GB2203550A - Electromagnetic interference suppression filters for thermocouple wires - Google Patents

Abstract

In a thermocouple system wherein two different emi suppression filters (16c, 16d) are employed to couple the dissimilar wires (12', 14') of a thermocouple (10') through an equipment bulkhead (19) to the equipment internal circuitry each filter provides a continuous d.c. path (21,23) through that filter of the same material as the respective thermocouple wire (12',14') to which it is connected. The electrical connections (A',C') between the terminations of the d.c. paths (21,23) through the filters and the thermocouple wires are made by mechanical means, as opposed to soldering, whereby to limit the generation of thermal emf's at said connections. <IMAGE>

Description

DESCRIPTION ELECTROMAGNETIC INTERFERENCE SUPPRESSION FILTERS FOR THERMOCOUPLE WIRES.

The present invention is concerned with filters of the type used for suppressing electromagnetic interference (EMI) in measurement systems employing thermocouples.

Electronic equipment and its circuitry is frequently subjected to the effects of electromagnetic interference. This may occur as direct electromagnetic radiation upon the equipment, or as conducted currents introduced to the equipment via interconnecting wires and cables.

A number of means are currently available whose purpose is to eliminate or minimise the effects of electromagnetic interference, which may extend over the frequency range of a few Hz, to tens of GHz.

Thus, for example, the effects of direct radiation from electromagnetic fields on circuitry may be reduced by endorsing or shielding the circuitry within a grounded conductive enclosure. Openings into the enclosure, e.g. for ventilation, may be protected by various means, for example the use of conductive honeycomb mesh.

Conducted interference present as circulating currents upon wires leading into an equipment may be reduced or diverted by the use of line filters. Such filters contain parallel capacitive and/or series inductive elements, intended to provide low shunt admittance or high series impedance to the currents which require suppression. Such filters are often low pass types, but may also be band-pass or band-stop types.

The present invention relates to a particular application ofxelectromagnetic interference (emi) suppression filters, where it is required to minimise the effects of interference upon thermocouple wires.

Thermocouple measurements rely on the Seebeck effect, whereby the Seebeck emf is measured, being an indication of temperature. The measurement technique relies on the generation of an emf between two dissimilar metals, and the comparison of that emf with the emf from a reference junction, usually in a differential measuring circuit.

There are- a number of possible sources of errors in such measurement circuits, particularly where intermediate metal/metal junctions are interposed.

Also, intermediate junctions may be a necessity when thermocouple wires are taken into an enclosure, especially where it is necessary to introduce emi suppress ion filters to reduce the possible undesirable effects of conducted interference upon the thermocouple wires.

Fig.l of the accompanying drawings shows part of a typical conventional thermocouple circuit where a chromel-alumel thermocouple pair is passed into an enclosure. The thermocouple junction is indicated by reference numeral 10, the wire 12 being chromel and the wire 14 being alumel. Conventional pi-section emi suppression filters 16a,16b are used as a means of securing the thermocuple wires 12,14, introducing the wires to the enclosure, and also providing emi suppression. These conventional pi-section filters 16a,16b have copper terminations with a silver finish.

It will be noted that Chromel/Silver and Alumel/Silver couples exist at the incoming junctions A and C, although the silver may commonly be replaced by copper, tin, gold, tin-lead alloys or other metals. Thermocouple pairs other than chromel/alumel would produce similar effects. Furthermore, second junctions B and D exist at the interior of the equipment, between the emi suppression filters, and the internal circuitry (not shown).

Potential temperature measurement errors occur due to: a) Differences in emf characteristics with temperature of the chromel/silver (or other metal) and alumel/silver (or other metal) couples. An error may occur even when all junctions are held at the same temperature, and also when only two junctions are held at fixed temperatures.

b) Differences in temperature between the equipment exterior and equipment interior junctions.

Substantial errors may occur due to temperature differences between the two junctions, whether or not the junctions are of similar metals. Much avionic equipment for instance may experience temperature differentials between exterior and interior of at least 30"C.

It is an object of the present invention to overcome the problems outlined hereinbefore.

In accordance with the present invention in its broadest aspect, two different emi suppression filters are employed to couple the dissimilar wires of a thermocouple through an equipment bulkhead to the equipment internal circuitry, each filter providing a continuous d.c. path through that filter of the same material as the respective thermocouple wire to which it is connected.

Preferably, the electrical connections between the terminations of said d.c. paths through the filters and said thermocouple wires are all made by welding or by a mechanical means, as opposed to soldering using a dissimilar metal, whereby to limit the generation of thermal emf's at said connections.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Fig.l shows a typical example of a known thermocouple circuit having emi suppress ion using pi-section filters; Fig.2 is a longitudinal cross-section of one embodiment of an emi suppression filter in accordance with the present invention; and Fig.3 shows a thermocouple circuit in accordance with the present invention.

