GB2306663A

GB2306663A - Probe with a heat-insulating neck part

Info

Publication number: GB2306663A
Application number: GB9621164A
Authority: GB
Inventors: Holger Schroeter; Kerstin Borchers
Original assignee: Gestra AG
Current assignee: Gestra AG
Priority date: 1995-10-27
Filing date: 1996-10-10
Publication date: 1997-05-07
Anticipated expiration: 2016-10-10
Also published as: DE19540035B4; GB2306663B; DE19540035A1; GB9621164D0

Description

Title: PROBE WITH A HEAT-INSULATING NECK PART The invention relates to a probe for monitoring medium in a container.

Very high operating temperatures, for example 300"C, sometimes prevail in containers on which such probes are to be used. There is often electronic circuitry in the top housings of the probes. The maximum permitted temperature for use of the top casing is therefore significantly lower than the operating temperatures of the containers, for example 70"C.

A known probe of this type (DE-CM 82 16 323) has, between the fastening casing and the top casing, a neck part which serves to provide a temperaturereducing interval between the container and the top casing. The radial cooling fins on the neck part of the known probe have not been found to be advantageous.

The fitting space available for the probes limits the length of the neck part. It has been found that the necessary low temperature is not attained at the top casing within the existing length limits at high operating temperatures despite the normal heat insulation of the container.

The present invention aims to provide a probe which is also suitable for use on containers having high operating temperatures.

According to the invention, there is provided a probe for monitoring medium in a container with a fastening casing for arranging the probe on the container, at least one sensor provided on the fastening casing, a top casing spaced apart from the fastening casing and a neck part connecting the top casing to the fastening casing and through which at least one electrical conductor extends, wherein the neck part has a first length portion which is adjacent to the fastening casing and consists of a material having temperature resistance suitable for high operating temperatures in the container and a second length portion which adjoins the end of the first length portion remote from the fastening casing and consists of a material of which the heat conductivity is lower than that of the material of the first length portion.

The first length portion of the neck part adjacent to the fastening casing has the necessary dimensional stability even if the container and therefore the fastening casing are at high temperatures. A temperature reduction occurs on the first length portion at its end remote from the container due to the discharge of heat to the environment. The second length portion also has the necessary dimensional stability at the lower temperature. Owing to the heat conductivity which is much lower than in the first length portion, a further, particularly great drop in temperature occurs over the remaining length of the neck part. The pronounced drop in temperature at the neck part results in reduced heat convection in the region of the neck part. All in all, a particularly great temperature difference is achieved between the container and the top casing.The probe is therefore equally suitable for use on containers with a low operating temperature and on those with a particularly high operating temperature.

Preferably, the neck part has a transverse wall spaced from its ends, the conductor(s) extending through said wall. The transverse wall divides the interior of the neck part into a container-side compartment and a top-casing-side compartment. Convection currents in the air in the neck part are restricted to the respective compartment and this further increases the temperature difference between the container and the top casing. Low heat conduction through the transverse wall is particularly advantageous if the transverse wall consists of a material of which the heat conductivity is lower than that of the material of the first length portion.

A passage for the conductor(s) may be provided in the transverse wall which is sealed and the first length portion may have a pressure-compensating orifice connecting its interior to the environment of the neck part. Should medium from the container penetrate the neck part, the medium is prevented from entering the top casing and causing disturbances or damage there by the sealed transverse wall.

The pressure-compensating orifice prevents a rise in pressure in the first compartment so the pressure-resistance of the transverse wall and of the conductor seal do not have to meet high requirements. The transverse wall may further have, in the region of the passage, a receiving chamber in which a casting compound is introduced as packing. Such a conductor seal is particularly advantageous in terms of production. For example, medium can penetrate the neck part if the sensor or its seal is not completely tight to diffusion against the medium in the container or if a leak occurs.

It is particularly desirable in terms of production and assembly if the transverse wall and the second length portion are combined to form a single component.

The transverse wall may form the end of the second length portion adjoining the first length portion. This arrangement is particularly advantageous for low heat convection in the neck part; this applies, in particular, if the second length portion extends over the remainder of the length of the neck part.

Alternatively, the second length portion may be designed as a spacer ring located between the first and a third length portion of the neck part. The second length portion is in this case restricted to a small proportion of the total length of the neck part but a great temperature difference between the container and the top casing is nevertheless achieved. The restricted overall length of the second length portion allows the use of a relatively expensive material for the second length portion without increasing the overall cost of the neck part. The first and third length portions of the neck part may consist of metallic tubes which are particularly simple and inexpensive to produce.

The neck part may further have, between the top casing and the third length portion, a fourth length portion which consists of a material of which the heat conductivity is lower than that of the material of the third length portion. This leads to a further increase in the temperature difference between the container and the top casing in that heat conduction in the contact region of neck part and top casing is reduced.

The transverse wall and/or the second and fourth length portions desirably consist of polyether ether ketone (PEEK). This is a particularly advantageous material for the regions of the neck part which are to exhibit particularly low heat conduction. The specified plastics material is distinguished not only by particularly low heat conductivity but also by relatively high temperature resistance.

