as claimed in Claim 6 in which the damping member comprises a wire loop.

9. A support arrangement as claimed in any one of Claims 1 to 5 in which the damping member is of substantially cylindrical form adapted to engage the filament at one or more points.

10. A support arrangement as claimed in any preceding Claim in which the damping member is of electrically insulating material or is electrically insulated from the filament.

11. A support arrangement as claimed in any preceding Claim in which the damping member is disposed adjacent the spring means.

1 2. A support arrangement as claimed in any preceding Claim in which the filament is embodied in a Pirani Vacuum Gauge.

1 3. A support arrangement substantially as herein described with reference to Fig. 1 or to Figs. 2 and 3 of the accompanying drawings.

14. A Pirani Vacuum Gauge including a filament support arrangement as claimed in any preceding Claim.