Sealing device for sealing a cold-end part of a thermocouple wire arrangement that is based on a mineral-insulated cable and thermocouple temperature sensing device

The invention is concerned with a sealing device for sealing a mineral-insulated cable that is used for sensing a temperature. The sealing device can seal a space between an outer sheath of the cable and at least one wire arranged inside the outer sheath. The invention is also concerned with a thermocouple temperature sensing device, i.e. a temperature sensing device based on a thermocouple which is provided in the form of a mineral-insulated cable .

A mineral-insulated cable (MIC) is an electric cable with an outer sheath that can be of tubular shape and that can be made of metal or an alloy. Inside the outer sheath at least one electrically conductive wire can be arranged and the space between the at least one wire and the outer sheath is filled with an electrically insulating mineral, like e.g. magnesium oxide powder or aluminum oxide. The insulating mineral can be a powder.

A mineral-insulated cable can be designed as a thermocouple cable by the choice of the material of the conductive wires inside the sheath and/or the outer sheath itself. One possible thermocouple is given by two wires comprising different nickel alloys, e.g. Nicrosil (NiCrSi) and Nisil (Nisi) . At one end of the miner al-insulated cable the two ends of the wires are electrically connected. This is the sensing end or hot end or hot-end part. At the other end of the mineral-insulated cable, the so-called cold end or cold-end part, the two wires must be connected to a measuring circuit in order to determine an electromagnetic force emf or an electric voltage that is generated by the thermocouple due to a temperature difference between the hot-end part and the cold-end part. In other words, only a relative temperature difference may be measured this way. For obtaining the absolute temperature of the hot-end part, the absolute temperature of the cold-end part must be known. Alternatively, the thermocouple wires of the mineral-insulated cable may be extended by means of a special thermocouple extension cable that also provides thermocouple properties. This allows for placing the temperature sensor for measuring an absolute temperature value away from the colt-end part onto a printed circuit board (PCB) of the measuring circuit. However, such an extension cable is expensive.

After cutting a mineral-insulated cable into length, the open ends of this cable must be sealed in order to keep the mineral powder inside the sheath and to prevent humidity from intruding the cable.

Such a sealed mineral-insulated cable is described in US 6,229,093 Bl, wherein the sealing of the cold end of the sheath of the cable comprises a liquid-tight sleeve for each wire projecting out of the sheath of the cable. However, for providing such sleeves, a sealing material must be applied to the wires and cured in order to seal the space between the wires and the sleeve. As sealing material, glass or epoxy raising or spinel is used. This requires the placing of the loose sealing material at the end of the mineral-insulated cable and curing the sealing material together with the end of the mineral-insulated cable which is a complex process.

Further sealing devices for mineral-insulated cables are de scribed in GB 1546883 A and GB 2227617 A.

US 4627744 A describes a thermocouple temperature sensor that is based on a mineral-insulated cable which in turn provides an electric resistor at the hot-end part of the mineral-insulated cable. A thermal noise of this resistor is used to calibrate the thermocouple-based temperature measurement.

For increasing the mechanical stability, a mineral-insulated cable may be arranged inside a support tube. Such a support tube may be a metal tube, e.g. a steel tube. It is an object of the present invention to provide a measurement of an absolute temperature value on the basis of a thermocouple that is provided in the form of a mineral-insulated cable.

The object is achieved by the subject matter of the independent claims . Advantageous embodiments with convenient and non-trivial further embodiments of the invention are specified in the following description, the dependent claims and the figures.

The invention provides a sealing device for hermetic sealing a cold end or a cold-end part of a thermocouple wire arrangement that is based on a mineral-insulated cable. The sealing device comprises a sealing element made of or containing an electrically insulating material. The sealing element can have the shape of a plate or a flat cylinder. The electrically insulating material can be, e.g., ceramics or glass. The sealing element comprises a respective through-hole for passing through a respective thermocouple wire of the mineral-insulated cable. A sealing ring that comprises metal or an alloy is arranged at an outer rim of the sealing element for fixing the sealing element to a sheath or a support tube of the mineral-insulated cable. As the sealing ring is made of metal or an alloy, the sealing ring may be fixed to the sheath or the support tube by soldering or welding. In other words, for sealing the mineral-insulated cable, the sealing element may be placed over the cold-end part of the thermocouple mineral-insulated cable such that the thermocouple wires are each passed through one individual through-hole of the sealing element. The sealing element may then be arranged at the end of the sheath or the support tube. There it may be fixed by soldering or welding the sealing ring to the sheath or the support tube. This provides for a gas-tight, hermetic sealing.

