JP2019174313A - Dense crack depth measuring method using electric resistance method - Google Patents

Abstract

To provide a measurement method with which it is possible to measure crack depth taking into account the effect on measurement accuracy of the length and the number of pieces of crack.SOLUTION: A dense crack depth measuring method using an electric resistance method includes a step for applying a measured value of the length of crack and/or the number of pieces of crack in an evaluation object and the measured value of a potential difference to an evaluation expression that indicates a correlation among the length of crack and/or the number of pieces of crack occurring on a member surface, a potential difference between one side and other side of the crack in a direction orthogonal to the length direction of the crack, and the depth of the crack, and finding the depth of the crack.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to a dense crack depth measurement apparatus and measurement method using an electrical resistance method.

  Conventionally, for example, digital X-ray inspection (accuracy: ± 0.3 mm) has been used as a method for nondestructively measuring the depth of a groove-like corrosion crack generated in a furnace wall tube of a land boiler. In this digital X-ray inspection, it is necessary to secure a space between the X-ray source and the object to be inspected, and since it takes time to adjust the position and move the X-ray source, a wide range of inspection is difficult. Certain restrictions arise, such as the inability to work in parallel with other constructions because it is necessary to leave workers other than the inspector.

  As a nondestructive inspection method for crack depth without using X-rays, a crack depth measurement method using an electrical resistance method is known. For example, Patent Document 1 discloses a method for obtaining a crack depth from an electrical resistance value of a CT test piece based on the intermittent energization with a crack length measurement current flowing through the CT test piece as intermittent energization of 1 second or longer. Yes. Further, in Patent Document 2, a display illumination unit is provided in each of a probe having a plurality of contact needles and a main body having a power source connected to a predetermined contact needle among the contact needles so that the energized state can be confirmed. An improved crack depth measuring instrument is disclosed.

Japanese Patent Laid-Open No. 4-194471 Japanese Utility Model Publication No. 4-43256

  However, neither Patent Document 1 nor Patent Document 2 discloses any consideration of the influence of the crack length and the number of cracks mixed between terminals when measuring the crack depth.

  In view of the above circumstances, at least one embodiment of the present invention aims to provide a measurement method capable of measuring the crack depth in consideration of the influence of the length and number of cracks on the measurement accuracy.

(1) A measurement method according to at least one embodiment of the present invention includes:
A method for measuring crack depth using an electrical resistance method,
Evaluation formula showing a correlation between at least one of the length or number of cracks generated on the surface of the member, a potential difference between one side and the other side of the crack in a direction orthogonal to the length direction of the crack, and the depth of the crack And determining the depth of the crack by applying the measured value of at least one of the length or number of the cracks in the evaluation object and the measured value of the potential difference.

As a result of diligent research by the present inventors, it has been clarified that, in the measurement of the crack depth using the electric resistance method, a measurement error may occur depending on the length and number of cracks. That is, when the crack length is short, a current path that bypasses the side of the crack is generated, and the electric resistance value is observed to be low, and the number of cracks existing between the input / output terminals and the measurement terminals of the power supply voltage is small. Increasing the depth tends to underestimate the depth and can affect measurement accuracy. Therefore, it has been found that it is effective to consider the length and number of cracks in order to reduce this measurement error.
According to the above method (1), in the crack depth measurement using the electrical resistance method, the crack depth is measured in consideration of the effect of at least one of the length and number of cracks that can affect the measurement accuracy. Therefore, the measurement error can be reduced and the crack depth can be measured with high accuracy. Moreover, it is not necessary to use radiation such as X-rays for measuring the crack depth, and the crack depth can be inspected in parallel with other operations, so that the work efficiency can be greatly improved. .

(2) In some embodiments, in the method of (1) above,
Measuring the length or number of the cracks to be measured;
A step of measuring a potential difference between the one side and the other side of the crack by arranging a pair of measurement terminals on the one side and the other side of the crack to be measured;
Further comprising
In the step of determining the depth of the crack, the measured value obtained in the step of measuring the length or number of the cracks and the measured value obtained in the step of measuring the potential difference are applied to the evaluation formula. The depth of the crack may be obtained.

  According to the method of (2) above, by applying the measured value of the length of the crack to be measured and the measured value of the potential difference between one side and the other side of the crack to the evaluation formula, Can be obtained with high accuracy.

