JP4411734B2 - Hot ultrasonic thickness gauge and thickness measurement method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hot ultrasonic thickness meter and a thickness measuring method using a pulse laser and an electromagnetic ultrasonic sensor.
[0002]
[Prior art]
In recent years, with the improvement in quality of steel products, high guarantee accuracy is required for the thickness of the iron plate and the thickness of the iron pipe. Therefore, in the manufacturing process, it is desired to measure the thickness immediately after rolling and reflect the measurement result on the manufacturing line. From such a demand, a γ-ray thickness meter has been invented, and this γ-ray thickness meter is installed in a production line such as a thick plate. However, since the γ-ray thickness meter is very expensive, it is not widely used in a line for manufacturing a product with a small added value. Further, in order to measure the thickness of the iron pipe, an advanced measuring method as disclosed in JP-A-6-160068 is required. Therefore, the γ-ray thickness meter is used only on a limited production line.
[0003]
Therefore, as a method for measuring the thickness hot at a low cost, a method for measuring the thickness of an object to be inspected with ultrasonic waves has been invented as disclosed in JP-A-60-53806. This measurement method using ultrasonic waves can be applied to the measurement of the thickness of a steel pipe, and further has an advantage that the equipment is not expensive like a γ-ray thickness gauge.
[0004]
However, the method of measuring the thickness of the object to be inspected using the ultrasonic waves as described above has the following problems.
[0005]
First, as a first problem, an electromagnetic ultrasonic sensor is used as an ultrasonic transmission / reception means, and the sensitivity is very low. In general, the transmission of ultrasonic waves by electromagnetic ultrasonic waves is inferior in sensitivity by about 1/100 to 1/1000 compared with a method using a piezoelectric element. Furthermore, in the transmission / reception of ultrasonic waves by electromagnetic ultrasonic waves, the sensitivity is lowered due to the square of the above-described poor sensitivity. Therefore, since the distance between the object to be inspected and the sensor (hereinafter referred to as lift-off) needs to be 2 mm or less, the electromagnetic ultrasonic sensor is very difficult to handle. On the other hand, in a rolling line for steel products, a pass line fluctuation of about 5 mm is unavoidable, so an electromagnetic ultrasonic sensor is not useful in a non-contact ultrasonic transmission / reception technology with a lift-off of about 2 mm. Therefore, in order to avoid the problem of sensitivity of the electromagnetic ultrasonic sensor, an attempt has been made to improve the transmission output of the electromagnetic ultrasonic wave by using a transmission voltage of several kilovolts. In addition, attempts have been made to improve the transmission / reception sensitivity of electromagnetic ultrasonic waves using a magnetizer that generates a 1 T DC magnetic field. However, the sensitivity improvement by these attempts has been insufficient. For this reason, the thickness meter using electromagnetic ultrasonic waves is not actually operated except when it is used in a state close to contact in a part of cold.
[0006]
As a second problem, when measuring the thickness of an object to be inspected using ultrasonic waves, the speed of sound changes depending on the temperature. In the method of measuring the thickness of an object to be inspected by propagating ultrasonic waves between heat, the thickness of the object to be inspected is measured from the ultrasonic wave propagation time between heats. For this reason, it is naturally necessary to measure the sound speed of the object to be inspected hot in advance, but the thickness of the object to be inspected calculated using the sound speed of hot is the object to be inspected in the hot state. It is the thickness of the body. Therefore, in the method disclosed in JP-A-60-53806, the thickness of the object to be inspected in the cold is calculated from the thickness of the object to be inspected in the hot using the linear expansion coefficient α of the material. ing. However, since the linear expansion coefficient α changes depending on the temperature, it is necessary to measure the linear expansion coefficient α in advance during the heat. That is, in the conventional method, it is necessary to measure in advance the sound velocity V (T) and the coefficient of thermal expansion α (T) at each temperature T during the heat, so that it takes time to measure the thickness of the object to be inspected. Furthermore, in actual measurement, since the thickness of the object to be inspected in the cold must be calculated from the thickness of the object to be inspected in the hot, the cold thickness is calculated in two steps. Therefore, there is a problem that an error occurs when calculating the thickness of the object to be inspected cold from the thickness of the object to be inspected hot.
