AU625154B2 - Temperature sensor - Google Patents

Description

ii.

625154 Form COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-69 COMPLETE SPECIFiCATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority: '0 related Art: 0 Al Name of Applicant: 0 0 Addeass of Applicant: Actual Inventor: 4 9 4cjdress for Service: 50 Bruningstrasse, D-6230 Frankfurt/Main Federal Republic of Germany HOECHST AKTIENGESELLSCHAFT WERNER GROH, JOCHEN COLUTANDIN, PFTER HERBRECHTSMEIER, JURGEN THEIS NMIXWXXK9CRSEKM LS5, Watennark Patent Trademark Attorneys 50 QUEEN STREET, MELBOURNE, AUSTRALIA, 3000.

Complete Specification for the invention entitled: TEMPERATURE SENSOR The following statement is a full description of this invention, including the best method of performing it known to us

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k HOECHST AKTIENGESELLSCHAFT HOE 88/F 182 Dr.DA/sch Description Temperature sensor The invention relates to a temperature sensor which comprises a temperature-sensitive element in the form of a liquid crystal and at least one optical waveguide.

Liquid crystals have long been used technically for temperature measurements. They have the property that their chiral molecular orientation, i.e. the Fitch of the helix, varies with temperature. As a rule the pitch becomes less as the temperature increases. If the pitch is adjusted so that at a specified lower temperature limit red light is totally reflected, the liquid crystal exhibits a red coloration. If the temperature is increased, the reflected light alters its color through yellow and green to blue (cf. Bacci et al., Applied Optics 25, pages 1079-1082 (1986); Smith et al., Applied Phys. Letters 24, pages 453-454 (1974)).

The disadvantage' of earlier liquid-crystal applications in temperature measurement technology is that the color, and consequently the temperature, are assessed visually and the measurement accuracy is insufficient.

The object was therefore to find a temperature sensor based on a liquid crystal which has a high measurement accuracy.

It was found that the object can be achieved by a temperature sensor which comprises a liquid crystal and at least one optical waveguide.

The invention relates therefore to a temperature sensor which comprises a protective cap which contains a chirally nematic liquid crystal with helical molecular orientation and in which at least one optical waveguide *i- 4 j

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-2terminates.

The temperature sensor according to the invention comprises a protective cap which contains the temperaturesensitive element and in which at least one optical waveguide terminates.

As a rule, the cap has the form of a cylindrical tube which is open at one end face and sealed at the other end face. The dimensions of the cap depend on the size of the temperature-sensitive element and on the diameter of the optical waveguide or waveguides. The wall thickness is 0.1 to 5, preferably 0.1 to 1 mm at the round sides and 0.1 to 5, preferably 0.1 to 1 mm at the sealed end face. The cap is composed preferably of a material which Sconducts heat well, for example a metal, preferably 15 aluminum. However, plastic or ceramic is also suitable as cap material provided the wall thickness is small S' enough. Suitable plastics are, for example, polyethylene, polypropylene, polyvinyl chloride, polyacrylates, polymethacrylates, polyethylene terephthalate, polytetrafluoroethylene, polyvinylidine fluoride or adhesives with a polymer base.

Suitable ceramics are, for example aluminum nitride or silicon carbide.

The temperature-sensitive element is a chirally nematic liquid crystal with helical molecular orientation.

Liquid crystals of the chirally nematic type mentioned are particularly suitable for the application since they are chemically and photochemically substantially more stable than cholesteric liquid crystals those based on cholesterol compounds), which are also used for temperature measurements.

The liquid crystal may be microencapsulated or unencapsulated.

i, t 1 1 1 J A 3 The liquid crystal is preferably contained in microcapsules which are embedded in a lacquer or a water-soluble polymer matrix, for example polyvinyl alcohol. It is also possible to use unencapsulated liquid crystals which are, however, more easily destroyed by external factors UV light) and are less easy to handle. The liquid crystal is contained as a layer in the interior of the cap between the inside of its sealed end wall and the end surface of the optical waveguide or guides and is 0.005 to 0.3, preferably 0.01 to 0.05 mm thick.

Preferably solvent-free or low-solvent formulations are suitable for the lacquer layer since solvents may alter or destroy the helix structure of the liquid crystal. To improve the path inertness, white pigment may be added 15 to the lacquer.

At least one optical waveguide terminates in the protec- Stive cap at the open end face opposite the temperature S sensitive element. It may be an individual optical waveguide, but it may also be a plurality of optical waveguides which may be bundled in groups. The bundling is preferably effected by pulling on a polymer shrink sleeve.

