JPS6370102A - Method for measuring thickness of conductor in non-destructive manner - Google Patents

Abstract

PURPOSE:To simply measure the thickness of a conductor, by measuring electric resistance of the surface of an article to be measured by four probes and estimating the thickness of the article to be measured from the measured value, shape correction factor and the volume intrinsic resistance of the article to be measured in a non-destructive manner. CONSTITUTION:When a current I flows between two probes 2, 5 positioned outside of four probes 2, 3, 4, 5 from a power source E, a current value is measured by an ammeter A. A voltmeter V measures the voltage between two probes 3, 4 positioned inside of four probes. Then, the thickness (t) of the conductive layer 1a of an article 1 to be measured is calculated from the volume intrinsic resistance of the conductive layer 1a of the article 1 to be measured, the current value I, the potential difference measured value between the probes 3, 4 and shape correction factor. By this method, the substantial thickness of the metal wall surface of water piping or a tank can be measured in a non-destructive manner.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for nondestructively measuring the thickness of conductors such as piping, metal walls of tanks, ship hulls, steel frames of buildings, metal wall surfaces, etc.

[Conventional technology]

Measuring the actual thickness of the metal walls of piping and tanks in which fluids flow or are stored for long periods of time is extremely important in predicting and confirming the lifespan and performance of piping and tanks.

Recently, accidents such as water leakage due to corrosion of the inner walls of water pipes and leakage of corrosive liquids due to corrosion of the inner walls of storage tanks have been occurring frequently. In particular, corrosion of the inner walls of water pipes has become a widespread social problem.

In order to prevent such accidents, it is extremely important to easily detect the internal situation.

Conventionally, such inspections have been carried out by inserting optical fibers or the like into piping or tanks and observing and inspecting their inner wall surfaces.

[Problem that the invention seeks to solve]

However, this method required expensive equipment and laborious testing, and quantitative measurements were difficult.

The present invention provides a method for easily, quickly, and non-destructively measuring the thickness of such a conductor.

[Means for solving problems]

In the present invention, four probes 2.3. ．． -4.5 method to measure the electrical resistance 1, the measured value, the shape correction coefficient and the object to be measured (sample 1
, lc, ld, le) non-destructively estimates the thickness of the metal or semiconductor layer of the measurement object (sample 1, lc, ld, le) from the volume resistivity ρ of the conductor thickness. It is a measurement method.

〔Example〕

Figure 1 (() (0) shows an example applied to a rectangular parallelepiped sample 1 in order to basically explain the principle of the present invention.
a is the vertical width of sample 1, b is the horizontal width of sample 1, t is the thickness of the conductive layer of sample I, and the shaded part is the insulating layer 1b below the conductive layer 1a.
It is. 2, 3, 4, and 5 are four probes in contact with the outer surface of sample 1. E is a power source, ■ is an ammeter, and ■ is a voltmeter (or potentiometer) with high internal impedance.

Next, the operation of this device will be explained.

The power source E is connected to the outer two probes 2.5 of the four probe probes 2, 3, and 4.5.
5, and a current of ■ flows, this current value is measured by an ammeter. Voltmeter ■ measures the voltage between the two inner probes 3 and 4 of the four probes.

Now, if the measured value of the potential difference between the two inner probes 3 and 4 is ΔV when a current ■ flows between the two outer probes 2 and 5, then the volume resistivity ρ and the thickness t of the sample 1, and the current value ■ and the potential difference Measured value Δ
There is the following relational expression between V.

Δ■ ρ= −t−Fc ・・・・・・・・・・・・・・・
(1) ■ (Fc (Correction Factor): Shape correction coefficient) Here, ΔV/I is the resistance value R. (It is not resistivity.) The shape correction coefficient Fc is the shape of sample 1, four probes 2° 3.4
．． 5 depending on the position on the sample surface. However, if the shape is constant and measurements are taken at the same position, the shape correction coefficient Fc can be determined by actually measuring the object with a known thickness t. In the case of a rectangular parallelepiped sample l, the shape correction coefficient Fc can also be solved mathematically.

On the other hand, the volume resistivity ρ has already been accurately measured using other methods.

For example, in the case of pure iron, it is 9.8 Xl0-'Ωam at room temperature, and in the case of copper, it is 1.6 Xl0-'Ωam at room temperature.
It is.

Feze3. Iron is oxidized due to corrosion. When Fe) is turned on, the volume resistivity ρ becomes 10'Ω (to) or more,
In comparison with iron, it may be considered an insulator.

Therefore, when the thickness t of the sample is unknown and the volume resistivity ρ is known, tFc=-7 can be found by measuring the resistance value R (.DELTA.V/I) using the four-probe method.

