US 3791195 A
A thermal conductivity detector element is mounted within a small diameter cavity and held by support posts, one of which extends closely adjacent to a metallic cavity wall. Electrical short circuiting between the support post and wall is prevented by providing the support post or at least the portion thereof adjacent to the cavity wall with electrically non-conductive heat resistant coating such as glass or a ceramic frit. The coating is preferably fused onto the post so that it becomes an integral part of the support post.
Claims available in
Description (OCR text may contain errors)
United States Patent [191 Loe [ THERMAL CONDUCTIVITY ELEMENT  Inventor: Winston C. Loe, 4851 DelMonte, La
Canada, Calif. 91011  Filed: Sept. 21, 1972  App]. No: 291,091
 US. Cl. 73/27 R, 117/212  Int. Cl. G0ln 31/00  Field of Search... 73/23, 27 R, 23.1; 23/232 E, 23/254 E; 29/592; 117/212, 37
 References Cited UNITED STATES PATENTS 2,720,076 10/1955 Sachara .l 117/37 R X 2,753,247 7/1956 Greanias et a1. 73/27 R 3,400,264 9/1968 Bull 73/23 X 3,537,914 11/1970 Cieplinski et a1. 713/27 R X 3,650,024 3/1972 Finney 29/592 X [451 Feb. 12, 1974 Primary Examiner-Richard C. Queisser Assistant ExaminerStephen A. Kreitman Attorney, Agent, or Firm-Robert C. Comstock  ABSTRACT A thermal conductivity detector element is mounted within a small diameter cavity and held by support posts, one of which extends closely adjacent to a metallic cavity wall. Electrical short circuiting between the support post and wall is prevented by providing the support post or at least the portion thereof adjacent to the cavity wall with electrically nonconductive heat resistant coating such as glass or a ceramic frit. The coating is preferably fused onto the post so that it becomes an integral part of the support post.
6 Claims, 5 Drawing Figures THERMAL CONDUCTIVITY ELEMENT BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a wire filainentor thermistor I type thermal conductivity detector element for gas chromatographs and thermal conductivity analysis instruments.
2. Description of the Prior Art In such instruments, the volume of the detector cavity is a predominant factor in determining the response time constant of the system. It is desirable to make this respect time as short as possible in order to provide sharp resolution of adjacent sample component peaks. This is especially important when using small bore high efficiency separating columns.
The detector response time is predominantly reduced by reducing the detector cavity volume. The cavity length cannot be reduced below approximately linch without adversely affectingthe high sensitivity of hot wire filament type detector elements. The cavity length may be reduced well below Ainch if thermistor type elements are used. Reducing the diameter below 0.080 inch is desirable when using either type of element, but this leaves little clearance between the cavity wall and the element support post. It is accordingly easy for the support post to contact and form an electrical short circuit with the metal cavity wall which may short out the element bridge circuit.
At the present time, somedetector elements seek to overcome this electrical shorting problem by running a filament concentrically along the longitudinal axis of the cavity, with the opposite ends of the filament being sealed in place. This structure is difficult and costly to build and to replace.
Ceramic insulating tubing cannot be used to surround or enclose the sensing element because even small movements such as those due to vibration would change the heat transfer characteristics of the cavity. For the same reason, it is not practical to place a sleeve over the support post to achieve electrical insulation of the post from the cavity wall.
SUMMARY OF THE INVENTION vide such a structure in which the support post is proand most likely to contact the cavity'wall) is covered vided with sufficient electrical insulation to prevent short circuiting even if the support post and cavity wall should move into close proximity or even physical contact due to vibration or other causes.
Another object of the invention is to provide such a structure which is economically feasible and practical in manufacture and use.
A further object of the invention is to provide such a detector element which permits simple and rapid replacement of the detector element in the field by relatively unskilled help. The invention also eliminates the necessity for suppliers to carry a stock of complete detector assemblies, since only the elements themselves need to be stocked.
The invention also comprises such other objects, advantages and capabilities as will later more fully appear and which are inherently possessed by the invention.
While I have shownin the accompanying drawings preferred embodiments of the invention, it should be understood that the same are susceptible of modifica tion and change without departing from the spirit of the invention.
"is'm ou OF TI-IE DRAWINGS FIG. 1 is a longitudinal sectional view of a detector element embodying the invention in use, with the filament and its support posts shown in elevation; pair of element leads are normally connected to the upper ends of the support posts 12 and 13.
FIG. 2 is an enlarged fragmentary view of the lower end of the filament and adjacent support post;
FIG. 3 is a sectional view taken on line 33 of FIG.
FIG. 4 is a view similar to FIG. 3 of an alternative embodiment of the invention;
FIG. 5 is a view similar to FIG. 1, showing another alternative embodiment of the invention utilizing a thermistor instead of a wire filament.
aiiibiibfior rite IREFERRED EMBODIMENTS first embodiment which has been sei2 and 13. A pair of elementdeads arenormally con- I nected to the upper ends of the support post 12 and 13.
The midportions of the support posts 12 and 13 extend through and are securely held within a seal 15 of circular cross sectional configuration. The seal 15 may be formed of glass or ceramic or other electrical nonconductive material.
Extending around the periphery of the seal 15 is a metal seal shell 16. The seal shell 16 has a vertical portion which surrounds and is secured to the periphery of the seal 15. It also has a horizontal portion which extends radially outwardly and overlies the lower edge 21 of the opening in which the seal retaining nut 11 is mounted. The lower edge of the nut l 1 is provided with an annular ring 17 which engages and exerts pressure against the horizontal portion of the seal shell 16 in order to provide an air and gas tight seal between the shell 16 and the adjacent edge 21.