In order to make the measurement of an absolute temperature possible, the sealing device comprises an electric temperature sensing element for sensing an absolute temperature of the sealing device and thus the cold-end part. The temperature sensing element comprises an electric property that is tem perature-dependent. In other words, the value of the electric property is a function of the temperature. By arranging the sealing device at the cold-end part of the thermocouple min eral-insulated cable, an electric temperature sensing element is thus implicitly arranged at the cold-end part.

The invention provides the advantage that no extension cable is needed for extending the thermocouple wires from the miner al-insulated cable to the electronic temperature measuring circuit. As the electric temperature sensing element is arranged directly at the cold-end part of the thermocouple miner al-insulated cable, the absolute temperature value can be measured at the cold-end part.

The invention also comprises embodiments that afford additional technical advantages.

In one embodiment the temperature sensing element comprises a resistance sensor. The electrical property that can be evaluated for sensing the absolute temperature can be the resistance value. The advantage of using a resistance sensor is that temperatures higher than 500°C can be measured. The resistance sensor can be, e.g., an NTC resistor (NTC - negative temperature coefficient) or a PTC resistor (PTC - positive temperature coefficient) or a P100 resistor.

In one embodiment the temperature sensing element comprises an inductive temperature sensor and/or a capacitive temperature sensor. The electrical property that can be evaluated for sensing the absolute temperature can be the inductance value for the inductive temperature sensor and the capacitance value for the capacitive temperature sensor. The advantage of using an in ductive temperature sensor is that it can be connected in series to one of the thermocouple wires such that no additional connecting wire is needed. The advantage of using a capacitive temperature sensor is that it can be run at a frequency of an AC voltage (AC - alternating current) that can be superimposed to the voltage generated by the thermocouple wires at the cold-end part. The inductive temperature sensor can be, e.g.,r a choke or an electric coil . The capacitive temperature sensor can be, e.g., a capacitor.

In one embodiment the temperature sensing element is integrated into the sealing element. In other words, the temperature sensing element is surrounded by a material of the sealing element. This provides for a mechanical protection of the temperature sensing element .

In an alternative embodiment the temperature sensing element is attached to the sealing element. In other words, the temperature sensing element is arranged at or fixed to a surface of the sealing element. This provides for the advantage that the temperature sensing element may be installed later, after the sealing element has been produced. The sealing element and the temperature sensing element may thus be produced independently from each other .

The invention comprises several embodiments regarding the electrical connection of the temperature sensing element for connecting the sensing element to an electronic temperature measuring circuit.

In one embodiment the temperature sensing element comprises two electrical contacts (e.g. for plus and minus polarity), wherein one of the contacts is electrically connected to one of the thermocouple wires of the mineral-insulated cable and the other contact is electrically connected to one separate electrical connecting element. This embodiment is also referred to as the single-wire connection. The electrical connecting element is different from any of the thermocouple wires of the miner al-insulated cable. This provides for the advantage that only one additional electrical connecting element is needed for con necting the mineral-insulated cable to the electronic measuring circuit for sensing the absolute temperature. The connecting element can be a wire, e.g. a copper wire or an iron wire. In an alternative embodiment the temperature sensing element is connected in series to one of the thermocouple wires of the mineral-insulated cable and a connecting element (e.g. a wire) for connecting the mineral-insulated cable to the electronic measuring circuit. In other words, the temperature sensing element is connected to one of the thermocouple wires of the mineral-insulated cable and the connecting element is connected to the thermocouple wire over the sensor element. This provides for the advantage that no additional wire for connecting the temperature sensing is needed. This embodiment is also referred to as the inline or zero-wire connection. The connecting element for connecting the thermocouple wire to the measuring circuit can be a copper wire or an iron wire.

