The present invention concerns a therrr neter or temperature alarm device making use of fibre optics.

It is necessary in numerous supervision tasks within industry and technology to measure temperature in such a way that an alarm is ϊ actuated when the teirperature has trans-gressed a given threshold value. Of course such threshold alarm can be obtained from a con¬ ventional electronic thermometer by providing electronic circuits for threshold detection so that an alarm is actuated if the preset teirperature is surpassed.

There are hσw v ^p er vari-ous situations in which one would prefer to forgo the use of a so-called electronic thermometer because such in-struments as a rule contain an electrically conductive measuring pick-up, or sensor, to which a voltage has been applied. Typical 0 objects of where this consideration is relevant are, for instance, warehouses cx^taining readily inflammable chemicals, e.g. oil tanks and cisterns, powder stores, etc. In such conditions the electrically condι_-σtive pick-up leads may give rise to sparking, and they may for instance carry lightning into the storage space, 5 with disastrous results.

It is possible, of course, in such situations to use so-called teπperature pick-ups utilizing fibre optics, which continuously follow the development of temperature and which can be programmed 0 to actuate an alarm if a given critical teirperature is surpassed. As is cα-πrt-only known, measuring -instruments, such as thermometers, using fibre cptics have the inherent advantage that the optic fibre itself is an insulator .and thus cannot cause any harmful electrical phenomena, such as .short c-Lrcuits or sparking, which 5 might cause dangerous situations in an environment susceptible to explosion.

Hcwever, fibre-optic thermometers of present art have turned out to be rather complicated and expensive, and this has limited their use. On the other hand, 13-ιermc-meters errplcying conventional galvanic ^■ technology, such as thermocouple.s, thermistors or NTC and PIC resistance elements, offer a rather inexpensive and reliable solu¬ tion to surveillance problems as long as there is no apprehension rega-rding risk of fire or explosion. It is possible especially with PIC and NTC resistors to implement a temperature detection which is specifically of the alarm type because the resistance of these resistors changes most powerfully as a function of te-rperature in various teirperature ranges.

Ihe fibre-cptic thermometers kncwn at present are based on the principle that light is carried with an optic fibre to a measuring cell and the material in this cell reacts in one way or another to the teirperature so that the change of teπperature can be observed by optical means.

One way kncwn in the art is to make use of the teirperature depen¬ dence of the fluorescence phenomenon caused by light. It is a well-kncwn fact that the intensity becomes less with increasing teπperature. Some of the c mπercially available measuring instru¬ ments are based on this phenomenon.

It is also kncwn that the properties of so-called liquid crystals are dependent on teirrperature in such a way that it is possible to construct thermometers using liquid crystals. Liquid crystals cause a kind of interference phencmena in thin films so that the colour of the liquid crystal film appears to change as a -function of temperature in desired manner. A type of paint ccarposed of liquid crystals is kncwn which can be used to coat those objects which are of interest, and one may then ctoserve the changing colour by those means which fibre optics afford. It has to be noted, hcwever, that liquid crystal materials have no particularly long life span: in a way, they are decomposed as they age.

' In the present invention is disclosed a fbre-optic thermometer differing frcm that which has been described above and which can be used to cbserve the temperature in a given space and to obtain alarm if the teπperature rises over a given critical value. These 5 critical values depend on the material that has been placed in the sensor itself and they are therefore highly reliable and reproduc¬ ible because in their case no thought need be given to the potential creep of electronically set threshold values. In-other words, the threshold values can be derived from the material parameters of 0. the sensor material itself. The pick-ups to be presented here are furthermore highly reliable, simple and inejφensive, as will become apparent later on.