A cylindrical cell or cavity 18 is of smaller diameter than the opening 20 and extends into the detector block 10 directly beneath the opening 20. The lower portions of the support posts 12 and 13 are removably mounted within the cavity 18. The support post 12 extends to adjacent the bottom of the cavity 18, while the other support post 13 terminates adjacent to the top of the cavity 18.
The bottom of the support post 12 is provided with a cylindrical projection 19. An elongated fine wire filament extends between the projection 19 and the lower part of the support post 13.
A gas inlet duct 26 is provided adjacent to the bottom of the cavity 18 for admitting gas to be analyzed. An outlet duct 27 adjacent the top of the cavity 18 is provided for the outward flow of gas.
Since the diameter of the cavity 18 may be 0.080 inch 0s less, it will be obvious that the amount of physical separation which is provided between the support post 12 and the adjacent wall of the cavity 18 is extremely small. Any slight variations in tolerance, vibration, etc., are likely to cause physical engagement and therefore electrical contact and short circuiting to occur between the support post 12 and adjacent portion of the wall of cavity 18.
In order to avoid such electrical short circuiting even if physical contact does occur, the support post 12 is provided along substantially its entire length with a discontinuous coating 28 comprising a plurality of small closely spaced projections of glass or ceramic frit or other suitable insulating material which will physically engage the wall of the cavity 18 before and instead of its being engaged by the support post 12, so that no electrical contact or short circuiting will occur. Because the coating 28 is discontinuous, it is possible for the support post 12 to flex without spalling off the coating 28.
The coating 28 may extend around only a portion of the periphery of the support post 12 as shown in FIG. 3 of the drawings. In an alternative embodiment, the coating 28a may extend around the entire periphery of the support post 12a, as shown in FIG. 4 of the drawings. In the embodiment shown in FIG. 3 of the drawings, the portion of the periphery carrying the coating 28 is the portion which is disposed directly adjacent to the wall of the cavity 18.
The metal used to form the post 12 and the glass, ceramic or other insulating material used to form the coating 28 should preferably have relatively close coefficients of expansion. One metal which is suitable for forming the post 12 is Kovar, which is an iron-nickel alloy made and sold by Westinghouse Corporation.
The post 12 may be heated to a bright red heat with a torch or in a furnace and then dipped into a powder of suitable glass or ceramic while still above the melting point of the powder. This causes a thin discontinuous layer of frit to adhere to the surface and permit further heating to fuse the frit firmly to the surface of the metal. Alternatively, the frit may be applied by making a slurry of the powder in a dispersing liquid such as water and applying the slurry to the desired surface portions by brushing or spraying, and then allowing the dispersing liquid to evaporate either before or during the heating operation required to fuse the frit to the mtal surface.
If the Kovar alloy is used, it is preferably heated suffrciently in the presence of air to oxidize its surface at the same time or prior to the time that the glass or ceramic material is fused to the surface. The two materials then form a bond so that the coating 28 of frit effectively becomes an integral part of the outer surface of the post 12.
With Kovar and possibly other material, it is not critical for the surface to oxidize, since the insulating material will stick without prior surface oxidization.
The coating might also be applied by diffusing the insulating material into an argon plasma, torch flame, or the like, and applying it to the support post. The insulating material is partially metled and melted molten particles will impringe on the surface and stick.
It should be understood that any material or method may be used which will result in the application of a ceramic, glass, vitreous or other heat resistant electrically non-conductive coating on the surface of the support post or a critical portion thereof.
The detector element must be capable of operating at temperatures of up to 500 C. At this temperature epoxy resin cannot be used as a coating for the support post because it would decompose and give off a gas and resulting signal. It would also tend to become noninsulating. No presently known organic material would in fact be capable of withstanding this high a temperature.
FIG. 5 of the drawings shows an alternative embodiment of the invention in which the invention is used with a thermistor type sensing element instead of a hot wire filament. In this embodiment, support posts 12b and 13b are used to support a relatively fine wire 25b which is connected to and carries a thermistor bead 25c which is cutomarily coated with a thin glass covering. The wire 25b and thermistor bead 25c are conventional. The opposite ends of the wire 25b may be spot welded to the support posts 12b and 13b.
When a thermistor is used, the length of the cavity and the wire 25b may be reduced, but it is still desirable to use a cavity having adiameter of less than 0.080
inches for time constant reduction.
With the present invention, economies can be achieved because it is possible to replace only the element and support posts instead of replacing the entire detector, as is required at the present time.
1. In a thermal conductivity analysis instrument having a sensing element mounted in a small diameter cavity in which said sensing element is held by a support post disposed closely adjacent to the metallic wall of said cavity, the invention which comprises a substantially permanent heat resistant electrically nonconductive coating covering at least the portion of said support post which is disposed adjacent to said cavity wall, said coating comprising discontinuous particles of glass or ceramic frit disposed on the exterior surface of said support post, said coating being adapted to prevent electrical discharge between said sensing element and cavity due to movement of said support post into close proximity or physical contact with said cavity wall.
2. The structure described in claim 1, said coating forming an integral part of said support post.
3.-The structure described in claim 1, in which the exterior surface of said support post is oxidized to prosupport post is formed of Kovar iron-nickel alloy.