In an alternative embodiment the temperature sensing element comprises two electrical contacts (e.g. for plus and minus polarity) , wherein the temperature sensing element is connected to a four-wire connection which provides two wires at each of the electrical contact of the temperature sensing element. In other words, at each end of the contacts of the temperature sensing element two wires are provided, one for conducting a sensing current and one for measuring the resulting voltage. This reduces the influence of the local resistance of the contact points where the sensing current enters and leaves the wires.

In one embodiment the electrically insulating material of the sealing element comprises glass or ceramics. The sealing element thus provides a high melting point over 400°C. The sealing element is also mechanically stiff which is advantageous for holding the temperature sensing element fixed at a high temperature greater than 400°C.

In one embodiment the sealing element provides sealing surfaces or sealing areas in each through-hole. The sealing areas provide a surface made of a metal or an alloy. A thermocouple wire arranged in such a through-hold may therefore be connected to the sealing element by means of soldering or welding. This allows for a hermetic or gas-tight sealing. The said sealing ring also provides sealing surfaces or sealing areas with a surface made of a metal or an alloy. The sealing element may therefore hermetically seal the mineral-insulated cable by soldering or welding the sealing ring to the sheath or the support tube of the mineral-insulated cable. The hermetic or gas-tight sealing protects the mineral powder of the mineral-insulated cable against water and moisture.

The invention also comprises a thermocouple temperature sensing device that comprises a mineral-insulated cable that comprises thermocouple wires. The mineral-insulated cable is sealed at a cold-end part by a sealing device according to the invention. The thermocouple wires are connected to an electronic temperature measuring circuit over a respective electric conducting element, e.g. a wire. The conductive elements for the thermocouple wires can be of the same material, e.g. copper or iron. The measuring circuit is designed to determine an absolute temperature of the cold-end part by measuring an electric property and/or electric quantity of an electric temperature sensing element of the sealing device. An electric property can be the resistance and/or the inductance and/or the capacitance. An electric quantity can be an electric current and/or an electric voltage. The ther mocouple temperature sensing device provides the advantage that no thermocouple extension cable is needed for connecting the mineral-insulated cable to the measuring circuit. The meas urement of the absolute temperature takes place at the cold-end part by means of the temperature sensing element of the sealing device. The electronic temperature measuring circuit may comprise a microcontroller. The measuring circuit may determine the value of the at least one electric property and/or electric quantity and it may determine a value for the absolute temperature on the basis of a function that relates the at least one value of the electric property and/or the electric quantity to a value of the absolute temperature. The function can be designed as a look-up table.

In one embodiment the measuring circuit comprises a switch or switching element for alternatingly determining the temperature of the cold-end part (absolute temperature) and a temperature of a hot-end part of the mineral-insulated cable (relative tem perature) . This reduces interference between the two meas urements. The switching element can be based on at least one transistor .

In one embodiment the measuring circuit is designed to provide a supply voltage for the thermocouple wires and to modulate the measurement voltage by means of a sensing voltage for the temperature sensing element. The measurement voltage can be, e.g., a DC-voltage (DC - durating current) . With this embodiment the temperature of the hot-end part and the cold-end part of the thermocouple mineral-insulated cable can be measured at the same time .

In one embodiment the measuring circuit is designed to provide a supply current for the temperature sensing element and measure a voltage U resulting over the temperature sensing element and to determine the temperature of the cold-end part as a function of the measured resulting voltage U. The resulting voltage U can be measured by connecting the temperature sensing element to the measuring circuit by means of at least one conducting element, e.g. at least on wire, that can be made of copper or iron.

The invention also comprises the combinations of the described embodiments as long as the embodiments are not explicitly stated to be alternatives to each other.