Therefore, if the relationship between t-Fc, the shape of the sample, and the position of the four probes is known (particularly if the other conditions are kept constant and the shape is a function of the thickness t), the thickness t can be found. It turns out.

Generally, there is a relationship between the thickness t and t-Fc0 as shown in the graph shown in the second figure.

When measuring the volume resistivity of a conductor using the normal two-probe method,
The contact resistance between the probe and the sample surface becomes large enough that it cannot be ignored, making accurate measurement difficult. However, if a high-impedance voltmeter is used in the four-probe method as in the present invention, the voltmeter
The influence of the current flowing into the sample is so small that the voltage drop due to the contact resistance can be ignored, and the measured value ΔV becomes a substantially accurate potential difference on the sample surface, making it possible to accurately measure the resistance.

FIG. 3 shows an embodiment in which the present invention is applied to the measurement of the wall thickness of a pipe-shaped sample 1c.

FIG. 4 shows an embodiment in which the present invention is applied to a nearly infinitely planar sample 1d, such as the wall of a large tank.

In the case of the rectangular parallelepiped-shaped sample l, as mentioned above,
Once the positions of the four probes 2.3 and 4.5 and the vertical and horizontal dimensions of the sample are determined, t-Fc can be determined accurately (the current flow pattern can be accurately calculated by solving Poisson's equation) can). Naturally, this principle can be applied to samples 1c and ld of other shapes even if t-Fc cannot be solved accurately. In the case of sample 1d as well, t-Fc can be mathematically solved accurately as a function of t by calculation using the mirror image method.

If the shape dependence tendency of tic is roughly confirmed, the thickness t can also be approximately estimated with a commensurate accuracy.

For practical purposes, that degree of accuracy is often sufficient to achieve the goal.

In a normal four-point probe, the distance S between each probe is constant and they are arranged equally in a straight line, but the relationship between t-Fc and t (i.e., Fc(!:
If the relationship between tQMs and t is known, there is no need to worry about the placement of the probe.
It is practical to change the

It is better to change the distance S between the probes 2.3 and 4.5 depending on the thickness of the object to be measured. Further, it is preferable to select the distance MS so that the slope of the graph shown in the second figure is steep because the estimated value will be more accurate. Thickness up to twice the distance S between the probes can be measured with relatively high accuracy.

Isotropic graphite 1e (upper and lower surfaces are 7 cm) as shown in Figure 5.
Measurements were carried out on 8 types of samples (+X 7 c+++ rectangular parallelepipeds with different thicknesses from 0.35 cm to 5.65 cm). Before making measurements, remove rust from the surface of the graphite 1e to lower the contact resistance with the electrode as much as possible to stabilize the contact situation. Next, as shown in Figure 5, four probes 2.3 and 4.5 were installed in the center at a distance of 10.5 cm so that the array was parallel to the sides, and the resistance value was measured.

The volume resistivity ρ of this graphite material has been determined by another method, and its value is 9. OX 10-'Ωcm.

On the other hand, when the relationship of Xt-wt to p is plotted on a graph using Fc calculated by Re5i Ca1c, it becomes as shown in FIG. Therefore, ρ/R was determined from the measured resistance value for each sample (actual thickness t), and the estimated thickness L1 obtained from the graph was shown in the table and compared with the actual measured thickness value t2.

The following findings can be obtained from the table.

1. Although the estimated thickness value t tends to be several percent lower than the actual value 1z, it can be estimated with an accuracy of 10% or less (it can be seen that there is no problem in practical terms with the accuracy of thickness estimation). 2. When the thickness becomes more than twice the distance between the needles (0,50!11), the accuracy drops dramatically.

If the thickness is greater than 3c+n, the estimated value t cannot be read from the graph. (The value of ρ/R does not intersect with the curve of Fc thickness) can be measured non-destructively.

[Brief explanation of the drawing]

Figure 1 (a) is an explanatory diagram showing the principle of the present invention, and (b) is (
2 is a graph showing the relationship between thickness t and t・Fc, FIG. 3 is a perspective view when the present invention is applied to pipe thickness measurement, and FIG. A perspective view when the present invention is applied to thickness measurement of a large metal wall surface, FIG. 5 is a perspective view of a measurement part in an experimental example of the present invention, and FIG. 6 shows the relationship between thickness t and tl'c in the experimental example. This is a graph showing. 1, lc, ld, 1e・” ---Object to be measured, 2, 3, 4.5... Four probes, ρ...
・Volume resistivity. -wl eye (a) C Ron salt 2 eye note 3 rho

Claims (1)

[Claims] Electrical resistance is measured on the surface of the object to be measured using the four-probe method.
A method for nondestructively measuring the thickness of a conductor, characterized by nondestructively estimating the thickness of a metal or semiconductor layer of an object to be measured from the measured value, a shape correction coefficient, and a volume resistivity of the object.