[0007]
[Problems to be solved by the invention]
The present invention has been made to solve the above-described problems, and an object of the present invention is a hot ultrasonic wave that is an inexpensive apparatus configuration and can transmit and receive ultrasonic waves in a non-contact and highly sensitive manner. It is to provide a thickness gauge. Another object of the present invention is to provide a thickness measuring method that can easily and accurately measure the thickness of an object to be inspected from the ultrasonic propagation time in the hot.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, the present invention uses the following means.
[0009]
The hot ultrasonic thickness meter according to the present invention includes an ultrasonic wave generating means for generating an ultrasonic wave in an object to be inspected, an ultrasonic wave receiving means for receiving an ultrasonic wave propagated through the object to be inspected, and the received ultrasonic wave. Propagation time calculation means for calculating the propagation time of the ultrasonic wave from the waveform, temperature estimation means for estimating the temperature T (where T> T ₀ ) of the measurement part of the object to be inspected, and the estimated temperature The apparent sound speed of the object to be inspected at T is calculated based on the sound speed calculation means based on the temperature characteristic of the apparent sound speed, and the propagation time and the apparent sound speed are multiplied by the temperature T ₀ of the object to be inspected. An ultrasonic thickness meter that measures the thickness of an object to be inspected at a temperature T ₀ of the object to be inspected provided with a thickness calculating means for calculating the thickness, wherein the temperature characteristic of the apparent sound velocity is expressed by a temperature T specimens of the same material as the object to be inspected with a thickness of WT _{0 0} Nitsu Te measures the ultrasonic wave propagation time of the test piece from the plurality of temperature of the hot temperature T _0, the thickness WT ₀ is divided by the propagation time of a plurality temperatures the measured, it is determined.
[0010]
In the above-described hot ultrasonic thickness gauge of the present invention, the ultrasonic wave generating means is a pulse laser, and the ultrasonic wave receiving means is an electromagnetic ultrasonic sensor.
In the above-described hot ultrasonic thickness meter of the present invention, the electromagnetic ultrasonic sensor may have a through hole through which pulsed laser light passes.
[0011]
In the thickness measurement method of the present invention, an ultrasonic wave is generated in an object to be inspected , an ultrasonic wave propagated in the object to be inspected is received, a propagation time of the ultrasonic wave is calculated from a waveform of the received ultrasonic wave, and a temperature is measured. The estimation means estimates the temperature T (here, T> T ₀ ) of the measurement part of the object to be inspected, and the apparent sound speed of the object to be inspected at the estimated temperature T is converted into the temperature characteristic of the apparent sound speed. based calculated, the a thickness measuring method for determining the thickness of the object to be inspected at a temperature T ₀ of the object to be inspected by multiplying the propagation time of the ultrasonic wave of the test subject and the acoustic velocity of the apparent , the temperature characteristic of the acoustic velocity of the apparent, the test piece of the same material as the object to be inspected in the thickness WT ₀ at the temperature T _0, ultrasonic wave propagation time of the test piece from the plurality of temperature of the hot temperature T ₀ And measuring the thickness WT ₀ by measuring the plurality of temperatures It is determined by dividing by each propagation time in degrees.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0013]
[First Embodiment]
The first embodiment of the present invention uses a transmission-type hot ultrasonic thickness meter to measure the thickness of an object to be inspected with ultrasonic waves, and without using a thermal expansion coefficient, It is characterized in that the thickness of the object to be inspected is calculated.
[0014]
First, the principle by which the thickness of the object to be inspected can be measured without using the linear expansion coefficient will be described.
[0015]
A test piece having a thickness L (T ₀ ) at a cold temperature T ₀ has a length L (T) at a hot temperature T, and this relationship is expressed by Expression (1). Here, α is a linear expansion coefficient, and the linear expansion coefficient α is expressed by Expression (2).
[0016]
L (T) = L (T ₀ ) + L (T ₀ ) × α × (T−T ₀ ) (1)
α = 1 / L × dL / dT (2)
On the other hand, when the ultrasonic wave propagation time is Δt (T) at the temperature T, the sound velocity V (T) is defined by (thickness / propagation time). For this reason, the sound velocity V (T) is expressed by the equation (3).
[0017]
V (T) = L (T) / Δt (T) (3)
However, the sonic velocity V ′ (T) using the thickness L (T ₀ ) at the cold temperature T ₀ is ignored by ignoring the thermal expansion of the test piece in the hot state, and is expressed by the equation (4).
[0018]
V ′ (T) = L (T ₀ ) / Δt (T) (4)
Therefore, the difference between the sound speed V (T) and the sound speed V ′ (T) is expressed by the expression (5) from the expressions (1), (3), and (4).