In the following an exemplary implementation of the invention is described. The figures show:

Fig. 1 a schematic illustration of an embodiment of the inventive mineral-insulated cable;

Fig. 2 a schematic illustration of a sealing device provided in the cable of Fig. 1; Fig. 3 a schematic illustration of another embodiment of the cable ;

Fig. 4 a schematic illustration of a sealing device provided in the cable of Fig. 3;

Fig. 5 a schematic illustration of another embodiment of the cable ;

Fig. 6 a schematic illustration of a sealing device provided in the cable of Fig. 5;

Fig. 7 a schematic illustration of another embodiment of the cable with a support tube;

Fig. 8 a schematic illustration of a sealing device provided in the cable of Fig 7;

Fig. 9 a schematic illustration of a thermocouple temperature sensing device according to the invention, the device comprising a four-wire connection for a temperature sensing element;

Fig. 10 a schematic illustration of a thermocouple temperature sensing device according to the invention, the device comprising a single-wire connection for the temperature sensing element; and

Fig. 11 a schematic illustration of a thermocouple temperature sensing device according to the invention, the device comprising an inline connection or zero-wire connection for the temperature sensing element.

The embodiment explained in the following is a preferred em bodiment of the invention. However, in the embodiment, the described components of the embodiment each represent individual features of the invention which are to be considered inde pendently of each other and which each develop the invention also independently of each other and thereby are also to be regarded as a component of the invention in individual manner or in another than the shown combination. Furthermore, the described em bodiment can also be supplemented by further features of the invention already described.

In the figures elements that provide the same function are marked with identical reference signs.

Fig. 1 shows a mineral-insulated cable 10 that can serve as a thermocouple element for measuring a temperature at a hot-end part (not shown) of the mineral-insulated cable. The cable 10 can comprise an outer sheath 11 that can be made of a metal or an alloy. The sheath 11 can be made of, e.g., steel or Inconel (R) . The sheath 11 can have the shape of a tube. Inside the sheath 11, wires 12 can be arranged that may constitute a thermocouple. At the hot-end part, the wires 12 can be electrically connected, e.g. by soldering the wires 12 together.

At end 13 of sheath 11, which serves as the cold-end part of the thermocouple element, the wires 12 protrude out of the sheath 11. Thus, the wires 12 can be connected to electrical connecting elements (not shown) for connecting the thermocouple element to an electronic measuring circuit. The wire 12 can be of different material. For example, one of the wires 12 by be made of Nisi and the other of the wires 12 of NiCrSi. A space 14 between wires 12 and sheath 11 is filled with an insulating mineral 15. The mineral 15 can be provided in the form of a powder. The mineral 15 can be, e.g., aluminum oxide or magnesium oxide.

In order to seal hermetically the end 13 in order to protect the mineral 15 against humidity, a sealing device 16 is fixed to the end 13. The sealing device 16 alone is shown in Fig. 2.

The sealing device 16 comprises a sealing element in the form of a sealing plate 17 for covering or closing or sealing the open end 13 of sheath 11. The sealing plate 17 can be made of glass or ceramics. For each wire 12 the sealing plate 17 provides a through-hole 18. In or at each through-hole 18 a ring element 19 can be arranged or provided that can be attached, e.g., to an inner rim 20 of each through-hole 18. The inner rim 20 represents an edge of the through-hole 18. Each ring element 19 is of the basic shape of a hollow cylinder through which the protruding end of the wire 12 can be pushed or stuck. Inside each ring element 19 a contact surface 21 is provided. Each ring element 19 can be made of nickel or nickel silicone or nickel chrome silicone or another alloy that is compatible to wire 12 with regards to CTE (co efficient of thermal extension) and their electrical compat ibility. The compatibility can be found in simple experiments and depends on the planned operational conditions of the cable 10. The contact surface 21 allows for producing a mechanical bonding between the ring element 19 and the wire 12 inside ring element 19. For example, the wire 12 and the ring element 19 can be welded or soldered or brazed together. The contact surfaces 21 are therefore sealing areas.

An outer rim 22 of the sealing plate 17 can be surrounded by a sealing ring 23. The sealing ring 23 can comprise a metal and/or an alloy. The sealing ring 23 can be fixed to the sheath 11 by means of a mechanical bonding 24 similar to the mechanical bonding

24 of the wire and the ring element 19. For example, the sealing ring 23 can be welded or brazed or soldered to the sheath 11. The sealing ring 23 therefore provides a further sealing area.