(3) In some embodiments, in the method of (2) above,
In the step of measuring the potential difference, the measurement terminals may be arranged such that only one crack is arranged between the pair of measurement terminals.

When a crack other than the object to be measured exists between measurement terminals that measure the potential difference on both sides across the crack, the electrical resistance increases, so the measured potential difference tends to increase.
In this regard, according to the method of (3) above, only one crack to be measured is arranged between the measurement terminals. Therefore, an increase in potential difference due to the presence of other cracks, that is, an error is suppressed, and the potential difference is accurately detected. Can be measured. Based on the measured potential difference, the depth of the crack can be obtained with high accuracy.

(4) In some embodiments, in the method according to any one of (1) to (3) above,
The measurement terminal may be arranged side by side at a position shifted in the length direction of the crack with respect to a pair of an input terminal and an output terminal connected to the input terminal via a power source.

In the measurement of crack depth using the electrical resistance method, if there is a crack other than the measurement target between the voltage input terminal or output terminal and the measurement terminal, the current flowing between the measurement terminals decreases. The potential difference between the two tends to decrease, and the measurement accuracy may decrease.
In this respect, according to the method of (4) above, the input terminal and the output terminal respectively disposed on one side and the other side of the crack to be measured, and the measurement terminal for measuring the potential difference are in the length direction of the crack. Therefore, the possibility that an extra crack is interposed between the input terminal or the output terminal and the measurement terminal can be eliminated as much as possible. Therefore, the measurement error can be reduced, and the potential difference on both sides of the crack and the depth of the crack can be determined with higher accuracy.

(5) In some embodiments, in the method according to any one of (1) to (3) above,
The measurement terminal may be arranged on a line connecting the input terminal and the output terminal between a pair of the input terminal and an output terminal connected to the input terminal via a power source.

  According to the method (5), the voltage input terminal and output terminal and the potential difference measurement terminal are arranged in a straight line.

(6) In some embodiments, in the method according to any one of (1) to (5) above,
The evaluation formula may be created for each number of cracks existing between the measurement terminals.

In the measurement of the crack depth using the electrical resistance method, the potential difference measured between the measurement terminals is affected by the number of cracks existing between the measurement terminals. That is, if the number of cracks existing between the measurement terminals is different, a different evaluation formula is required, and an error is increased in measurement without considering the number of cracks.
In this respect, according to the method of (6) above, an evaluation formula is created for each number of cracks existing between the measurement terminals. Therefore, by using an evaluation formula corresponding to the number of cracks to be measured, cracks to be measured Can be obtained with high accuracy. In the method (6), for example, when the voltage input terminal and output terminal pair and the measurement terminal pair are arranged in a straight line, the distance between the terminals is relatively long, and a plurality of the terminals are arranged between the terminals. This is particularly effective for reducing measurement errors when cracks can be present.

(7) In some embodiments, in the method according to any one of the above (1) to (6), a crack depth measurement method using an electrical resistance method is:
A surface treatment step for cleaning an outer surface around the crack to be measured may be further provided.

  According to the method (7), the surface of the member that may adversely affect the potential difference measurement can be removed in the surface treatment step. Therefore, the measurement error can be suppressed, and the potential difference on both sides of the crack, that is, the depth of the crack to be measured can be measured with higher accuracy.

(8) In some embodiments, in the method according to any one of the above (1) to (7), a method for measuring a crack depth using an electrical resistance method includes:
The method may further include a step of obtaining the evaluation formula in advance by measuring a potential difference using a calibration test piece.

  According to the method of (8) above, based on the potential difference measurement using the calibration test piece, based on the crack formed in the test piece of the same material as the member where the crack to be measured, the length or number of the cracks, An evaluation formula indicating the correlation between the potential difference between one side and the other side of the crack and the depth of the crack can be acquired in advance. By applying the length or number of cracks to be measured and the potential difference between one side and the other side of the crack to the evaluation formula thus obtained, the depth of the crack to be measured can be obtained with high accuracy. Can do.

(9) In some embodiments, in the method of (3) above,
The evaluation object may include furnace wall piping.

  According to the above method (9), the depth of the crack generated on the surface of the furnace wall pipe can be obtained with high accuracy by using the electric resistance method. Therefore, it is not necessary to use a measuring device that uses radiation such as X-rays for measuring the crack depth, and the crack depth can be inspected in parallel with other operations, greatly improving work efficiency. Can be improved.