The optical waveguide comprises a core and a cladding, S' the cladding material having a slightly lower refractive index than the core material. Core and cladding may be composed of glass, preferably quartz glass, or plastic.

Preferred combinations are glass core, plastic cladding and also plastic core, plastic cladding. Particularly preferably, core and cladding are composed of plastic.

Suitable plastics for the core are, for example, polymethyl methacrylate, polystyrene, polycarbonate, polyfluoroacrylates or polyflouromethacrylates.

Suitable plastics for the cladding are, for example, :i polyfluoroacrylates or polyfluoromethacrylates. These 4may be copolymers of vinylidine fluoride and tetrafluoroethylene.

The diameter of the optical waveguide is 0.05 to 3, preferably 0.5 to 1.0 mm.

As a rule, the polished end surface of the optical waveguide makes contact with the surface of the liquid crystal. In some case, however, it is possible and necessary for a gap to remain between optical waveguide and liquid crystal whose size is equivalent to two to seven times the diameter of an optical waveguide.

The temperature sensor according to the invention may be S. produced in various ways.

a In one method the liquid crystal capsules, encapsulated in an aqueous polymer solution, are applied to the 15 polished end surface of the optical waveguide. After the separation, the protective cap, which is blackened on the inside, is pushed over the optical waveguide and the optical waveguide is cemented into the protective cap with a suitable sealing compound, for example an epoxy i20 adhesive. It is also possible to deposit the liquid crystal on the inside of the closed end wall of the protective cap. Preferably, the liquid crystal is embedded in microencapsulated form in a lacquer whose composition has already been described. This lacquer is applied either to the end face of the optical waveguide or to the inside of the closed end wall of the protective cap.

As a protection against UV light, which may originate from the measurement illumination, it is advisable to provide a UV-absorbing filter. This absorbing filter is advantageously arranged between light source and optical waveguide.

i i. For the purpose of temperature measurement, the temperature sensor according to the invention is connected by means of the optical waveguide to a measuring instrument which contains a light source (light emitter), a light detector and an evaluation and indicating device.

Advantageously, a coupling is arranged between temperature sensor and measuring instrument. Inside or outside the measuring instrument, the optical waveguide branches in order to connect the temperature sensor both to the light source and also to the light detector. This branching may also be situated in the temperature sensor, with the result that the optical waveguide runs to the measuring instrument as a multiple guide.

The white light originating from the light source is reflected by the liquid crystal layer (LC layer) and <guided via the branching to the light detector. Filters fitted in front of the detector ensure that only the intensity of certain wavelengths is measured, and it is possible, for example, for one wavelength to be used as reference.

Examples of other possibilties of producing a temperature-independent reference are a) the use of the intensity back-reflected at the front fiber end surface (Fresnel L.flection), b) the Fresnel reflection at a transparent protective lacquer layer on the liquid crystal and c) the reflection at a non-black background in front of B which the LC layer is situated.

For simple temperature measurements without a claim to good path inertness and long-term stability it is also possible to choose, for example, a light-emitting diode with suitable emission wavelength instead of the white- light source.

The following advantages result for temperature measurement with the aid of the temperature sensor according to 6 the invention: simple, and therefore very cheap, principle, high measurement accuracy no temperature hysteresis, high sensitivity and short response time as a result of the very small quantity of liquid crystal used and suitable sensor construction, very small, compact sensor tip and point measurements therefore possible, the place of measurement does not have to be visible as in the case of previous applications of liquid crystal temperature indicators; place of measurement and indication may be spacially separated, the electronic evaluation makes it possible also to evaluate reflected light components even outside the visible spectral range, with the result that the measurable temperature range is extended both upwards to shorter wavelengths and also downwards to longer wavelengths, in contrast to temperature sensors based on fluorescent compounds, the measuring light obeys the physical laws of reflection in the case of the sensor according to the invention and returns to the evaluation unit in the measuring fiber with high efficiency, the sensor is insensitive to electromagnetic fields and static electric and magnetic fields, measurement corruptions as a result of an oblique angle of view such as may occur in the visual observation of unencapsulated liquid crystals, are not 30 possible since the illumination of the liquid crystal and the return of the measuring light take place in a defined manner via the optical waveguide; the spectral purity of the color may be controlled by means of the nuierical aperture, it is also possible to assess the color visual via the optical waveguide.

By suitable choice of the liquid crystal mixture, it is possible to choose a measurable temperature range in wide 1I -7limits which is suited to the measurement problem.

Examples of the application of the temperature sensor according to the invention are: temperature measurements in the field of medicine, for example microwave treatment of tumors; skin surface temperature measurements; tomographic examinations; temperature measurements in an explosive environment; temperature measurements with microwave heating, for example in industrial processes or in the home.