The mechanical bonding 24 provides a watertight sealing such that no humidity and/or gas can intrude the sheath 11 and reach the mineral 15. The sealing device 16 may therefore hermetically seal the end 13 of mineral insulated cable 10.

The ring elements 19 can be of the shape of a tube with a length

25 greater than a thickness 26 of the sealing plate 17. Thus the ring elements 19 provide a support tube for the end of the wire 12.

Further, an outer surface 27 of each ring element 19 can be cone shaped thus that the mechanical bonding 24 between the wire 12 and the ring element 19 can be produced by welding or soldering or brazing with less heat energy at a far end 28 of the ring element 19 with regard to sealing plate 17.

Sealing device 16 can also comprise a temperature sensing element T. The sensing element T can be attached to a surface of the sealing plate 17 or the sensing element T can be partly or fully integrated into the material of sealing plate 17.

The temperature sensing element T can comprise an electrical property that is temperature-dependent. In other words, a value of the electrical property is a function of the absolute temperature of the sensing element. By measuring the electrical property or an electrical quantity that depends on the electrical property, the absolute temperature of the sensing element T can be measured. The sensing element T can be or comprise a resistance sensor (e.g., NTC or PTC) and/or an inductive temperature sensor (e.g. a choke) and/or a capacitive temperature sensor (e.g. a capacitor) .

Fig. 3 shows a mineral-insulated cable 16 that is built in a comparable way with regard to the mineral-insulated cable of Fig. 1. For this reason, only the elements that differ are explained.

The cable 10 shown in Fig. 3 and the corresponding sealing device 16 shown in Fig. 4 differ from the embodiment shown in Fig. 1 and Fig. 2 in that a length or thickness 28 of the sealing ring 23 is larger or greater than the thickness 26 of the sealing plate 17. Thus, a collar 29 is provided by sealing ring 23 such that the end 13 of sheath 11 is supported or covered or surrounded by the collar 29. The mechanical bonding 24 between sheath 11 and sealing ring 23 can be made more robust as a larger contact surface 30 is available as compared to the embodiment of Fig. 1 and Fig. 2.

The Sealing device 16 can also comprise a temperature sensing element T. The sensing element T can be attached to a surface of the sealing plate 17 or the sensing element T can be partly or fully integrated into the material of sealing plate 17. The sensing element T can be of the same type as described in connection with Fig. 1 and Fig. 2.

In Fig. 5 a mineral-insulated cable 10 is shown that is based on the embodiment of Fig. 3. The sealing device 16 of the cable 10 of Fig. 5 is shown in Fig. 6.

The difference between the embodiments of Fig. 5 and Fig. 6 on one side and Fig. 3 and Fig. 4 on the other side is that the ring elements 19 are provided as tubes with different length 25 in the embodiment of Fig. 5 and Fig. 6.

The sealing device 16 can also comprise a temperature sensing element T. The sensing element T can be attached to a surface of the sealing plate 17 or the sensing element T can be partly or fully integrated into the material of sealing plate 17. The sensing element T can be of the same type as described in connection with Fig. 1 and Fig. 2.

Fig. 7 illustrates a cable with a support tube 31 for mechanically supporting the sheath 11. The sheath 11 is arranged inside the support tube 31. The support tube 31 and the sheath 11 may be provided as two coaxially arranged cylinders that touch each other . A sealing device 16 may be fixed or connected to the support tube 31 in the already described manner. The sealing device 16 may be arranged at a distance to sheath 11 such that a gap or free space or cavity 32 is provided between the sealing device 16 on one side and the end 13 of the sheath 11 on the other side. Fig. 8 shows the sealing device 16 used for the cable 10 of Fig 7.

The sealing device 16 can also comprise a temperature sensing element T. The sensing element T can be attached to a surface of the sealing plate 17 or the sensing element T can be partly or fully integrated into the material of sealing plate 17. The sensing element T can be of the same type as described in connection with Fig. 1 and Fig. 2. Thus, the mineral-insulated cable 10 as shown in Fig. 1, Fig. 3 and Fig. 5 can be sealed at an colt-part open end 13 by means of a sealing device 16 according to Fig. 2, Fig. 4, Fig. 6 and Fig. 8. Each sealing device 16 is a termination element that consists of one external sealing ring 23 with the same dimensions as the end 13 of the cable or sheath 11 or the support tube 31 and with two tubes as ring elements 19 to pass wires or conductors going through. The elements can be welded or soldered to fix the sealing device 16 to the wires 12 and the sheath 11.