According to a preferred embodiment of the invention, a first electrical conductor connected to the sensor is arranged in the first length portion of the neck part and a second electrical conductor is arranged in the remainder of the neck part and extends to the first length portion, the two conductors being provided with mating electrical plug-in connections at their mutually adjacent ends.

The plug-in connections between the electrical conductors in the first and second length portion of the neck part provide an electrical connection for the sensor which is particularly convenient to assemble.

The invention will now be further described, by way of example, with reference to the drawings, in which: Fig. 1 is a vertical section through one embodiment of a probe according to the invention; and Fig. 2 is a vertical section through a second embodiment of a probe according to the invention.

In Figs. 1 and 2, the respective probe has a fastening casing 1 with which it is arranged on a container 2 having external heat insulation 3. A sensor 4 which extends into the container 2 and with which the level of the medium in the container 2 is to be monitored is provided on the fastening casing 1. Outside the container 2, the fastening casing 1 carries a neck part 5 which extends away from the container 2, projects from the heat insulation 3 and on whose end remote from the container a top casing 6 is arranged. Two electrical conductors 7, 8 extend through the neck part S into the top casing 6. They are connected to an electronic circuit unit 9 in the casing 6 which in turn is connected to an electric connecting line 10.

The tubular neck part 5 in Fig. 1 has two length portions 11, 12 of different materials. The first length portion 11 adjacent to the fastening casing 1 consists of a metallic tube portion having a pressure-compensating orifice 13 outside the heat insulation 3. The container-side end of the first length portion 11 is held firmly on a peg-shaped end of the fastening casing 1. The second length portion 12 which is also tubular but is adjacent to the head casing 6 consists of plastics material, preferably polyether ether ketone (PEEK). The end of the second length portion 12 facing the top casing has a base 14 on which the head casing 6 is fastened (fastening means not shown). The base 14 is provided with a central through-orifice 15 for the two conductors 7, 8.The other end of the second length portion 12 is held firmly by its peg-shaped end in the end of the first length portion 11 remote from the container. At that end, the second length portion 12 has a transverse wall 16 through which the two conductors 7, 8 extend. A receiving chamber 17 provided in the transverse wall 16 on the side facing the top casing 6 is filled with a casting compound 18, preferably silicone. The end of the conductor 8 projecting toward the first length portion 11 is designed as a plug 19.

The sensor 14 is connected to an electrical conductor 20 having a contact socket 21 at its end remote from the container in the first length portion 11.

During assembly, the conductor 7 (earth potential) is initially connected to the first metallic length portion 11 in an electrically conductive manner, for example by welding. The conductor 8 is inserted through the transverse wall 16 into the second length portion 12 until the plug 19 has reached its proposed position. The conductor 7 is also pulled through the transverse wall 16. The two length portions 11, 12 are then connected to one another, preferably by adhesion. The casting compound 18 is introduced into the receiving chamber 17; this can be achieved through the through-orifice 15, for example by means of a portioning syringe. The neck part 5 is then placed on the fastening casing 1, producing a firm, electrically conductive connection between the two parts 1, 5.At the same time, the plug 19 and the contact socket 21 come into engagement and produce a reliable electrical connection between the sensor 4 and the conductor 8. The top casing 6 is finally fastened on the neck part 5.

If a high operating temperature prevails in the container 2, the heat insulation 3 prevents the undesirable discharge of heat from the container 2 to the environment. However, the fastening casing 1 and the adjoining first length portion 11 heat up due to heat conduction; their materials have the necessary dimensional stability even at high operating temperatures. As the interval from the container 2 increases, the temperature of the first length portion 11 drops due to the discharge of heat to the environment. At its end remote from the container, the temperature has a value at which the material of the second length portion 12 also has the necessary dimensional stability. Owing to its heat conductivity, which is significantly lower than that of the first length portion 11, a correspondingly great drop in temperature occurs on the second length portion 12.

A continuous convection current cannot flow between container 2 and top casing 7 in the interior of the neck part 5 owing to the transverse wall 16. Rather, the convection current in the interior 22 of the first length portion 11 is separated from that in the interior 23 of the second length portion 12. The low heat conductivity of the transverse wall 16 also limits the heat exchange between the two interiors 22, 23. The pronounced drop in temperature produced by the second length portion 12 on the neck part 5 also reduces the convection current externally along the neck part 5. All these factors together mean that the low temperature required for perfect operation over a prolonged period prevails in the container 2 on the top casing 6 despite high operating temperatures.

The casting compound 18 closes the passage for the two conductors 7, 8 through the base 14 so the two interiors 22, 23 of the neck part 5 are sealed from one another in a medium-tight manner. Should medium pass from the container 2 into the interior 22 - for example because the seal of the sensor 4 in the fastening casing 1 is not completely tight to diffusion of the medium in the container or a leak has occurred - the penetrating medium can escape through the pressurecompensating orifice 13 outwardly into the environment of the neck part without a significant rise in pressure occurring in the interior 22. The base 14 which is tight to medium prevents medium from entering the interior 23 and passing from there into the top casing 6. Therefore, it cannot interrupt operation in the top casing 6 or damage the circuit unit 9.As there is no discernible rise in pressure in the interior 22, the base 14 and the casting compound 18 do not have to have significant resistance to pressure.

