GB2401190A

GB2401190A - A reference gas leak

Info

Publication number: GB2401190A
Application number: GB0309268A
Authority: GB
Inventors: Ronald Douglas Collins
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-04-24
Filing date: 2003-04-24
Publication date: 2004-11-03
Anticipated expiration: 2023-04-24
Also published as: GB0309268D0; GB2401190B

Abstract

A reference gas leak is described consisting of an inert powder 4, eg. stainless steel contained within an impervious tube 1 the ends of which form the inlet and outlet ports of the leak. Methods for effecting calibration of the leak, for cleaning and for stabilisation of the finished leak are described.

Description

2401 90 Developments in reference gas 'KS

Background

Many products today have to be leak tested during manufacture, and also sometimes during use.

The traditional methods of leak testing using a liquid bubble test or dye penetrant are being rapidly replaced by electronic devices (sniffers) which are more sensitive and convenient to use.

These 'sniffer' leak detectors invariably require a test gas different from air as a tracer to trigger then. In use, a pressure differential is created across the leak surface to encourage the tracer gas to escape.

Calibration of leak detectors is achieved through the use of reference gas leaks which provide a pre-defined test gas flow rate. Various types of reference leak are available, based upon the use of quartz glass, crimped capillaries, membranes and tenon permeation.

Capillaries are commonly made front fine-bore glass tubes. Such capillaries are invariably hand- made and require considerable skill in their manufacture and calibration. Manufacture involves drawing a fine glass tube down to a pre-selected diameter, testing the leakage rate so achieved' and either rejecting the tube if the rate is too low, or reheating and further drawing down if the rate is too high. Rejection rates are high, and the finished capillary is exceptionally fragile, often having a diameter of only a few tenths of a millimeter. Even when the capillary is encased in a protective housing, excessive gas pressures and/or harsh handling can still damage it.

Membranes are likewise fragile arid unsuitable for use with Urge pressure differentials.

Leaks made using Teflon permeation are prone to poisoning and are again rather fragile.

None of the known technologies are suitable for the construction of reference leaks for use in rugged' industrial environments, and are all too expensive for some applications such as the appliance servicing industry where a very low cost, rugged reference leak is needed.

nt- Suction L The objective of this invention is to provide a design for a reference gas leak comprising of an inert powder contained in an impervious tube, the ends of which form the inlet and outlet ports of the leak. Methods will be described for effecting calibration of the leak, for cleaning and for stabilization of the finished leak.

Do- fiction According to the invention, for which we pray a patent may be granted, a short length of tube is filled with an inert powder and capped at both ends by plugs each containing a hole through which the trace gas may enter and exit. The powder provides a significant obstacle to the free flow of trace gas such that only a very small quantity of gas may travel from inlet to outlet when a pressure differential is created across opposite ends of the tube.

In a preferred embodiment of the invention, and illustrated in the sectional view of figure l, a stainless steel tube (1) has tight fitting plugs (2) and (3) fitted one in each end, trapping between them a stainless steel powder (4). Plugs (2) and (3) optionally contain fine axial borings (S) to allow gas to flow into tube (l), through powder (4) and eventually exit. For very low leak rates, sufficient gas escapes between the plugs and the walls oftube (1) to avoid the need for axial borings (5). The assembly shown in figure I is a completed reference leak.

Manufacturing of the leak begins by inserting plug (2) into tube ( I) and then upending the assembly with plug (2) at the bottom, to permit a precise quantity of powder (4) to part fill tube (l). Plug (3) is then inserted into the upper end of tube (l) and the assembly placed in a press, which applies an axial compressive force upon the two plugs, compacting powder (4). At this point in the manufacturing cycle, the part-finished leak is tested to ascertain if the requisite leakage rate has been achieved. Reduction in leakage rate may be achieved by further compaction of powder (4).

It will be appreciated that the actual leakage rate achieved will be a function of the internal bore of tube (1), the type, grain size and quantity of powder (4) and the degree of compaction achieved during manufacture. Experience gained during the manufacture of a number of such leaks enables a skilled operator to gauge the force required to achieve a target leakage rate, to compress plugs (2) and (3) to a level slightly less than this, to confirm the actual leakage rate from test, and to then subsequently compress a little more to reach the target leak rate in one additional operation. '1'his process results in a very low reject rate, such that the majority of the leaks produced have leakage rates within desired tolerances.

Once the target leakage rate has been achieved, the assembly is sintered in vacuum. The process of sintcring causes powder (4) to coagulate, so 'fixing' it. Some variation in leak rate may occur during the sintering process, which variation requires to be taken into account when setting the targets for leak rate during the earlier stages of manufacturing.

1 he high temperatures inherent in the sintering process also clean the complete assembly, preventing subsequent contamination of the leak detector by foreign emissions.

Claims

1 A reference leak consisting of an impervious tube containing an inert powder 2 A reference leak as described in claim 1 whereby the powder may comprise particles of dissimilar size.

3 A reference leak as described in claim 1 whereby the aforementioned powder may be compacted during manufacture to effect variation of the leak rate.