[0019]
V (T) = V ′ (T) × {1 + α × (T−T ₀ )} (5)
Now, when a certain object to be inspected is measured, it is assumed that the temperature is T and the propagation time of the one-way ultrasonic wave is Δt ′. Accordingly, the thickness WT (T) of the inspection target is expressed as in Equation (6) using the sound velocity in Equation (3).
[0020]
WT (T) = V (T) × Δt ′ (6)
Since the thickness WT (T) is the thickness at the temperature T, the thickness WT (T ₀ ) at the cold temperature T ₀ needs to be calculated using the linear expansion coefficient α. Accordingly, the thickness WT (T ₀ ) at the cold temperature T ₀ is expressed by the equation (7).
[0021]
WT (T ₀ ) = WT (T) / {1 + α × (T−T ₀ )} (7)
When this equation (7) is rewritten, equation (8) is obtained.
[0022]
WT (T ₀ ) = V (T) × Δt ′ / {1 + α × (T−T ₀ )}
= V ′ (T) × {1 + α × (T−T ₀ )} × Δt ′ / {1 + α × (T−T ₀ )}
= V ′ (T) × Δt ′ (8)
That is, if the apparent sound speed V ′ (T) and the ultrasonic propagation time Δt ′ at the temperature T necessary for the measurement are examined in advance, the cold thickness WT (T ₀₎ without using the linear expansion coefficient. ) Can be measured.
[0023]
Next, a hot ultrasonic thickness meter for measuring the thickness of an object to be inspected by ultrasonic waves will be described.
[0024]
Fig.1 (a) shows the block diagram of a hot ultrasonic thickness meter. FIG.1 (b) shows the block diagram which applied the radiation thermometer 7 to the temperature estimation part 13 of Fig.1 (a). Note that the temperature estimation method is not limited to using the radiation thermometer 7, and any method can be used as long as the temperature of the object to be inspected can be estimated by heat transfer calculation or measurement using a contact-type thermometer.
[0025]
1 (a) and 1 (b), 1 denotes an object to be inspected, 2 denotes a pulse laser, and 3a denotes an electromagnetic ultrasonic sensor. In the first embodiment, since the ultrasonic wave is received by the transmission method, the electromagnetic ultrasonic sensor 3a is arranged on the opposite side of the pulse laser 2 with the object 1 to be inspected in between. Moreover, the radiation thermometer 7 shown in FIG. 1 (b) is connected to the device under test 1 by a fiber 6.
[0026]
When such a hot ultrasonic thickness meter is used, the thickness of the object to be inspected is measured by a laser ultrasonic method. In this laser ultrasonic method, an object to be inspected is irradiated with pulsed laser light or modulated light, and the surface of the object to be inspected is subjected to a reaction of ablation or thermal stress to generate ultrasonic waves. The generation intensity of this ultrasonic wave can be arbitrarily changed depending on the conditions of the pulse laser beam to be irradiated. Therefore, the generation intensity of the ultrasonic wave can be about 10 times the normal intensity that can be generated by the piezoelectric element. When such an ultrasonic wave is received by an electromagnetic ultrasonic sensor, the ultrasonic wave can be received with a sensitivity 1000 to 10,000 times higher than that of a conventional electromagnetic ultrasonic wave transmission / reception method.
[0027]
Hereinafter, a method for measuring the thickness of the object to be inspected by the above-described laser ultrasonic method will be described in detail.
[0028]
First, a pulse laser beam 5 having a light density of 10 MW / cm ² or more is emitted from the pulse laser ² . Here, the pulse laser beam 5 has a pulse energy of, for example, 200 mJ and a pulse width of, for example, 5 ns. The pulse laser beam 5 is condensed to 2 mm, for example, and irradiated on the object 1 to be inspected such as a steel plate. As a result, the surface 1b of the inspection object 1 is ablated, and ultrasonic waves 4 are generated by the reaction of this ablation. The ultrasonic wave 4 is reflected by the back surface 1a or the front surface 1b of the inspection object 1 and reciprocates a plurality of times within the inspection object 1. Here, an electromagnetic ultrasonic sensor 3 a is disposed on the back surface 1 a of the inspection object 1. For this reason, every time the ultrasonic wave 4 reaches the back surface 1a of the inspection object 1 on the electromagnetic ultrasonic sensor 3a side, the ultrasonic wave 4 is detected from the electromagnetic ultrasonic sensor 3a. The electromagnetic ultrasonic sensor 3a includes a magnet having a magnetic flux density of, for example, 3500 gauss and a coil having, for example, 10 turns.