In Fig. 2, the tubular neck part 5 of the probe has four length portions 11, 24, 25, 26. Like the first length portion 11, the third length portion 24 also consists of a metallic tube portion. The mutually facing ends of these two length portions 11, 24 are spaced apart from one another. The second length portion 25 is designed as a spacer ring between the first and third length portions and two conductors 7, 8 extend through the a transverse wall 16 of the portion 25 which is also provided with a receiving chamber 17 and a casting compound 18. The fourth length portion 26 has a base 14 with a through-orifice 15 and is arranged as a spacer member between the top casing 6 and the adjacent end of the third length portion 24. The second and fourth length portion 25, 26 both consist of plastics material, preferably polyether ether ketone (PEEK).

Once the conductor 7 (earth potential) has been connected in an electrically conductive manner to the first, metallic length portion 11 in the probe according to Fig. 2, the two conductors 7, 8 are drawn through the transverse wall 16 of the second length portion 25. Once the plug 19 in the transverse wall 16 has reached the proposed position, the second length portion 25 is inserted into the first length portion 11 by one peg-shaped end and the casting compound 18 poured into the receiving chamber 17. The third length portion 24 is then placed on the other peg-shaped end of the second length portion 25 and the fourth length portion 26 inserted into the third length portion 24 by its peg-shaped end. The four length portions 11, 24, 25, 26 of the neck part 5 are firmly connected to one another, for example by adhesion.The remainder of the assembly corresponds to that described with reference to Fig. 1.

The first and third length portions 11 and 24 can be produced very inexpensively from common semifinished tubing. The second and fourth length portions 25 and 26 can easily be produced from semifinished rods by turning, particularly in small numbers. The amount of material required for them is relatively small owing to their relatively small lengths in comparison with the overall length of the neck part 5, making the material costs relatively low. As the receiving chamber 17 is readily accessible, introduction of the casting compound 18 is particularly simple.

The neck part 5 is therefore particularly convenient to produce and inexpensive.

The two length portions 25, 26 with particularly low heat conductivity in combination with the other two length portions 11, 24 mean that the necessary great temperature difference also exists between the container 2 and the top casing 6 in this probe.

As an alternative to the illustrated vertical arrangement of the neck part, the probe can also be fitted with an obliquely or horizontally extending neck part (not shown) if necessary. Furthermore, the invention is not restricted to probes with level sensors but is also suitable for probes with other sensors for monitoring medium in a container such as temperature sensors, pressure sensors, conductivity sensors.

Claims

1. A probe for monitoring medium in a container with a fastening casing for arranging the probe on the container, at least one sensor provided on the fastening casing, a top casing spaced apart from the fastening casing and a neck part connecting the top casing to the fastening casing and through which at least one electrical conductor extends, wherein the neck part has a first length portion which is adjacent to the fastening casing and consists of a material having temperature resistance suitable for high operating temperatures in the container and a second length portion which adjoins the end of the first length portion remote from the fastening casing and consists of a material of which the heat conductivity is lower than that of the material of the first length portion.

2. A probe according to claim 1, wherein the neck part has a transverse wall spaced from its ends, the conductor(s) extending through said wall.

3. A probe according to claim 2, wherein the transverse wall consists of a material of which the heat conductivity is lower than that of the material of the first length portion.

4. A probe according to any one of the preceding claims, wherein a passage for the conductor(s) is provided in the transverse wall which is sealed and the first length portion has a pressure-compensating orifice connecting its interior to the environment of the neck part.

5. A probe according to claim 4, wherein the transverse wall has, in the region of the passage, a receiving chamber in which a casting compound is introduced as packing.

6. A probe according to any one of the preceding claims, wherein the second length portion and the transverse wall are combined to form a single component.

7. A probe according to claim 6, wherein the transverse wall forms the end of the second length portion adjoining the first length portion.

8. A probe according to one of the preceding claims, wherein the second length portion consists of polyether ether ketone (PEEK).

9. A probe according to any one of the preceding claims, wherein the second length portion is designed as a spacer ring located between the first and a third length portion of the neck part.

10. A probe according to claim 9, wherein the third length portion of the neck part consists of a metallic tube.

11. A probe according to claim 9 or claim 10, wherein the neck part has, between the top casing and the third length portion, a fourth length portion which consists of a material of which the heat conductivity is lower than that of the material of the third length portion.

12. A probe according to claim 11, wherein the fourth length portion consists of polyether ether ketone (PEEK).

13. A probe according to claim 3, wherein the transverse wall consists of polyether ether ketone (PEEK).

14. A probe according to any one of the preceding claims, wherein the first length portion of the neck part consists of a metallic tube.

15. A probe according to any one of the preceding claims, wherein a first electrical conductor connected to the sensor is arranged in the first length portion of the neck part and a second electrical conductor is arranged in the remainder of the neck part and extends to the first length portion, the two conductors being provided with mating electrical plug-in connections at their mutually adjacent ends.

16. A probe substantially as described herein with reference to the drawings.