GB2171516A - Temperature sensors - Google Patents

Abstract

An optical fibre (3) extends through a capillary tube (1) coupled to a reservoir (2) containing a temperature responsive fluid (5) which extends along a length of the capillary tube predetermined by the temperature of the fluid. Optical power (6) is input to one fibre end and the amount of optical power output from the other fibre end is related to the fluid temperature. If the fibre is unclad and the fluid is mercury the transmitted power will increase with temperature increase, owing to the expansion of the mercury and the increased area of the reflective coating provided thereby for the fibre. <IMAGE>

Description

SPECIFICATION Temperature sensor This invention relates to temperature sensors.

According to the present invention there is provided a temperature sensor comprising a capillary tube, an optical fibre extending through the capillary tube, a temperature responsive fluid disposed in a reservoir coupled to the capillary tube, which fluid extends into the capillary tube for a length thereof determined by the temperature of the fluid, and wherein in use optical power is input into one fibre end and the amount of optical power output from the other fibre end is related to the temperature of the fluid.

An embodiment of the invention will now be described with reference to the accompanying schematic, and partially in section, drawing.

The temperature sensor shown in the drawing comprises a capillary tube 1 with a reservoir 2 containing mercury 5 connected thereto. An unclad optical fibre 3 extends through the capillary tube 1. Where the fibre 3 extends out of the tube 1 adjacent the reservoir 2 the tube is closed and sealed to the fibre. At the other end 4 the tube 1 also closed and sealed to the fibre, there is an airspace above the mercury 5 in the capillary tube, light output from a laser or LED 6 is input to one end of the fibre 3, which may as indicated by the dashed section of fibre be at a position remote from the tube, and light as output from the other end of the fibre 3 is detected by a detector 7, which may as indicated by the corresponding dashed section of fibre be at a position remote from the tube.

Since the fibre is unclad, at least within the tube, light transmitted into it will be lost by radiation. As the mercury 5 expands upon a temperature increase it will extend further into the capillary tube reducing the airspace and the optical power transmitted through the fibre will increase, owing to the increased area of mercury in contact with the fibre and thus in view of the increase in the area of reflective coating at the mercury/fibre interface. Assuming a constant optical power input the detected optical power will thus be directly related to the mercury temperature and the detector output can be calibrated in terms of temperature.

Since the capillary tube can be remote from the light source 6 and detector 7, no electrical connections are necessary in the vicinity of the capillary tube and the temperature sensor is thus particularly suitable for use in combustible environments. The capillary tube may be of glass so that the sensor may be considered to be somewhat like a conventional mercury in glass thermometer but with an optical fibre extending therethrough. Whereas the fibre has been described as unclad, at least in the capillary tube, it may alternatively be comprised by a leaky clad fibre provided sufficient sensitivity of response to temperature changes can be achieved. Whereas mercury is obviously a first choice for the liquid in the capillary tube in view of its expansion and reflection properties, other liquids may be employed.For example a clad fibre, which may be leaky, may be combined with an index matching fluid so that with increased temperature more optical power is removed from the fibre due to the increased area of the fluid in contact with the fibre than at a lower temperature.

1. A temperature sensor comprising a capillary tube, an optical fibre extending through the capillary tube, a temperature responsive fluid disposed in a reservoir coupled to the capillary tube, which fluid extends into the capillary tube for a length thereof determined by the temperature of the fluid, and wherein in use optical power is input into one fibre end and the amount of optical power output from the other fibre end is related to the temperature of the fluid.

2. A temperature sensor as claimed in claim 1, wherein the optical fibre is unclad and the fluid is mercury.

3. A temperature sensor as claimed in claim 1, wherein the optical fibre is clad and the fluid is an index matching fluid.

4. A temperature sensor as claimed in any one of the preceding claims, and including a light source and a light detector which are each connected to the optical fibre in the capillary tube by respective lengths of clad optical fibre whereby the capillary tube can be disposed at a position remote from the light source and the detector whereby to sense the temperature at said remote position.

5. A temperature sensor substantially as herein described with reference to and as illustrated in the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Temperature sensor This invention relates to temperature sensors. According to the present invention there is provided a temperature sensor comprising a capillary tube, an optical fibre extending through the capillary tube, a temperature responsive fluid disposed in a reservoir coupled to the capillary tube, which fluid extends into the capillary tube for a length thereof determined by the temperature of the fluid, and wherein in use optical power is input into one fibre end and the amount of optical power output from the other fibre end is related to the temperature of the fluid. An embodiment of the invention will now be described with reference to the accompanying schematic, and partially in section, drawing. The temperature sensor shown in the drawing comprises a capillary tube 1 with a reservoir 2 containing mercury 5 connected thereto. An unclad optical fibre 3 extends through the capillary tube 1. Where the fibre 3 extends out of the tube 1 adjacent the reservoir 2 the tube is closed and sealed to the fibre. At the other end 4 the tube 1 also closed and sealed to the fibre, there is an airspace above the mercury 5 in the capillary tube, light output from a laser or LED 6 is input to one end of the fibre 3, which may as indicated by the dashed section of fibre be at a position remote from the tube, and light as output from the other end of the fibre 3 is detected by a detector 7, which may as indicated by the corresponding dashed section of fibre be at a position remote from the tube. Since the fibre is unclad, at least within the tube, light transmitted into it will be lost by radiation. As the mercury 5 expands upon a temperature increase it will extend further into the capillary tube reducing the airspace and the optical power transmitted through the fibre will increase, owing to the increased area of mercury in contact with the fibre and thus in view of the increase in the area of reflective coating at the mercury/fibre interface. Assuming a constant optical power input the detected optical power will thus be directly related to the mercury temperature and the detector output can be calibrated in terms of temperature. Since the capillary tube can be remote from the light source 6 and detector 7, no electrical connections are necessary in the vicinity of the capillary tube and the temperature sensor is thus particularly suitable for use in combustible environments. The capillary tube may be of glass so that the sensor may be considered to be somewhat like a conventional mercury in glass thermometer but with an optical fibre extending therethrough. Whereas the fibre has been described as unclad, at least in the capillary tube, it may alternatively be comprised by a leaky clad fibre provided sufficient sensitivity of response to temperature changes can be achieved. Whereas mercury is obviously a first choice for the liquid in the capillary tube in view of its expansion and reflection properties, other liquids may be employed.For example a clad fibre, which may be leaky, may be combined with an index matching fluid so that with increased temperature more optical power is removed from the fibre due to the increased area of the fluid in contact with the fibre than at a lower temperature. CLAIMS

1. A temperature sensor comprising a capillary tube, an optical fibre extending through the capillary tube, a temperature responsive fluid disposed in a reservoir coupled to the capillary tube, which fluid extends into the capillary tube for a length thereof determined by the temperature of the fluid, and wherein in use optical power is input into one fibre end and the amount of optical power output from the other fibre end is related to the temperature of the fluid.

2. A temperature sensor as claimed in claim 1, wherein the optical fibre is unclad and the fluid is mercury.

3. A temperature sensor as claimed in claim 1, wherein the optical fibre is clad and the fluid is an index matching fluid.

4. A temperature sensor as claimed in any one of the preceding claims, and including a light source and a light detector which are each connected to the optical fibre in the capillary tube by respective lengths of clad optical fibre whereby the capillary tube can be disposed at a position remote from the light source and the detector whereby to sense the temperature at said remote position.

5. A temperature sensor substantially as herein described with reference to and as illustrated in the accompanying drawing.