For better understanding the way in which the pick-up operates,

15 the following situation shall be considered in which e.g. in a test tube of glass has been placed snow. When for instance a narrow laser light beam is directed on this test tube, it will be fo-und that no light can pass through the test tube: the light is all scattered frcm the sncw surface. Snew is white, a well-known fact,

20 because it scatters nearly all of the incident light. But if the contents of the test tube are warmed so that the sncw therein melts, all light can pass through and hardly anything is scattered back frcm the surfa-ce of the test tube. One knows in that case that the teirperature has definitely increased to be higher than

25 the melting point of sncw, i.e., 0°C. It follows that in this instance light transmission can be taken as threshold indication of 0°C teirperature. It is obvious, of course, that if the light is introduced with the aid of an optic fibre and its transmission is observed with.another cptic fibre, it becomes possible to construct

-- a fibre-optic threshold detector.

In the respβnt invention a separate sensor housing is used, which is filled with a material of the above-described kind and the optical properties of which are monitored with cptic fibres. Cptic fibre transmitters and receivers are kncwn in the art in themselves, but all the same they constitute an important c-carponent part in

^' the present invention. Said sensor housing may be filled with a material which melts and turns transparent above a certain tempera¬ ture which is characteristic for the material in -question but is solid and scatters back a great part of the light incident on it, at temperatures below said melting point. Stearine may be mentioned as an example. Below its melting point stearine is white and kind of milky, and it scatters back all liφt. When it melts, stearine turns transparent and clear like water and transmits all light. Stearine in solid state contains microcrystals of a kind in great * ^" numbers, which scatter liφt and make for instance a candle appear white. On the other hand, stearine has a clearly defined melting point (about 58°C) , above which it becomes transparent. There is available a rather great number of stearines with different melting points so that appropriate alarm threshold te-rperatures are easy enough to find. There are several other similar materials, such as paraffins, waxes, certain fats and oils. Seme of the plastics also turn clear from milky condition when they melt (for instance, - polyethylene) . All told there exists a quite ample selection of suitable materials and suitable melting or phase transformation teirperatures. It is also possible to admix light-scattering crystals to certain oils, which crystals as they melt either let the light pass through or cease to scatter light. In the present invention fibre optics are employed to observe the change in light scattering and the change of light transmission at the melting point or other phase transformation point of the respective material.

It should be clear from the preceding description that a pick→αp of this type would be rather simple, inexpensive and reliable in operation owing to its being-based on a simple natural phenomenon.

The thermometer or heat alarm device of the invention making use of fibre optics is characterized in that the measuring -Instrument consists of a fibre-optic-ii t transmitter, which sends cut liφt along an cptic fibre tc a measuring head provided on the end of the transmitting fibre, and of a fibre-optic receiver, which is connected by an optic fibre to the same measuring head, and that

the measuring head consists of a chamber or hollow enclosure which has been filled with a liφt-scattering substance of the kind which below a given threshold temperature scatters the liφt earning throuφ the transmitting fibre back into the receiving fibre but above the threshold temperature daracteristic of the material loses its liφt scattering capacity, or becomes transparent, so that the liφt arriving throuφ the transmitting fibre is no longer scattered to the receiving fibre, whereby transgression of the threshold teirperature characteristic of said -substance is observable in the fibre-optic receiver as a di -inishi-ng of the back-scattered liφt and of the signal which it produces.

For better understanding of the invention, the design solutions therewith associated shall be gone throuφ in greater detail in the following. An initial observation: the fibre-optic transmitters and receivers are in themselves kncwn in the art and are commer¬ cially obtainable as completed units; therefore they are not more closely described here althouφ they constitute part of the inven¬ tion.

The invention is described in detail with reference to certain advantageous en-bediments of the invention, presented in the figures of the appended drawing, to which however the invention is not meant to be exclusively confined.

Fig.1 presents an advantageous embodiment of the thermometer, or teirperature alarm device, of the invention in which two separate cptic fibres are used, in the form of a -scheatical diagram.

Fig.2 presents a second advantageous em-κdi-ment of the 1-hermαmeter, or temperature alarm device, of the invention in which one single optic fibre is used, in the form of a schematical diagram.