4 A reference leak as described in claim 3 whereby compaction may be achieved by applying an axial force to plugs inserted into the open ends of the tube.

A reference leak as described in claim 4 whereby the aforesaid plugs may be left in place after compaction, becoming an integral part of the reference leak 6 A reference leak as described in claim 4 whereby each plug contains an axial boring.

7 A reference leak as described in claim 3 whereby the walls of the tube may be deformed during manufacture to effect compaction of the powder.

8 A reference leak as described in claim 1 whereby the achieved leak rate is measured during assembly and the powder further compressed to reduce the leak rate if _31;_ 1"lle=.

9 A reference leak as described in claim 1 whereby the complete assembly may be sintered following assembly in order to both clean and 'fix' the powder.

A reference leak as described in claim 9 whereby the sintering temperature may be varied to effect adjustment of the leak rate.

11 A reference leak as described in claim 1 whereby the reference leak may be heated during use to avoid contamination by trace gasses.

12 A reference leak as described in claim 1 whereby a number of leaks may be combined in either series and/or parallel to achieve different leak rates.

13 A reference leak as described in claim 4 whereby at least one of the aforementioned plugs is capable of being moved axially in use in order to compress or decompress the powder.

14 A reference leak as described in claim 1 which may also function as a fixed conductance.

A conductance as described in claim 14 which may be combined with others in order to effect blending of different gases.

16 A conductance as described in claim 14 which may be used to provide controlled venting of a pressure/vacuum vessel.

7 A reference leak as described in claim 1 which may also be used as a precision filter against the entry of dust and debris.

18 A reference leak as described in claim [which may also be used to atomise liquids or gases held under pressure.

Amendments to the claims have been filed as follows:

1. A method of manufacturing a reference leak comprising the steps of providing an impervious tube; inserting an inert powder in the tube; and compacting the powder in the tube to provide the reference leak 2 The method of claim 1 wherein the step of compacting the powder in the tube comprises inserting plugs into the open ends of the tube and applying an axial force to the plugs to compress the powder between the plugs.

3. The method of any preceding claim wherein the step of compacting the powder in the tube comprises inserting plugs into the open ends of the tube and moving at least one of the plugs axially to compress or decompress the powder between the plugs.

4. The method of claim 2 or claim 3 wherein each plug comprises an axial . boring.

5. The method of claim 4 comprising supplying a gas through the axial bore of one of said plugs into the tube such that the gas flows through the powder in the tube and out of the tube through the axial bore in the other plug, and determining the rate at which the gas leaks from the tube through said other plug.

6 The method of any preceding claim comprising selectively compacting the powder to try to achieve a target leakage rate 7. The method of any preceding claim further comprising testing the leak after compaction of the powder to determine whether a target leak rate has been achieved 8 The method of claim 7 comprising compacting the powder to a level which provides a leakage rate below a target rate, determining the leakage rate of the part- finished leak, and further compacting the powder to achieve the target leak rate 9 The method of any preceding claim further comprising sintering the tube containing the power after compaction of the powder to "fix" the compacted powder in the tube l O. The method of claim 9 further comprising selecting the sintering temperature appropriately to try to effect a desired adjustment of the leak rate achieved after compaction of the powder.

l l. The method of claim 9 or claim l O wherein the sintering of the powder in the tube is carried out in a vacuum.

" ,( ]5 12 A reference leak produced in accordance with the method of any preceding . claim 13. A reference leak comprising an open ended tube and a compacted powder fixed between the ends of the tube, whereby a gas applied to one end of the tube ( passes through the powder and leaks from the other end of the tube at a given rate 14 The reference leak of claim 13 wherein the compacted powder is a sintered compacted powder.

15. The reference leak or method of any preceding claim wherein the tube is a stainless steel tube.

16 The reference leak or method of any preceding claim wherein the powder is a stainless steel powder.

17 The use of a reference leak in accordance with any of claims 12 to 16 as a fixed conductance.

] 8 A method of using a reference gas leak as claimed in any of claims 12 to 16 comprising combining the reference gas leak with other such reference gas leaks in order to effect blending of different gases 19 The use of a reference gas leak as claimed in any of claims 12 to l 6 to provide controlled venting of a pressure/vacuum vessel.

20. The use of a reference leak as claimed in any of claims 12 to 16 as a precision filter against the entry of dust and debris 21 The use of a reference leak as claimed in any of claims 12 to 16 to atomise . . ' 15 liquids or gases held under pressure 22 A method of using a plurality of reference leaks in accordance with any of . claims 12 to 16 comprising combining the leaks in either series and/or parallel to " achieve different leak rates. ' 20 .

23 A method of manufacturing a reference leak comprising inserting a plug into one end of an open-ended tube, part- filling the tube with a quantity of powder; inserting a further plug into the other end of the tube; placing the assembly in a press and applying an axial compressive force to the two plugs to compact the powder between the plugs and testing the reference leak obtained to determine whether a target leakage rate has been achieved.