  According to at least one embodiment of the present invention, there is provided a measurement method capable of measuring the crack depth in consideration of the influence of the length and number of cracks on the measurement accuracy.

It is a flowchart which shows the measuring method of the crack depth which concerns on one Embodiment of this invention. It is the schematic for demonstrating the measurement principle of the crack depth using an electrical resistance method. It is the schematic for demonstrating the measurement principle of the crack depth using an electrical resistance method, (a) is the top view seen from the crack 14 upper direction, (b) shows the figure seen from the crack length L1 direction. . It is a figure similar to FIG. 2, and shows the case where the crack length is shorter than FIG. It is a figure similar to FIG. 3, and shows a case where the crack length is shorter than FIG. It is a perspective view which shows the state which arrange | positions an input terminal, an output terminal, and a measurement terminal pair in series, and measures an electrical potential difference. An input terminal, an output terminal, and a measurement terminal pair are arranged in series to show a state of measuring a potential difference. (A) shows one crack between measurement terminals, (b) shows two cracks between measurement terminals, c) shows a state in which one crack exists between the measurement terminals and between the measurement terminal and the input terminal. It is a perspective view which shows the state which arrange | positions an input terminal, an output terminal, and a measurement terminal pair in parallel, and measures a potential difference. The state in which an input terminal, an output terminal, and a measurement terminal pair are arranged in parallel to measure a potential difference is shown. (A) shows one crack between measurement terminals, (b) shows two cracks between measurement terminals, c) shows a state in which one crack exists between the measurement terminals and outside the measurement terminal pair. It is a figure which shows the relationship between the estimated value and actual value of a crack depth by the crack depth measuring method which concerns on one Embodiment of this invention. It is a figure which shows the relationship between the number of the cracks contained between measurement terminals, and an electrical potential difference.

Hereinafter, some embodiments of the present invention will be described with reference to the accompanying drawings. However, the dimensions, materials, shapes, relative arrangements, etc. of the components described in the embodiments or shown in the drawings are not intended to limit the scope of the present invention, but are merely illustrative examples. Absent.
For example, expressions expressing relative or absolute arrangements such as “in a certain direction”, “along a certain direction”, “parallel”, “orthogonal”, “center”, “concentric” or “coaxial” are strictly In addition to such an arrangement, it is also possible to represent a state of relative displacement with an angle or a distance such that tolerance or the same function can be obtained.
In addition, for example, expressions representing shapes such as quadrangular shapes and cylindrical shapes not only represent shapes such as quadrangular shapes and cylindrical shapes in a strict geometric sense, but also within the range where the same effect can be obtained. A shape including a chamfered portion or the like is also expressed.
On the other hand, the expressions “comprising”, “comprising”, “comprising”, “including”, or “having” one constituent element are not exclusive expressions for excluding the existence of the other constituent elements.

FIG. 1 is a flowchart showing a crack depth measuring method according to an embodiment of the present invention.
As shown in FIG. 1, a measurement method according to at least one embodiment of the present invention is a method for measuring the depth of a crack 14 (hereinafter, also referred to as crack depth D) using an electrical resistance method, At least one of the length (hereinafter also referred to as crack length L) or the number of cracks 14 generated on the surface 12 and the potential difference V between one side and the other side of the crack 14 in the direction orthogonal to the length direction of the crack 14. And the evaluation formula indicating the correlation between the depth D of the crack 14 and the measured value of at least one of the length or number of the cracks 14 in the evaluation object 10 and the measured value of the potential difference, the depth of the crack 14 is determined. Step S5 for obtaining is provided.

First, the principle of measuring the crack depth using the electrical resistance method will be described. Since the principle is well known in the art, details thereof will not be described in detail, and only points relating to the present invention will be described.
FIG. 2 is a schematic diagram for explaining the principle of measuring the crack depth using the electric resistance method, and is a view seen from a direction orthogonal to the length L1 direction of the crack 14. FIG. 3 is a schematic diagram for explaining the principle of crack depth measurement using the electrical resistance method, where (a) is a plan view seen from above the crack 14 and (b) is from the direction of the crack length L1. FIG. In FIG. 2, for example, a part of piping viewed from the tube axis direction A is shown as the evaluation object 10 that is an object for evaluating the depth D <b> 1 of the crack 14.
As shown in FIG. 2 and FIG. 3, for example, when obtaining the depth of the crack 14 generated in the direction orthogonal to the tube axis direction A, the member of the evaluation object 10 corresponding to one side of the crack 14 in the tube axis direction A The power supply voltage input terminal 2 is brought into contact with the surface 12, and the power supply voltage output terminal 4 is brought into contact with the other member surface 12 of the crack 14. When a voltage is applied between the input terminal 2 and the output terminal 4, a constant current flows from the input terminal 2 to the output terminal 4 through a route passing under the crack 14 (see, for example, FIG. 3B). Then, the measurement terminal pairs 6 and 6 respectively connected to the potentiometer 8 are brought into contact with one side and the other side of the crack 14, so that the potential difference between both sides (the one side and the other side) of the crack 14 is increased. It is measured with a potentiometer 8. Since this potential difference and the crack 14 are represented by a linear relationship, the depth of the crack 14 can be obtained by measuring the above potential difference.

