GB2406967A - Ceramic thermocouple unit - Google Patents

Abstract

A thermocouple head unit 6 comprises an insulating housing 10 formed from ceramic material. The housing 10 has ports for metal thermocouple signal terminals 7, the terminals 7 having a one piece construction. The terminals 7 are joined to the housing 10 using a ferrule 13, which is brazed to a metalised portion of the ceramic housing 10. The housing 10 is attached to a metal flange 11 via a second ferrule 14, the flange 11 being used to attach the thermocouple unit 6 to a surface.

Description

Ceramic Thermocouple Unit The present invention is concerned with

thermocouple units.

In particular, it is concerned with the mounting terminal head of a thermocouple unit.

Thermocouples typically comprise two wires of different metals joined at their ends to form a loop. A temperature difference between the joined ends unbalances the contact potentials and causes a current to flow around the loop. The difference in temperature between the two ends (the hot and cold ends) can be determined by measuring the current flowing in the loop, or more usually the potential difference set up when the circuit is open. If the temperature of the cold end is known, one can then determine the temperature of the hot end (or vice versa).

A thermocouple unit consists of one, two or more thermocouple elements. A typical thermocouple element is mineral insulated and protected by a metal sheath. The measuring end of the thermocouple element is placed in a gas stream, while the other end is placed in a thermocouple head with cold ends of the wires connected to a measuring circuit. When two or more elements are used, they are normally connected in parallel to provide average temperature measurements. The elements' wires are connected together inside the thermocouple head and then connected to two output terminals made from the same material as the wires. Several thermocouple units can be connected together using a harness connected to the unit terminals.

The thermocouple head has to fulfill the following functions: - to protect fragile thermocouple wires - to house additional components such as ballast resistors - to provide mounting for the thermocouple unit.

- to provide connection to the harness using terminals As discussed above, the loop is typically enclosed within a protective tube or sheath. The sensing (typically hot) end of the loop is placed in the location where temperature is to be measured (e.g. a gas stream in an aircraft engine or a gas turbine). The other remote or measuring end of the loop is held within a thermocouple head unit which includes means for connecting the wire ends to measuring circuitry. Typically the head also acts as a mounting head and is mounted on the outside of the element enclosing the location which temperature is being measured or monitored (e.g. the outside of an aero engine or gas turbine).

Known thermocouple heads (see figure 2 and, e.g., US 3,405,225) are made from metal. The head of a typical thermocouple head unit such as that shown in figure 2 is manufactured from stainless steel welded to a stainless steel flange. The thermocouple signal terminals for connecting the cold, measuring or remote end of the thermocouple to the measuring or monitoring circuitry are vacuum brazed metal/ceramic assemblies welded into ports in the body. The known arrangements have ceramic insulation between the thermocouple terminals and the thermocouple head body. The known thermocouple heads include a high number of separate parts and their production or manufacture is multi- operation with vacuum brazing and TIC (Tungsten Inert Gas) welding.

They are therefore relatively difficult and expensive to make.

The present invention provides a thermocouple unit as defined in claims 1 and 5 to which reference should now be made.

Preferred features of the invention are defined in dependent claims 2 to 4.

The present invention includes less parts than the known systems and is simpler to assemble. There is no need to provide insulation between the thermocouple terminals and the thermocouple head body.

Preferred embodiments of the invention will be described, by way of example only, with reference to the attached figures.

The figures and accompanying description are only for the purposes of illustrating one or more preferred embodiments of the invention and are not to be construed as unifying the invention, limiting the invention or limiting the appending claims. The skilled man will readily and easily envisage alternative embodiments of the invention.

In the figures: Figure 1 is a diagram illustrating a system for monitoring temperature using a thermocouple having a pair of thermocouple elements; Figure 2 illustrates a known thermocouple head construction; and Figure 3 illustrates an embodiment of the invention.

Referring to figure 1, this shows in schematic form a thermocouple unit 1 having two thermocouple elements 2 of, e.g., type K. In use, the pair of thermocouple elements is housed in a probe 3. The probe 3 is mounted, for example, in an aero engine or gas turbine such that the tip 4 constitutes a hot junction located inside the gas stream in the engine or turbine to measure the gas temperature in the engine or turbine. For an aero engine the temperature might typically be of the order of 800 C to 1200 C.

