System for acoustically measuring the length of a pipe

A calibration pipe having a predetermined length is connected to an end of a pipe to be measured. A sound emitter and a sound receiver are connected to a base end of the calibration pipe. The sound emitter emits a sound into the calibration pipe and the pipe to be measured, and the sound receiver receives a sound reflected from a discontinuous portion of the pipe. A calculator is provided for calculating a period of time from emitting of the sound to receiving of the reflected sound. The length of the pipe is calculated based on speed of the sound in the calibration pipe.

BACKGROUND OF THE INVENTION 
The present invention relates to a system for acoustically measuring the 
length of a pipe disposed under a road by using sound wave, and more 
particularly to a calibration system provided for calibrating change of 
speed of sound caused by pressure, temperature and component of gas in the 
pipe. 
A pipe of a pipeline such as a gas main is disposed under a road for a long 
time so that the pipe may be corroded or broken by external force. In such 
a case, there is a method for lining an inner surface of a distributing 
pipe branched from the pipe-line with coating material such as resin. In 
order to repair the distributing pipe, there has been developed a method 
in which the inner surface of the pipe is repaired without digging the 
pipe. 
In such a method, if the amount of resin exceeds a desired value, the resin 
is unnecessarily used. If the amount of resin is less than the desired 
value, the pipe is insufficiently lined. Accordingly, it is necessary to 
accurately measure the length of the distributing pipe to be repaired. 
FIG. 3 shows a conventional measuring system for acoustically measuring the 
length of the distributing pipe. The measuring system is adapted to 
measure a distributing pipe 28 having a bent portion branched into a 
residential lot of a customer, from a gas main 29 which is laid under a 
street. One end of the distributing pipe 28 is connected to the gas main 
29 by way of a joint. The other end of the pipe 28 is connected with a 
service riser, to an end of which a gas meter 15 is normally connected. 
The measuring system comprises a pipe length measuring unit 21 and a 
cylindrical connecting member 16 for connecting the unit 21 to the pipe 28 
through a lead 16b. The connecting member 16 is mounted on the end of the 
distributing pipe 28 by removing the gas meter 15 therefrom. 
As shown in FIG. 4, the connecting member 16 has an inner threaded portion 
16a formed at an end thereof to be engaged with an outer threaded portion 
formed on the end of the distributing pipe 28. On the other end of the 
connecting member 16, a sound emitter 25 and a sound receiver 26 are 
provided to be exposed to the pipe. A temperature sensor (thermistor) 27 
is provided in the connecting member 16 adjacent to the sound emitter and 
sound receiver for measuring the temperature in the pipe. 
The pipe length measuring unit 21 comprises a CPU 22, an amplifier 17 
connected to the sound emitter 25, an amplifier 18 connected to the sound 
receiver 26, a D/A converter 19 connected to the amplifier 17 and to the 
CPU 22, an A/D converter 20 connected to the amplifier 18 and to the CPU, 
a memory 24 selectively connected to the CPU through a switch S, and a 
display 23 connected to the CPU. The temperature sensor 27 is connected to 
the memory 24. 
In an measuring operation, the CPU 22 produces a command signal in the form 
of pulses for emitting a sound wave. The signal is applied to the D/A 
converter 19 to be converted into an analog signal. The analog signal is 
amplified by the amplifier 17. The sound emitter 25 is operated to emit a 
sound wave into the pipe 28. The sound wave is reflected at discontinuous 
positions such as joint, bent portion and opening portion. The reflected 
sound is received by the sound receiver 26 and amplified by the amplifier 
18. The amplified signal is converted into a digital signal at the A/D 
converter 20 and applied to the CPU 22. The CPU measures a period of time 
from emitting of the sound in the pipe 28 to receiving the reflected sound 
in accordance with a time measuring circuit provided therein and 
calculates the length of the pipe 28 based on the measured period of time 
and the speed of the sound. The length of the pipe is displayed on the 
display 23. 
The memory 24 stores data of the speed of the sound corresponding to the 
type of gases (such as air or natural gas) and the temperature detected by 
the sensor 27. The operator operates to change the switch S in dependency 
on the type of gas and the detected temperature so as to set a standard 
value of the speed of the sound. 
However, the speed of sound largely changes in dependency on the 
temperature and component of gas in the pipe. 
Therefore, for the pipeline of town gas, speeds of sound are previously 
measured at every one or a few grades of the temperature between 0 and 30 
degrees under a constant pressure for every kind of gas. The data of the 
relationship between the speed of sound and the temperature and component 
of gas are stored in the memory 24. The switch S is operated for selecting 
data in accordance with conditions. Alternatively, an equation of the 
relationship is provided for correcting the measured value based on the 
temperature. 
Accordingly, the handling of the system is complicated because of the 
measurement of temperature, the handling of the switch and others. 
Moreover, the length of the pipe can not be measured with accuracy because 
of measurement error. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a system for acoustically 
measuring the length of a pipe which may eliminate measuring errors caused 
by temperature and component of the gas in the pipe, thereby obtaining the 
length of the pipe with accuracy. 
According to the present invention, there is provided a system for 
acoustically measuring a length of a pipe, the system having a sound 
emitter to be connected to an end of the pipe so as to emit a sound into 
the pipe, a sound receiver for receiving a sound reflected from a 
discontinuous portion of the pipe, calculator means for calculating a 
period of time from emitting of the sound to receiving of the reflected 
sound. 