The figures show possibilities for embodying the temperature sensor according to the invention. In thesa 1 denotes the protective cap, 2 denotes the temperature-sensitive element, 3 denotes the optical waveguide, 4 denotes the sealing and denotes a shrunk-on sleeve Figure 1 shows a temperature sensor which contains only a single optical waveguide. At the inside of the closed end face of the protective cap is a liquid crystal The optical waveguide is pushed into the cap far enough for its end surface to make contact with the liquid crystal The sealing protects the j liquid crystal against atmospheric factor-. i The temperature sensor according to Figure 2 contains two optical waveguides which terminate in front of the liquid crystal at a distance equivalent to about three times the diameter of the optical waveguide and are held together by a shrunk-on sleeve Figure 3 shows the use of an entire bundle of optical waveguides In other respacts, the structure of the temperature sensor in this figure is the same as that of the sensor in Figure 1.

The temperature sensor according to Figure 4 has the same -8difference that the two optical waveguides have been replaced by a plurality of bundles of optical waveguides.

Example 1 A inicroencapsulated liquid crystal was embedded in a lacquer which was applied directly to the end surface of a polymer OWG (1 mm dia.). The measurable temperature range set by the liquid crystal is 25 0 C to 45 0 C. After the lacquer had cured, the sensitive sensor tip was protected by a protective cap composed of a blackened PTFE. The base of the cap was only 0.1 mm thick in order to have a good thermal contact to the object of measurement. The sensor was connected to a measuring instrument. The white light emerging from a lamp was fed via the OWG to the liquid crystal layer. The selectively reflected light of a particular wavelength was a measure of the temperature at the tip of the fiber. It was guided via a Y junction to a detector, preceded by filters, which measured the light intensity at 630 nm and 800 nm. While the intensity at 630 nm was strongly dependent on the temperature, the intensity at 800 nm was temperature-independent and served as reference signal for the light-source variations or changes in the OWG transmission (path inertness). The quotient 1(630 nm)/I(800 nm), which is dependent only rn the temperature, was the measured quantity which it was possible to calibrate directly on the temperature scale.

Example 2 Two quartz-glass multimode optical waveguides (each 125 pm dia.) which were connected to a light source and a detector in a measuring instrument were held at one end by means of a plastic shrink sleeve. After shrinking, the sleeve firmly enclosed the optical waveguides. A black-lacquered aluninum cap, on the inside of which a thin unencapsulated liquid crystal layer was deposited, was pushed on to the combined end of the optical waveguides. The liquid crystal layer was at a distance of k H 9 9 300 1m from the two fiber end surfaces in order to ensure a good launch of the reflected intensity into the OWG leading to the detector. In order to protect the liquid crystal layer against UV light, the UV light emitted by the lamp had to be removed by a UV-absorbing filter positioned upstream. The liquid crystal was sensitive in the range -10 0 C to 10 0 C. The measurement accuracy achieved with the sensor was 0.1°C. As a result of the high thermal conductivity and low mass of the aluminum cap, the sensor had a short response time of 1 to 2 seconds.

Example 3 A construction analogous to Example 2 was chosen, but two 500 um dia. polymer optical waveguides and a microencapsulated LC layer were used. To improve the path inertness, white pigment which produced a low additional light reflection was added to the lacquer mixture which contained the liquid crystal. This was independent of the temperature and it was possible to use it as a reference value for improving the long-term stability of the temperature sensor.

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Claims (6)

1. A temperature sensor having a measurement accuracy of at least 0.10C which comprises a protective cap which contains a chirally nematic liquid crystal with helical molecular orientation and in which at least one optical waveguide terminates, which is connectable to a measuring instrument containing a light source, a light detector and an evaluation and indicating device.

2. The temperature sensor as claimed in claim 1, wherein the protective cap has the form of a cylindrical tube which is open at one end face and sealed at the other end face.

3. The temperature sensor as claimed in claim 1, wherein the protective cap is composed of metal, polymer or ceramic.

4. The temperature sensor as claimed in claim 1, wherein the liquid crystal is situated at the inside of the sealed end face of the protective cap.

The temperature sensor as claimed in claim 1, wherein the liquid crystal is V contained in microcapsules in a lacquer layer.

6. The temperature sensor as claimed in claim 1, wherein the optical waveguide terminates at the open end face of the protective cap opposite the liquid crystal. DATED this 9th day of April, 1992. HOECHST AKTIENGESELLSCHAFT WATERMARK PATENT TRADEMARK ATTORNEYS THE ATRIUM 290 BURWOOD ROAD HAWTHORN VICTORIA 3122 AUSTRALIA DBM:KJS:BB f :i S" B