[0029]
Next, the signal of the ultrasonic wave 4 detected by the electromagnetic ultrasonic sensor 3 a is amplified to 40 to 80 dB by the broadband amplifier 8. Of the amplified signal, only the frequency band of the ultrasonic wave that passes through the material is passed through the band-pass filter 9. Thereafter, the A / D converter 10 obtains an ultrasonic multiple reflection waveform as shown in FIG. At this time, the lift-off of the electromagnetic ultrasonic sensor 3a is, for example, 8 mm, the gain of the broadband amplifier 8 is, for example, 80 dB, and the band of the band-pass filter 9 is, for example, 1 to 5 MHz.
[0030]
Since the multiple reflection waveform shown in FIG. 2 is obtained by the arrangement of the transmission method, the first echo peak P1 in the figure is a peak when the ultrasonic wave 4 first reaches the electromagnetic ultrasonic sensor 3a. is there. The second echo peak P2 is one round trip from the first echo peak P1. The third echo peak P3 is one round trip from the second echo peak P2. Here, when the time interval Δt between the second echo peak P2 and the third echo peak P3 was measured, Δt = 7.744 μs. When the multiple reflection waveform shown in FIG. 2 was obtained, the indicated value of the radiation thermometer 7 (shown in FIG. 1B) used as the temperature estimation unit 13 was 872 ° C.
[0031]
FIG. 3 shows the relationship between the speed of sound and the temperature of an inspection object made of the same material as the inspection object 1. According to FIG. 3, it can be seen that the sound speed depends on the temperature so that the sound speed decreases as the temperature increases. A detailed description of FIG. 3 will be described later. When the sound speed is obtained using the graph of FIG. 3, the sound speed when the temperature is 872 ° C. is 4970 m / s.
[0032]
Therefore, from the equation (9), when Δt = 7.744 μs and the sound velocity V = 4970 m / s, the thickness WT of the object to be inspected in the cold is 19.24 mm.
[0033]
WT = Δt × V ÷ 2 (9)
In this way, the thickness WT of the object to be inspected in the cold is calculated in the thickness calculation unit 11, and the result is displayed on the display unit 12.
[0034]
In addition, the same experiment was performed many times and compared with the measurement by a micrometer. As a result, the difference between the thickness measured by the micrometer and the thickness WT calculated by the method described above was all less than 10 μm.
[0035]
By the way, when the propagation time of ultrasonic waves is originally measured and the thickness of the object to be inspected is calculated using the hot sound speed, the thickness of the object to be inspected in the hot state, that is, the inspected object that has been thermally expanded. You will know the thickness. However, in the present invention, the thickness of the object to be inspected is directly calculated at normal temperature. This is because the speed of sound shown in FIG. 3 is not true.
[0036]
The relationship between temperature and sound speed as shown in FIG. 3 was derived as follows. First, using a micrometer, the thickness WT ₀ at a normal temperature T ₀ ° C. of the material whose sound speed is desired is measured. Next, the ultrasonic propagation time Δt ₀ is measured by a hot ultrasonic transmission / reception method. From the thickness WT ₀ and the propagation time Δt ₀ , the sound velocity V ₀ at room temperature T ₀ ° C. is obtained. Next, while sequentially heating the material, the propagation times Δt ₁ , Δt ₂ , Δt ₃ ,... Are measured at the temperatures T ₁ , T ₂ , T ₃ ,. Here, the thickness used for the calculation of the sound velocity is not the thickness that has been thermally expanded, but the thickness WT ₀ at room temperature T ₀ ° C. In this way, the relationship between the sound speed and temperature as shown in FIG. 3 is derived. Therefore, if the speed of sound and the propagation time between heats according to FIG. 3 are used, the thickness at normal temperature can be calculated immediately.
[0037]
In the first embodiment, the sound velocity is directly calculated using the graph of FIG. 3 and the thickness at normal temperature is measured, but the present invention is not limited to this. If the relationship between the speed of sound and temperature is derived using the method described above using the thickness at any temperature, the thickness at any temperature can be calculated from the measurement of the propagation time of materials in different temperature ranges without correcting for thermal expansion. can do.
