JPS6054612B2 - Low temperature detection method using optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention detects the occurrence of a temperature change at an arbitrary position or section in a long distance or large area, localizes the position or section, and The present invention relates to a sensor device for detecting temperature, and particularly relates to a low-temperature detection method using an optical fiber as a sensor element, which exhibits a steep increase in optical transmission loss in a low-temperature region.

Generally, techniques for detecting and localizing the position or area where a temperature change occurs, as described above, and detecting the temperature in the area where the temperature change occurs are important in various fields. For example, leakage of liquid or gas due to an airtight failure in pipelines, tanks, or other equipment can lead to serious accidents, and early detection of this is extremely significant. Conventionally, various methods have been put into practical use or proposed as devices for detecting the location or region where a temperature change occurs, as well as the temperature in that region. The point-sensing method used a point-sensing method in which a large number of temperature sensors were placed point-like in a temperature or area, and temperature changes in that position or area were detected. When trying to detect the location or area where a change occurs, as well as the temperature in the area, it is necessary to place many sensors on a line or surface, and for safety reasons, the sensors must have explosion-proof performance. However, there were problems such as the method itself being extremely expensive.

The object of the present invention is to provide a low temperature detection method as mentioned in the opening paragraph, which eliminates the disadvantages of conventional devices as described above. In order to achieve this objective, in the present invention, an optical fiber exhibiting a sharp increase in optical transmission loss in a low temperature region is arranged in a temperature measurement area, and the light input end of the optical fiber is connected to a light emitting source. connected to the optical fiber, measures the amount of backscatter light relative to the incident light at the light input end of the optical fiber, and detects a temperature change occurrence position from a delay time corresponding to a steep change in the amount of light; The present invention is characterized in that a differential characteristic of the amount of light is determined, and from the differential characteristic, the period in which the temperature change occurs and/or the temperature are detected.

Hereinafter, the present invention will be described in detail based on embodiments with reference to the drawings.

FIG. 1 shows the temperature characteristics of optical transmission loss per unit length of the optical fiber used in the present invention.

When the area around the optical fiber is cooled, light is emitted to the outside of the optical fiber due to minute bends in that part, and optical transmission loss shows an extreme increase. There is a certain relationship between ambient temperature and optical transmission loss per unit length, as shown in Figure 1, and if you know the optical transmission loss per unit length, you can determine the ambient temperature.
Can be used as a temperature sensor. FIG. 2 shows the structure and operating principle of the low temperature detection system using optical fibers of the present invention.

This detection method basically consists of an optical fiber 1, which is a sensor element, and a detection section 2 connected to one end of the optical fiber 1. The detection section 2 has a light emitting source 3, a half mirror, and a light receiving detector 5. When the pulsed light generated by the light emitting source 3 is input into the optical fiber 1 (indicated by the solid line arrow), the incident light returns to the input end by a back scatter and is emitted (indicated by the dotted line arrow). 5. The delay time tl from when a light pulse is emitted until the backscattered light from an arbitrary position e returns to the input end is given by tl = 2r11/C, where the speed of light in vacuum is C1 and the refractive index of the optical fiber is n. Represented and obeyed. By detecting the delay time tl, l can be calculated and the light emitting position of the back scatter can be localized.

Now, the horizontal axis shows the generation position 1 of the back scatter light determined from the delay time 1, and the value P=-1010g when the amount of incident light is Pinl and the amount of light at the incident end of the back scatter light from position 1 is P1. When (Pln/Pl) is plotted on the vertical axis, P is a function of 1 and is represented by a graph G having an approximately linear slope as shown in FIG. 2B.

Note that the peak ER indicates the amount of light emitted from the input end of the reflected light at the tip E of the optical fiber 1. As described above, the optical fiber 1 in the detection method of the present invention has the characteristic that the optical transmission loss increases steeply in the low temperature region. When this change occurs, backscatter light returning to the input end from various parts of the optical fiber that are farther from the position or section where the abnormal cooling occurs, as viewed from the light input end, will be greatly attenuated.

That is, if the optical fiber 1 shown in FIG. 2 is abnormally cooled in the range of section By detecting the delay time 1 at this time, it is possible to detect and localize an arbitrary distance 1x from the incident end to the abnormal cooling occurrence section using the method described above.