At the other end 5, the thermocouple wires terminate in a head 6 mounted on the exterior of the, e.g., aero engine or gas turbine. For an aero engine, the outside is typically at a much lower temperature (of the order of 300 C to 600 C)to the inside.

The thermocouple wires 2 are connected to thermocouple terminals 7 in the thermocouple head 6. Wires 8 are connected to the terminals to connect the thermocouple wires to measuring circuitry 9. The measuring circuitry determines the temperature difference between the tip 4 and the cold junction of the thermocouple from the voltage or potential difference between the thermocouple wires.

A number of thermocouple units may be connected in parallel in the manner described in GB 2,344,892.

The known thermocouple heads (see figure 2) comprise a metal (e.g. stainless steel) body 10 welded to a metal (e.g. stainless steel) flange 11. The thermocouple signal terminals 7 are vacuum brazed metal/ceramic assemblies welded into ports on the metal body. The ceramics in the terminal assemblies insulate the signal output of the thermocouple elements.

The vacuum brazed signal terminal subassemblies for providing connections to wires and hence to the analytical circuitry are located and TIC welded into ports in the head 6. The terminal connection bushes are insulated from the metal thermocouple head body using a ceramic collar 12.

A thermocouple head embodying the present invention is shown in figure 3. Where appropriate, the same reference numerals to those of figures 1 and 2 are used to denote the equivalent elements. This head unit has a ceramic body 10' suitable for high temperature operation such as high purity Alumina or Alumina and Zirconia and the thermocouple signal terminals are directly inserted in ports in the ceramic body.

The terminals 7 are made from the same material as the thermocouple wires to which they are joined to the body using a ferrule (ring). For a type K thermocouple element (K+ and K-), one terminal is made from e.g. Alumel_ (Alumel is a metal based alloy containing about 5 per cent Aluminium Alumel is a trade mark of Concept Alloys LLC and has the Alumel_ wire(s) connected thereto while the other is made from, e.g. Chromel_ and has the Chromel_ (Chrome! is a nickel-based alloy containing about 10% chromium Chromel is a trade mark of Concept Alloys LLC wire(s) connected, thereto. The ferrule is made from a metal such as Kovar_ (an iron based alloy with nickel and cobalt - Kovar is a trade mark of Kovar GmbH) that has a small expansion coefficient matching that of the ceramics used to make the body.

A part of the ceramic body 10' is metalised - using e.g. the well known molybdenum manganese process during the body manufacturing. Then the ferrule 13 is brazed to this part of the ceramics. The ferrule is flexible to accommodate differential expansion of the metal terminals and the body.

A similar technique is used to attach the body to the metal flange 11. In this case a larger ferrule 14 is attached to the ceramic body lO'using brazing or welding process, next the flange 11 is brazed to the metal ferrule 14.

The ceramic body 10' attached to a metal flange 11 which may be screwed, bolted or otherwise attached to a surface (e.g. the outside of a turbine casing).

As discussed above the conductive materials which make up the two thermocouple wires (e.g. Chromel and Alumel) are each used to make a terminal 7. Each terminal is one half of a male/female connection. A terminal 7 could be in the form of a bolt or a threaded socket (hole). For bolt terminals a harness is attached using nuts, for socket terminals a harness is attached using bolts. In the embodiment of figure 3, the terminals 7 are female and are capable of receiving male (bolt)connectors. In an alternative embodiment, they could be male and for insertion into female connectors.

A protective sheath 16 surrounds the different wires 16, 17 of a K-type thermocouple element 2. The wires of the thermocouple element 2 pass through the surface on which the head 6 is mounted and finish at the sensing tip 4 (not shown in figure 3) at the location which temperature is being monitored. The wires of the two thermocouple elements are connected in parallel to respective thermocouple terminals 7. '

Claims (5)

1. A thermocouple head unit for connecting a thermocouple to measurement or monitoring apparatus or circuitry, comprising: a body made from an insulating material and having a port or ports for receiving one or more thermocouple signal terminal elements, one or more conductive thermocouple signal terminals in said port or ports, the signal terminals each having a first portion for connection to a thermocouple wire and a second portion, electrically connected to the first portion, for connection to said measurement or monitoring apparatus or circuitry.

2. A head unit according to claim 1 wherein the head unit body is made from a ceramic material.

3. A head unit according to any preceding claim wherein the terminal has a one piece construction.

4. A head unit according to any preceding claim wherein the terminal is made from metal.

5. A head unit substantially as hereinbefore described with reference to figures 1 and 3.