This provides a simple way to seal a mineral-insulated cable by an extension part that can be manufactured independently and can be provided as ready-made or pre—manufactured part. A further advantage is that by choosing the material of the ring elements 19 and of the sealing ring 23, CTEs can be adapted to the materials used for the wires 12 and the sheath 11. Moreover, as no melting of sealing material (like glass or ceramics) is needed close to the mineral powder (like magnesium oxide) of the miner al-insulated cable, no gas evaporates from the mineral powder which would result in bubbles inside the molten sealing material.

By providing a temperature sensing element T, the absolute temperature of the cold-end part (end 13) of the miner al-insulated cable can be measured. Together with sensing the voltage difference or electromagnetic force emf between the wires 12, which provides a value for the relative temperature between the cold-end part and the hot-end part of the mineral insulated cable 10, the absolute temperature of the hot-end part can be determined. Thus, there is no need to use a thermocouple extension cable for connecting the wires 12 to an electronic measuring circuit .

Fig. 9, Fig. 10 and Fig. 11 illustrate, how the wires 12 and the temperature sensing element T may be electrically connected to an electronic temperature measuring circuit 33.

Fig. 9 illustrates a four-wire connection 34 for connecting the temperature sensing element T to the electronic measuring circuit 33. The sensing element T may comprise 2 electrical contacts 35 to each of which to wires 36, 37 can be connected. A current source 38 may drive a sensing current I through wires 36. An electric voltage U can be measured between wires 37. The voltage U is a function of the absolute temperature of sensing element T. The voltage U can be received by an analog-digital-converter 39 of measuring circuit 33 a microcontroller 40 may evaluate the received voltage value of voltage U. Temperature sensing element T can be, e.g., an NTC or a PT100 resistor. The wires 36, 37 can be wires made of copper or iron. The wires 36, 37 can be of the same material.

The thermocouple wires 12 of the mineral-insulated cable 10 can be connected to measuring circuit 33 by wires 41. The wires 41 for each wire 12 can all be of the same material, e.g., copper or iron. Generally, wires 41 do not need to provide thermocouple properties .

Fig. 10 illustrates a 1-wire connection 42 of temperature sensing element T with the measuring circuit 33. In other words, only one additional wire 37 is used for measuring voltage U. One of the wires 41 that is used for connecting the thermocouple wires 12 to the measuring circuit 33 is used as a second wire. The temperature sensing element T can be, e.g., an NTC resistor or a PT 100 resistor.

Fig. 11 illustrates a 0-wire connection, which means, that the temperature sensing element T is connected to the measuring circuit 33 by the same wires 41 that are also used for connecting the thermocouple wires 12 to the measuring circuit 33. The temperature sensing element T used for this connection 43 can be, e.g., an inductance temperature sensor. Measuring circuit 33 may comprise a switching element 44 for switching between the measurement of voltage U and the measurement of the voltage or electromagnetic force generated by the thermocouple wires 12. For measuring the voltage U, a driving circuit 45 may be used for creating a resonance effect at a resonant frequency that depends on the inductance a value of sensing element T and a capacitor C and a resistor R of a driving circuit 45. The measurement of voltage U for deriving the absolute temperature of sensing element T is therefore performed at a different frequency than the measurement of the voltage difference between the ther- mocouple wires 12.

Thus, the temperature measurement is possible at the thermocouple cold-end on the basis of sealing device 16. This avoids the usage of expensive extension thermocouple wires for connecting wires 12 to the measuring circuit 33. Additionally, sealing device 16 provides a simple way to seal hermetically the mineral-insulated cable. By choosing the materials, different CTEs can be con sidered . Overall, the examples show how the invention can provide a mineral insulated cable sealing by a termination part with glass or ceramcis and with an integrated temperature sensor.