On the other hand, the horizontal axis shows the generation position 1 of the back scatter light determined from the delay time t1, and the differential value DPl/d determined from the change in the light amount Pl of the back scatter light from an arbitrary position 1.
1 on the vertical axis, it is represented by a nearly straight graph H as shown in FIG. 2D, and if the range of section Since the temperature changes as shown in FIG.

On the other hand, as shown in FIG. 2B, the difference Px in the amount of backscatter light returning to the incident end from each point at both ends of the abnormal cooling occurrence section length X is proportional to the increase in transmission loss in that section. The optical transmission loss per unit length in the abnormal temperature section is determined from the abnormal cooling occurrence section length X and Px, and using the temperature characteristics of the optical transmission loss per unit length in Fig. 1, It is possible to determine the temperature in the area where cooling occurs.

In addition, when determining the abnormal cooling section length X, the optical fiber 1
By injecting light from both ends of the
It goes without saying that by detecting and localizing x'', the abnormal cooling occurrence section length X can be determined from X=1-1x-1x'' based on the known optical fiber length 1. According to the low temperature detection method using an optical fiber of the present invention, it is possible to detect the location where a temperature change occurs at any position or section within the measurement area as described above, the detection localization of the section where the temperature change occurs, and the temperature of the section where the temperature change occurs. It is possible to do so.

A typical example is the detection of airtightness in vibration lines and tanks. For example, as shown in FIGS. 3 and 4, if the optical fiber 1 is placed along the vibration line 10 or surrounding the base of the tank 11, When low-temperature liquid or gas leaks at any position or section within the area, the ambient temperature of the optical fiber 1 at that position or section decreases, and therefore the leak location or section can be detected and localized using the method described above. , as well as the temperature in that area. The optical fiber 1, which is a sensor element, can be arranged in various ways depending on the region to be measured and the structure and shape of the object to be measured.

Some examples are shown in FIGS. 5-7. The example in Fig. 5 is a planar optical fiber 1 bent in a zigzag pattern on the left and right sides, the example in Fig. 6 is a planar optical fiber 1 arranged in a spiral shape, and the example in Fig. 7 is an optical fiber 1 bent in a horizontal zigzag pattern. 1 is spirally wound three-dimensionally. In this way, desired measurement accuracy can be obtained by arranging optical fibers sparsely or densely over a long distance or a wide area to be measured. The features of the low temperature detection method using an optical fiber of the present invention explained above can be summarized as follows.

(b) Detects the occurrence of a temperature change at any position or section in a long-distance or wide-area measurement area, localizes that position or section, and continuously detects the temperature in that section. Can be done. (b) Since optical fiber is used as the sensor element, there is no danger of it becoming a source of ignition for flammable gases, etc., and it is not affected by the negative effects of dust or moisture, making it extremely safe and reliable. expensive.

(c) Applicable to objects to be measured with various structural shapes.

(d) The structure is simple and inexpensive.

As described above, the present invention has many advantages and provides a highly practical and excellent low temperature detection method.

[Brief explanation of the drawing]

FIG. 1 is a characteristic diagram showing the relationship between the optical transmission loss per unit length of an optical fiber for low temperature temperature detection and the ambient temperature. Fig. 2 is an explanatory diagram of the configuration and working principle of the low temperature detection method using optical fibers of the present invention, Figs. 3 and 4 are diagrams schematically showing different usage examples of the low temperature detection method using optical fibers, respectively, and Fig. 5 , FIG. 6, and FIG. 7 are diagrams showing different arrangement configuration examples of optical fibers, respectively. 1... Optical fiber, 2... Detection unit, 3... Light emitting source, 4... Half mirror, 5... Light receiving detector , 10...vibration line, 11...
··tank.

Claims (1)

[Claims] 1. An optical fiber exhibiting a sharp increase in optical transmission loss in a low temperature region is placed in a temperature measurement area, the light input end of the optical fiber is connected to a light emitting source, A low-temperature inspection method using an optical fiber, characterized in that the amount of backscatter light relative to the incident light is measured at the light input end of the optical fiber, and the position where a temperature change occurs is detected from the delay time corresponding to a steep change in the amount of light. 2. A low temperature detection method using an optical fiber according to claim 1, characterized in that a differential characteristic of the amount of light of the backscatter light is determined, and the interval and/or temperature in which the temperature change occurs is detected from the differential characteristic.