The system comprises a calibration pipe which has a predetermined length 
and is to be connected to the end of the pipe at a front end thereof, the 
sound emitter and sound receiver being connected to a base end of the 
calibration pipe, the calculator means being arranged to calculate the 
length of the pipe based on a period of time from the emitting of the 
sound into the calibration pipe connected to the pipe to the receiving of 
the sound reflected at the front end of the calibration pipe. 
The speed of the sound in the calibration pipe and the length of the pipe 
are calculated as follows. 
Speed of sound=(length of the calibration pipe.times.2) .div.(reciprocating 
period of time from emitting sound to receiving sound in the calibration 
pipe). 
Length of pipe=(length of the calibration pipe.times.2) .div.(reciprocating 
period of time from emitting sound to receiving sound in the calibration 
pipe).times.(reciprocating period of time from emitting sound to receiving 
sound in the distributing pipe).div.2-(the length of the calibration 
pipe). 
These and other objects and features of the present invention will become 
more apparent from the following detailed description with reference to 
the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a measuring system of the present invention has a 
calibration pipe 10 provided between a connecting member of a measuring 
unit and the distributing pipe 28. The system comprises a pipe length 
measuring unit 3 and a connecting member 1 connected to the unit 3 through 
a lead 1a. 
The connecting member 1 has a sound emitter 4 and a sound receiver 5 both 
of which are the same as the conventional one, a discharge valve 14, and a 
connecting portion 7 to which a base end of the calibration pipe 10 is 
connected. 
The calibration pipe 10 has a spiral form and a predetermined length, for 
example a length between 3 m and 10 m. 
A connecting device 12 having operating handles 13 is provided for 
detachably connecting the front end of the calibration pipe 10 to the 
distributing pipe 28. Another connecting device may also be provided on 
the connecting portion 7. 
Referring to FIG. 2, the pipe length measuring unit 3 has approximately the 
same structure as the conventional one. The pipe length measuring unit 3 
comprises CPU 22, amplifier 17 connected to the sound emitter 4, amplifier 
18 connected to the sound receiver 5, D/A converter 19 connected to the 
amplifier 17 and to the CPU 22, and A/D converter 20 connected to the 
amplifier 18 and to the CPU. A memory 30 connected to the CPU 22 stores 
data necessary for calculating a period of time from emitting the sound to 
receiving the first reflected sound as a standard value for calculating 
the length of the pipe 28 at the CPU 22. 
In measuring operation, the discharge valve 14 of the connecting member 1 
is opened so as to discharge air in the calibration pipe 10 with aid of 
pressure of gas in the distributing pipe 28 to the atmosphere, while 
introducing the gas in the distributing pipe 28 in the calibration pipe 
10. When the calibration pipe 10 is filled with the gas, the discharge 
valve 14 is closed. Since the pipe 10 is filled with the same gas as in 
the pipe 28, the speed of sound in the pipe 10 is equal to that in the 
pipe 28. 
The CPU 22 produces a command signal in the form of pulses for emitting a 
sound wave. The signal is applied to the D/A converter 19 to be converted 
into an analog signal. The analog signal is amplified by the amplifier 17. 
The sound emitter 4 is operated to emit a sound wave into the pipe 28. The 
sound wave is reflected at a discontinuous position of the pipe such as 
joint, bent portion and opening portion in the pipe 28. 
The emitted sound is first reflected at the front end of the calibration 
pipe 10. Thus, the entire length of the pipe 10 is in proportion to the 
period of time from the emitting of the sound to receiving the first 
reflected sound. 
If a small projection is provided in the front end, the reflection of sound 
is ensured. 
The speed of sound in the calibration pipe 10, and hence distribution pipe 
28 is calculated as follows. 
Speed of sound=(length of the calibration pipe.times.2) .div.(reciprocating 
period of time from emitting sound to receiving sound in the calibration 
pipe). 
It is not necessary to correct the speed of sound, since the speed is the 
same as the speed in the pipe 28. Thus, an accurate length of the pipe can 
be obtained based on the calculated speed of the sound. 
The length of the pipe 28 is calculated as follows. 
Length of pipe=(length of the calibration pipe.times.2) .div.(reciprocating 
period of time from emitting sound to receiving sound in the calibration 
pipe).times.(reciprocating period of time from emitting sound to receiving 
sound in the distributing pipe).div.2-(the length of the calibration 
pipe). 
Thus, the length of the pipe is measured irrespective of the temperature, 
pressure and component of gas. 
A ratio of the reciprocating time of sound in the calibration pipe 10 to 
the reciprocating time of the reflected sound in the pipe 28 is equal to a 
ratio of the length of the calibration pipe 10 to a distance from the base 
end of the pipe 10 to a position where a sound is reflected in the pipe 
28. Thus, the sound reflected position in the pipe 28 can be easily 
calculated. 
In accordance with the present invention, the calibration pipe having a 
simple structure is provided for eliminating measured errors caused by 
temperature, pressure and component of gas. Consequently, the length of 
the pipe is measured with accuracy. 
Since the memory for correcting the speed of sound is omitted, the 
structure of the unit is simplified. 
While the presently preferred embodiment of the present invention has been 
shown and described, it is to be understood that this disclosure is for 
the purpose of illustration and that various changes and modifications may 
be made without departing from the scope of the invention as set forth in 
the appended claims.