[0038]
In the first embodiment, the sound velocity is obtained from the surface temperature of the device under test 1. This is because when the object to be inspected 1 is allowed to cool, the temperature difference generated between the surface temperature and the internal temperature is less than about 10 ° C., so the thickness calculation error due to the temperature distribution cannot be 0.2% or more. Accordingly, the surface temperature of the device under test 1 is represented as the material temperature.
[0039]
Further, the surface 1b of the inspection object 1 is ablated with the pulsed laser beam 5, but the amount of the surface melting by one ablation is about 1 μm in depth, and does not deteriorate the quality of the product.
[0040]
According to the hot ultrasonic thickness meter according to the first embodiment, ultrasonic waves are used as means for measuring the thickness of the object to be inspected. For this reason, it is possible to provide a hot ultrasonic thickness meter that is safer and less expensive than that using γ rays.
[0041]
Moreover, since the thickness is measured using ultrasonic waves, the ultrasonic waves 4 can be received with a sensitivity 1000 to 10,000 times higher than that of a conventional transmission / reception method using electromagnetic ultrasonic waves. Therefore, since ultrasonic waves can be transmitted and received with high sensitivity, even if the lift-off of the electromagnetic ultrasonic sensor 3a is separated by 10 mm or more, it can be received with an S / N ratio of 2 or more. In this way, if the lift-off can be made large, it becomes easy to provide a cooling mechanism in the electromagnetic ultrasonic sensor 3a, so that the influence of radiant heat can be reduced. Furthermore, if the lift-off can be made large, it is possible to afford to follow the line fluctuation of the inspection target, and there is no need to always maintain a constant lift-off. Therefore, transmission / reception of ultrasonic waves can be easily and stably performed on a hot test object.
[0042]
According to the thickness measurement method according to the first embodiment, the thickness of the object to be inspected at room temperature can be known from the propagation time between heats without using the thermal expansion coefficient. Therefore, the thickness of the object to be inspected can be measured easily and accurately.
[0043]
[Second Embodiment]
The second embodiment of the present invention uses a reflection-type hot ultrasonic thickness meter to measure the thickness of an object to be inspected with ultrasonic waves, and without using a thermal expansion coefficient, It is characterized in that the thickness of the object to be inspected is calculated. In the second embodiment, the description of the same structure as that of the first embodiment is omitted, and only a different structure will be described.
[0044]
Fig.4 (a) shows the block diagram of a hot ultrasonic thickness meter. FIG. 4B shows a configuration diagram in which the radiation thermometer 7 is applied to the temperature estimation unit 13 of FIG.
[0045]
4 (a) and 4 (b), 1 denotes an object to be inspected, 2 denotes a pulse laser, and 3b denotes an electromagnetic ultrasonic sensor. In the second embodiment, since the ultrasonic wave is received by the reflection method, the electromagnetic ultrasonic sensor 3b is arranged on the same side as the pulse laser 2 with respect to the object 1 to be inspected. Further, a through hole 14 is formed at the center of the electromagnetic ultrasonic sensor 3b so that the pulse laser beam 5 can pass through the center of the electromagnetic ultrasonic sensor 3b. A radiation thermometer 7 shown in FIG. 1 (b) is connected to the device under test 1 by a fiber 6.
[0046]
Hereinafter, a method for measuring the thickness of the object to be inspected using the above-described hot ultrasonic thickness meter will be described.
[0047]
First, as in the first embodiment, a pulse laser beam 5 having a light density of 10 MW / cm ² or more is emitted from the pulse laser ² . The pulse laser beam 5 is condensed to 2 mm, for example, and passes through the through hole 14 to irradiate the inspection object 1 such as a steel plate. As a result, the surface 1b of the inspection object 1 is ablated, and ultrasonic waves 4 are generated by the reaction of this ablation. The ultrasonic wave 4 is reflected by the back surface 1a or the front surface 1b of the inspection object 1 and reciprocates a plurality of times within the inspection object 1. Here, an electromagnetic ultrasonic sensor 3 b is disposed on the surface 1 b of the inspection object 1. For this reason, whenever the ultrasonic wave 4 reaches | attains the surface 1b of the to-be-inspected object 1 by the side of the electromagnetic ultrasonic sensor 3b, the ultrasonic wave 4 is detected from the electromagnetic ultrasonic sensor 3b. Since the following is the same as that of the first embodiment, description thereof is omitted.
