Field of the Invention
The present invention relates to an acoustic pipe length measuring apparatus for measuring the length of a pipe.
Some of pipe lines such as a gas pipe line and a signal cable pipe line are used underground for a long time.
A gas pipe line as one example is composed of a branch pipe line laid underground, and feed pipe lines branched from the branch pipe line for homes or firms and each connected thereto with a gas meter in a part which is exposed from the ground.
FIG. 1 is a view which shows the positional relationship among the main branch pipe, the feed pipes and the gas meter.
In a section A where the gas pipe line is laid underground (for example, under a road), a feed pipe line 2a branches from a main branch pipe line 1, and is curved by elbows 3 or the like at several positions. Each of the elbow 3 is a joint pipe for connecting feed pipes to each other with an angle therebetween, having a small radius of curvature. In a part B of the gas pipe line which is exposed from the ground, a gas meter 4 is connected to feed pipes 2b and 2c, which is secured to, for example, the wall of a building which is not shown, and a feed pipe 2d is laid inside of the building.
If an underground gas pipe is corroded or is broken, it is required to replace or repair the gas pipe. In this case, there is used a method having the steps of digging up the ground, and repairing the exposed gas pipe, and a non-digging type inner surface repairing method in which fluid resin is externally poured into a feed pipe to be repaired, and then the pipe is subjected to a vacuum so as to harden the poured resin in order to coat the inner surface of the pipe with the resin. The latter method is advantageous in view of the cost and the preservation of law and order, and accordingly, it is desired that the development of this method is promoted. However, in this method, an excessive quantity of resin to be externally poured into the pipe blocks the pipe, but a less quantity of resin causes the coating to be insufficient. Accordingly, in such a non-digging type inner surface repairing method, the calculation of the volume of a pipe is beforehand required in order to estimate the quantity of resin to be poured into the pipe. Since the inner diameter of the pipe is already known, it is only required to measure the overall length of the feed pipe.
Conventionally, the following methods using sound waves have been known in the technical field of measuring the length of a feed pipe (refer to lecture theses of Japanese Acoustic society).
(1) Long Pipe Length Measuring System
As shown in FIG. 2, one speaker 6 and two microphones 7, 8 are coupled to one end of a long pipe 5, and an oscillator 9 is connected to the speaker 6 while an oscilloscope 10 is connected to the microphones 7, 8. In this system, a pulse-like sound wave emitted from the speaker 6, is received by the microphones 7, 8, and the wave forms of the sound waves are observed through the oscilloscope 10 so as to calculate a sonic velocity. Further, the time required for the sound wave to propagate from the front end 5a to the rear end of the long pipe 5 is obtained, and accordingly, the length of the long pipe 5 can be measured.
(2) Polyethylene Pipe Length Measuring System
As shown in FIG. 3, a speaker 12 is attached to one end of a hose-like polyethylene pipe 11, and a microphone 13 is attached to the other end of the pipe 11. The speaker 12 and the microphone 13 are connected to a measuring device 14. In this system, the length of the pipe 11 can be measured with the use of the time from the emission of a pulse-like sound wave at a several kHz by the speaker to the receipt thereof at the microphone 13 and the sonic velocity. It is noted that the speaker 12 and the microphone 13 actually make close contact with the pipe 11 although FIG. 3 shows that they are separated from the pipe 11.
In general, in the case of measuring the length of a closed pipe with the use of a sound wave with the systems (1) and (2) as mentioned above, since the sonic velocity varies in accordance with the kind, components and temperature of gas flowing through the pipe, a measuring device for measuring the length of a pipe in accordance with the time between the emission of a sound wave into the pipe and the receiving of reflected-back sound waves gives a measured value which differs under various conditions since the propagation of the sound wave varies.
Meanwhile, in the system (1), the frequency of a sound wave may be set at a value of about 1 kHz if the bore diameter of a long pipe to be measured is large and the radius of curvature of a curved part in the pipe is large. If, for example, the bore diameter of a pipe such as a feed pipe is small (25 mm), or if a joint pipe such as a tee or an elbow is present in the curved part, the reflection of a sound wave becomes large at that part, and accordingly, the distinction between the reflected wave at the rear end of the feed pipe and the reflected wave at the curved part is difficult.
Further, if the frequency of a sound wave is decreased (the wave length thereof is increased), it is likely that measurement is affected by noise, resulting in that the margin of error in measurement becomes larger.
Further, since the system (2), the speaker 12 and the microphone 13 have to be arranged respectively on both ends of the pipe 11 to be measured, the length of the pipe can hardly be measured without unearthing the pipe which is laid underground.
Further, since the measurement is actually difficult unless both ends of a pipe to be measured are near to each other to a certain degree, the measurement of, for example, a long straight pipe is substantially impossible.