Here, as a result of diligent research by the present inventors, it has been clarified that, in the measurement of the depth of the crack 14 using the electrical resistance method, a measurement error may occur depending on the length and number of the cracks 14.
FIG. 4 is a view similar to FIG. 2, and shows a case where the crack length L2 is shorter than the crack length L1 shown in FIG. 2 (L1> L2). FIG. 5 is a view similar to FIG. 3 and shows a case where the crack length L2 is shorter than the crack length L1 shown in FIG.
As shown in FIGS. 4 and 5, when the length L of the crack 14 is short, a current path that bypasses the side of the crack 14 (see, for example, FIGS. 5A and 5B) occurs. Therefore, the potential difference, that is, the electric resistance value is observed to be low (R = V / I). Therefore, when the crack depth D is estimated based only on the potential difference measured by the potentiometer 8 without considering the crack length L, the ratio of the measurement error with the actually measured value of the crack depth D increases. On the other hand, when the crack depth D is estimated in consideration of the crack length L in the potential difference measured by the potentiometer 8, the error ratio is greatly reduced, and the measurement error from the actual value of the crack depth D is, for example, It has become clear that it is generally within the range of ± 0.3 mm, which is the same level as the digital X-ray inspection (see, for example, FIG. 10).

FIG. 6 is a perspective view showing a state in which a potential difference is measured by arranging an input terminal, an output terminal, and a measurement terminal pair in series. FIG. 7 shows a state in which a potential difference is measured by arranging an input terminal, an output terminal, and a measurement terminal pair in series. (A) shows one crack between measurement terminals, and (b) shows a crack between measurement terminals. 2 and (c) show a state in which one crack exists between the measurement terminals and between the measurement terminal and the input terminal.
As shown in FIGS. 6 and 7, when the number of cracks 14 existing between the input / output terminals 2 and 4 and the measurement terminals 6 and 6 of the power supply voltage increases (for example, FIGS. 7A and 7B). Reference), the electrical resistance increases, so the potential difference measured by the potentiometer 8 increases. For this reason, the measurement accuracy of the crack depth D may be affected (for example, see FIG. 11).
Therefore, it has been found that it is effective to consider the length L and the number of cracks 14 in order to reduce the measurement error as described above.
Note that there is a correlation between at least one of the length L or the number of cracks 14, the potential difference V between one side and the other side of the crack 14 in the direction perpendicular to the length direction of the crack 14, and the depth D of the crack 14. May be obtained by measuring in advance according to the length of the crack 14, the number of cracks 14 included between the measurement terminal pairs 6, 6, or the material used as the evaluation object 10. Good.
According to the above method, in the measurement of the crack depth using the electrical resistance method, the crack depth D can be measured in consideration of the influence of at least one of the length L or the number of the cracks 14 on the measurement accuracy. As a result, the measurement error can be reduced and the crack depth D can be measured with high accuracy. Moreover, it is not necessary to use radiation such as X-rays for the measurement of the crack depth D, and the inspection of the crack depth D can be performed in parallel with other work, so that the work efficiency is greatly improved. Can do.