[0048]
According to the second embodiment, the same effect as that of the first embodiment can be obtained. Furthermore, in the second embodiment, a through hole 14 is provided at the center of the electromagnetic ultrasonic sensor 3b so that the pulse laser beam 5 can pass therethrough. For this reason, it is possible to transmit and receive ultrasonic waves not only by a transmission method such as γ-rays but also by a reflection method. Therefore, the hot ultrasonic thickness meter according to the second embodiment can be applied to an object to be inspected such as a steel pipe.
[0049]
In addition, the present invention can be variously modified and implemented without departing from the scope of the invention.
[0050]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a hot ultrasonic thickness meter that has an inexpensive apparatus configuration and can transmit and receive ultrasonic waves in a non-contact and high sensitivity. Further, it is possible to provide a thickness measuring method that can easily and accurately measure the thickness of the object to be inspected from the ultrasonic propagation time in the hot.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a hot ultrasonic thickness meter using a transmission method according to a first embodiment of the present invention.
FIG. 2 is a diagram showing an example of a multiple reflection waveform obtained by a hot ultrasonic thickness meter according to the first embodiment of the present invention.
FIG. 3 is a graph showing the relationship between the speed of sound and temperature of an object to be inspected used in the first embodiment of the present invention.
FIG. 4 is a configuration diagram of a hot ultrasonic thickness meter using a reflection method according to a second embodiment of the present invention.
[Explanation of symbols]
1 ... Inspected object,
1a: back side of the object to be inspected
1b: surface of the object to be inspected,
2 ... pulse laser,
3a, 3b ... electromagnetic ultrasonic sensors,
4 ... Ultrasound,
5 ... Laser light,
6 ... Fiber,
7 ... Radiation thermometer,
8 ... Broadband amplifier,
9 ... Bandpass filter,
10: A / D conversion unit,
11 ... thickness calculation part,
12 ... display part,
13 ... temperature estimation part,
14 ... through hole.

Claims (4)

Ultrasonic generation means for generating ultrasonic waves on the object to be inspected;
Ultrasonic receiving means for receiving ultrasonic waves propagated through the body to be examined;
Propagation time calculation means for calculating the propagation time of the ultrasonic wave from the received ultrasonic waveform;
Temperature estimation means for estimating the temperature T (where T> T ₀ ) of the measurement part of the object to be inspected;
A sound speed calculating means for calculating an apparent sound speed of the object to be inspected at the estimated temperature T based on a temperature characteristic of the apparent sound speed;
The thickness of the object to be inspected at the temperature T ₀ of the object to be inspected is provided with thickness calculation means for calculating the thickness of the object to be inspected at the temperature T ₀ by multiplying the propagation time and the apparent sound speed. An ultrasonic thickness meter to measure,
The temperature characteristic of the apparent sound velocity is that the ultrasonic propagation time of the test piece is measured at a plurality of temperatures higher than the temperature T ₀ for a test piece of the same material as the object to be inspected at the temperature T ₀ and the thickness WT _0. A hot ultrasonic thickness meter, wherein the thickness WT ₀ is determined by dividing the thickness WT ₀ by each propagation time at the measured plural temperatures.   The hot ultrasonic thickness meter according to claim 1, wherein the ultrasonic wave generating means is a pulse laser, and the ultrasonic wave receiving means is an electromagnetic ultrasonic sensor.   The hot ultrasonic thickness meter according to claim 2, wherein the electromagnetic ultrasonic sensor has a through hole through which a pulse laser beam passes. Generate ultrasonic waves in the object to be inspected, receive the ultrasonic waves propagated in the inspected body, calculate the propagation time of the ultrasonic waves from the waveform of the received ultrasonic waves,
Estimating the temperature T (here, T> T ₀ ) of the measurement part of the object to be inspected by the temperature estimation means ,
An apparent sound speed of the object to be inspected at the estimated temperature T is calculated based on a temperature characteristic of the apparent sound speed ,
A thickness measurement method for obtaining the thickness of the object to be inspected at a temperature T ₀ of the object to be inspected by multiplying the apparent sound speed and the propagation time of the ultrasonic wave in the object to be inspected,
Temperature characteristics of the acoustic velocity of the apparent, the test piece of the same material as the object to be inspected in the thickness WT ₀ at the temperature T _0, the ultrasonic wave propagation time of the test piece at a plurality of temperatures of the high temperature than the temperature T ₀ A thickness measurement method characterized in that it is determined by measuring and dividing the thickness WT ₀ by each propagation time at the plurality of measured temperatures.

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