As indicated diagrammatically in Fig.3, in the illustrated thermocouple circuit in accordance with the present invention, the two dissimilar wires 12',14' (e.g. of chromel and silver) leading to a thermocouple junction 10' are connected respectively to two filter devices 16c,16d mounted in a bulkhead wall 15, the filters having continuous d.c.

through-paths defined by metal pins 21,23 made of the same metal as the wires 12',14' to which they are connected. As explained in more detail hereinafter, the connections made between the wires 12',14' and the pins 21 at A' and C', and preferably also at B' and D' inside the enclosure, are made mechanically by crimping, clamping or welding.

The filter illustrated in Fig.2 comprises either a chromel or alumel wire 18 around which are coaxially disposed a tubular ferrite inductor 20, a tubular capacitor 22 having inner and outer metalised surfaces, and a tubular outer housing of a metal such as brass. The outer periphery of the brass housing is screw-threaded at 24 and carries a fixing nut 26 and spring washer 28. Crimp fitted around the lead wire 18 at both ends of the ferrite inductor 20 are a pair of gold-plated brass ferrules 30a,3Cb which have enlarged end portions lying within overhanging end portions of the tubular capacitor 22 and abutting the two ends of the ferrite inductor, respectively. The two ends of the capacitor are sealed by respective masses of an epoxy resin indicated at 32 and 34. The outer circumferences of the enlarged end portions of the brass ferrules are soldered to the surrounding inner surfaces of the tubular capacitor 22, the outer periphery of the tubular capacitor being soldered to the inner periphery of the brass housing 24.

It will be noted that a continuous d.c. path of thermocouple material, i.e. chromel or alumel, exists through the filter. As a result, no thermocouple emf's are generated when similar thermocouple wires are connected to the filter. This applies to both the internal and external junctions A',B',C' and D' in Fig.3 (equivalent to A, B, C and D in Fig.l), whether or not there is a large temperature difference between internal and external junctions.

It should, of course, be noted that where other thermocouple metals are used, e.g. platinum/rhodium alloys, the metal of the wire 18 would be changed accordingly, i.e. would be platinum or rhodium in that example.

In order to avoid the introduction of thermal emf errors, it is preferred that no additional surface finish to the terminations of the wire 18 be employed, e.g. as would normally be applied for corrosion protection, or improving solderability.

In use of the present filters, the optimum way of connecting the filter to a thermocouple wire is by a mechanical means, rather than, for example, by soldering, which may itself introduce thermal emf errors.

Thus, in the case of the Fig.2 embodiment, the chromel wire 18 leading through the filter is connected to a chromel wire (corresponding to wire 12 of Fig.l) of the thermocouple by means of a mechanical connection, such as crimping, clamping and the like or by welding, so that connections between dissimilar metals are avoided. Advantageously, the two chromel ends to be joined are mechanically butted or twisted together to improve the electrical connection therebetween. Similar connections apply to the other filter which, in this case, employs an alumel through-wire 18.

Where the thermocouple wires are non-solderable, or only solderable with the use of highly active fluxes, the internal electrical joints of the filter are preferably made as shown in Fig.2 by crimping, or other convenient mechanical methods.

By the use of the present invention, it is possible to avoid the generation of spurious thermal emf's at emi suppression filter/thermocuple interfaces, even in the presence of substantial (e.g.

50"C to 100"C) temperature gradients at opposite ends of the filter.

Claims (8)

1. A thermocouple system wherein two different emi suppression filters are employed to couple the dissimilar wires of a thermocouple through an equipment bulkhead to the equipment internal circuitry, each filter providing a continuous d.c.

path through that filter of the same material as the respective thermocouple wire to which it is connected.

2. A thermocouple system as claimed in claim 1, wherein the electrical connections between the terminations of said d.c. paths through the filters and said thermocouple wires are all made by mechanical means, as opposed to soldering, whereby to limit the generation of thermal emf's at said connections.

3. A thermocouple system as claimed in claim 1, wherein the electrical connection between the terminations of said d.c. paths through the filters and said thermocouple wires are made by welding.

4. A thermocouple system as claimed in any of claims 1 to 3, wherein said continuous d.c. path through each filter is formed by a metal pin having no surface finish of a dissimilar metal.

5. A thermocouple system as claimed in any of claims 1 to 4, wherein each thermocouple comprises a metal pin forming said continuous d.c. path, a tubular ferrite inductor coaxially embracing the pin, a tubular capacitor having inner and outer metallised surfaces and coaxially embracing said tubular ferrite inductor, means electrically connecting the inner metalised surface of the capacitor to the metal pin and an outer tubular metal housing coaxially embracing and in electrical contact with the outer metalised surface of the capacitor.

6. A thermocouple system as claimed in claim 5, wherein the electrical connection between the metal pin and the inner metalised surface of the capacitor is achieved by two metal ferrules which are soldered to the respective ends of said inner metalised surface of the capacitor and are mechanically crimped to the metal pin.

7. A thermocouple system as claimed in claim 6, wherein said ferrules are made of gold-plated brass.

8. A thermocouple system substantially as hereinbefore described with reference to and as illustrated in Figs. 2 and 3 of the accompanying drawings.