In some embodiments, the method of measuring the depth of the crack 14 using the electrical resistance method includes, for example, step S3 of measuring the length or number of the cracks 14 to be measured, as shown in FIG. A step S4 of measuring a potential difference V between one side and the other side of the crack by arranging a pair of measurement terminals on one side and the other side of the target crack may be further provided.
In step S3, for example, the crack length L may be measured using a ruler such as a caliper (not shown), or the number of cracks 14 may be counted visually. Further, for example, the length and the number of the cracks 14 may be measured based on the scattering of the laser light irradiated on the member surface 12.
In step S4, for example, the potential difference V on both sides of the crack 14 may be measured using the potentiometer 8 as described above. The input terminal 2, the output terminal 4, and the measurement terminals 6 and 6 may be provided as a portable probe mounted on the carrier 24, for example. The center positions of the measurement terminal pairs 6 and 6 are, for example, ± 10 in the direction connecting the input / output terminal pairs 2 and 4 from the line along the crack length direction passing through the center positions of the input / output terminal pairs 2 and 4. % May be arranged at the position of%. The distance between the measurement terminals 6 and 6 may be, for example, 0.7 to 1.5 times the distance between the input / output terminal pair 2 and 4.
And in step S5 which calculates | requires the depth D of the crack 14, the measured value obtained by step S3 which measures the length L or the number of the cracks 14, and the measured value obtained by step S4 which measures the electrical potential difference V are used. You may obtain | require the depth D of the crack 14 by applying to an evaluation type | formula. In this way, by applying the actual measurement value of the length L of the crack 14 to be measured and the actual measurement value of the potential difference V between the one side and the other side of the crack 14 to the evaluation formula, The depth of 14 can be obtained with high accuracy.

  In some embodiments, in step S4 for measuring the potential difference V, for example, as shown in FIGS. 6, 7A, 8, and 9A, a crack 14 is formed between the pair of measurement terminals 6. The measurement terminals 6 may be arranged so that only one is arranged.

If there is a crack 14 other than the object to be measured between the measurement terminals 6 that measure the potential difference on both sides across the crack 14, the electrical resistance increases, so the measured potential difference tends to increase.
In this respect, according to the above method, only one crack 14 to be measured is disposed between the measurement terminal pairs 6 and 6, so that an increase in potential difference due to the presence of other cracks 14, that is, an error is suppressed. The potential difference V can be measured with high accuracy. Based on the measured potential difference V, the depth D of the crack 14 can be obtained with high accuracy.

  In some embodiments, for example, as shown in FIGS. 8 and 9, the measurement terminal 6 is connected to a pair of an input terminal 2 and an output terminal 4 connected to the input terminal 2 via a power supply 16. Alternatively, the cracks 14 may be arranged side by side at positions shifted in the length direction.

In the measurement of the crack depth D using the electrical resistance method, if there is a crack 14 other than the object to be measured between the voltage input terminal 2 or the output terminal 4 and the measurement terminal 6, the current flowing between the measurement terminals 6 decreases. For this reason, the potential difference between the measurement terminals 6 tends to decrease, and the measurement accuracy may decrease.
In this regard, according to the above-described method, the input terminal 2 and the output terminal 4 respectively disposed on one side and the other side of the crack 14 to be measured, and the measurement terminal 6 for measuring the potential difference are the length of the crack 14. Since they are arranged side by side at positions shifted in the vertical direction (that is, arranged in parallel), the possibility that an extra crack 14 is interposed between the input terminal 2 or the output terminal 4 and the measurement terminal 6 can be eliminated as much as possible. it can. Accordingly, the measurement error can be reduced, and the potential difference V on both sides of the crack 14 and the depth D of the crack 14 can be obtained with higher accuracy.

  In some embodiments, for example, as shown in FIGS. 6 and 7, the measurement terminal 6 is placed between a pair of an input terminal 2 and an output terminal 4 connected to the input terminal 2 via a power supply 16. Alternatively, the input terminal 2 and the output terminal 4 may be arranged on a line. In this way, the voltage input terminal 2 and output terminal 4 and the potential difference measurement terminal 6 are linearly arranged (that is, arranged in series).

  In some embodiments, the method of measuring the crack depth D using the electrical resistance method is a surface that cleans the outer surface (member surface 12) around the crack 14 to be measured, for example, as shown in FIG. Processing step S2 may be further provided. In this way, dirt on the member surface 12 that may adversely affect the measurement of the potential difference V can be removed in the surface treatment step S2. Therefore, it is possible to suppress the measurement error and measure the potential difference V on both sides of the crack 14 and thus the depth D of the crack 14 to be measured with higher accuracy.