Fig.3 presents a third advantageous eπibod-Lment of the thermometer, or temperature alarm device, of the invention in which the liφt transmission is directly utilized with the aid of optic fibres, in

the form of a schematical diagram.

In Fig. 1, a fibre-cptic liφt transmitter transmits liφt at a suitable wavelength; in many cases the visible liφt from a so- called LED lamp is sufficient, tut for instance near infra-red liφt may also be used. The liφt from the fibre-cptic liφt trans¬ mitter 1 proceeds along the fibre 2 to the -measuring head 5, which is for instance a measuring cell 6, which has been closely filled with a suitable substance 7, for instance stearine. The liφt arriving in the transmitter fibre 2 is scattered back from the stearine in such manner that part of it enters the second optic fibre 4 carried to the measuring cell 6 and along which fibre the scattered liφt reaches the fibre-cptic receiver 3, the liφt arriving there being converted to an electronic signal by means kncwn in themselves in electro-optics. When new the melting point of stearine is reached, the crystallinity of the substance dis¬ appears, and this is indicated by the electronic circuit of the fibre-cptic receiver 3 as an alarm signal. It should be noted that in an alarm device of this type a signal is received in the receiver 3 all the time while the teirperature is below the alarm threshold. This can ce characterized as a so-calle.d σlo-=ed-ci_rcuit (continuous current) principle: everything is OK if a return liφt signal is received; if the liφt signal has vanished, then either the threshold teirperature has been surpassed or the cptic fibre has been broken and the condition requires attention in every case.

In Fig. 2 is presented a design which contains only one fibre and which may therefore be better usable in some situations. In this design the fibre-cptic liφt transmitter 8 as well as the fibres cptic receiver 9 has been connected to one and the same fibre. ^' •_!, using a so-called semitranspareπt mirror 10. The liφt proceeding along the fibre 11 arrives at a similar measuring cell 12 as in the precedine embodiment, and if conditions are belcw the thr -shold temperature, the stearine or equivalent liφt-scattering su-υstance in the measuring cell 12 scatters liφt back along the fibre 11 to the receiver 9, where the liφt signal, or its absence, can be

electronically detected in ways kncwn in the art. It should be noted that the seπ transparεπt mirror 10 used in this embodiment constitutes a design known in itself in electro-optics and therefore needs no special description.

In Fig. 3 is presented a third embod-imerrt, wherein a fibre-optic transmitter 13 has been connected to a fibre-optic receiver 14 with the aid of a measuring cell 16 and an cptic fibre 15 in such manner that wϊien the material in the measuring cell 16 melts or

10. otherwise becomes transparent the liφt from the transmitter 13 can proceed thrσuφ the measuring cell 16 all the way to the receiver 14, whereby the threshold teπperature causing melting can be electronically recorded in the receiver 14.

15 it may furthermore be noted that many waxes, like paraffin and stearine, have melting temperatures which occur in rather in-teresting ranges. Very c-cmmαn melting points of stearine are the temperatures 58, 68 and 78°^, which are located in highly useful ranges in view of practical monitoring operations, for instance.

20 So-called micrccrystalline stearine waxes are also available which have melting points in even higher ranges. Also paraffins, for instance, have a number of useful melting points. These melting points are usually quite stable, a d changes of barometric press¬ ure, for instance, have little if any effect en them. Stearines

9 ^~

^- and other waxes are also in themselves very durable and stable substances, with minimal chemical changes in the course of time even if they -should remain in molten state for prolonged periods. Moreover, the measuring cell may be sealed so tiφtly that other solvent chemicals cannot destroy the specimens or cause them to

30 change. It is understood that submersion of the pick-up in any oil naturally calls for extra protection, but this c--rcumstance is no essential part of the invention in itself. Stearines and waxes are also insensitive to electric and magnetic fields. It may also be noted that the melting and solidifying process e.g. of waxes incor¬

35 porates a certain hysteresis phenomenon, which is nothing but an advantage in alarm actuating applications.