  In some embodiments, the method for measuring the crack depth D using the electrical resistance method includes, for example, a step of obtaining an evaluation formula in advance by measuring a potential difference using a calibration test piece (not shown) as shown in FIG. S1 may be further provided. In this case, the length of the crack 14 is determined based on the crack 14 formed in the calibration test piece of the same material as the member (evaluation object 10) where the crack 14 to be measured is measured by the potential difference measurement using the calibration test piece. An evaluation formula indicating a correlation between the length L or the number, the potential difference V between one side and the other side of the crack 14, and the depth of the crack 14 can be acquired in advance. By applying the length or number of the cracks 14 to be measured and the potential difference between one side and the other side of the cracks 14 to the evaluation formula thus obtained, the depth of the cracks 14 to be measured can be accurately determined. Can be requested.

  In some embodiments, an evaluation formula may be created for each number of cracks 14 present between the measurement terminals 6 (see, for example, FIG. 11).

As described above, in the measurement of the crack depth D using the electric resistance method, the potential difference V measured between the measurement terminals 6 is affected by the number of cracks 14 existing between the measurement terminals 6. That is, if the number of cracks 14 existing between the measurement terminals 6 is different, a different evaluation formula is required, and an error becomes large in measurement without considering the number of cracks.
In this respect, according to the above-described method, an evaluation formula is created for each number of cracks 14 existing between the measurement terminals 6. Therefore, by using an evaluation formula corresponding to the number of cracks 14 to be measured, The depth D of the crack 14 can be obtained with high accuracy. In addition, the above-described method is, for example, when the pair of the voltage input terminal 2 and the output terminal 4 and the measurement terminal pair 6 and 6 are arranged in a straight line. This is particularly effective for reducing measurement errors when a plurality of cracks 14 can be present.

  In some embodiments, the evaluation object 10 may include a furnace wall tube 10A (see, for example, FIG. 2). If it does in this way, the depth D of the crack 14 which arose on the surface of 10 A of furnace wall pipes can be calculated | required with high precision using an electrical resistance method. Therefore, it is not necessary to use a measuring device that uses radiation such as X-rays for the measurement of the crack depth D, and the inspection of the crack depth D can be performed in parallel with other operations. It can be greatly improved.

  According to the configuration described above, the crack depth D can be measured in consideration of the influence on the measurement accuracy due to the length L and the number of cracks.

  The present invention is not limited to the above-described embodiments, and includes forms obtained by modifying the above-described embodiments and forms obtained by appropriately combining these forms.

1 Measuring device (crack depth measuring device)
2 Input terminal 4 Output terminal 6 Measurement terminal (Measurement terminal pair)
8 Potentiometer 10 Object to be evaluated (piping / furnace wall pipe)
12 Member surface 14 Crack 16 Power supply 24 Carrier A Tube axis direction

Claims (9)

A method for measuring crack depth using an electrical resistance method,
Evaluation formula showing a correlation between at least one of the length or number of cracks generated on the surface of the member, a potential difference between one side and the other side of the crack in a direction orthogonal to the length direction of the crack, and the depth of the crack And measuring the depth of the crack by applying at least one measurement value of the length or number of the cracks in the evaluation object and the measurement value of the potential difference. Measuring the length or number of the cracks to be measured;
A step of measuring a potential difference between the one side and the other side of the crack by arranging a pair of measurement terminals on the one side and the other side of the crack to be measured;
Further comprising
In the step of determining the depth of the crack, the measured value obtained in the step of measuring the length or number of the cracks and the measured value obtained in the step of measuring the potential difference are applied to the evaluation formula. The measurement method according to claim 1, wherein the depth of the crack is obtained. The measuring method according to claim 2, wherein in the step of measuring the potential difference, each of the measurement terminals is arranged such that only one crack is arranged between the pair of measurement terminals. The measurement terminal is arranged side by side at a position shifted in the length direction of the crack with respect to a pair of an input terminal and an output terminal connected to the input terminal via a power source. The measuring method as described in any one of Claims 1 thru | or 3.   The measurement terminal is disposed on a line connecting the input terminal and the output terminal between a pair of the input terminal and an output terminal connected to the input terminal via a power source. The measuring method as described in any one of thru | or 3. The measurement method according to claim 1, wherein the evaluation formula is created for each number of cracks existing between the measurement terminals.   The measurement method according to claim 1, further comprising a surface treatment step of cleaning an outer surface around the crack to be measured.   The measurement method according to any one of claims 1 to 7, further comprising a step of obtaining the evaluation formula in advance by measuring a potential difference using a calibration test piece. The measurement method according to claim 3, wherein the evaluation object includes a furnace wall tube.