Electrical conductivity measurement provides a fast, non-destructive way to evaluate how well a material conducts electrical current. For manufacturers and quality teams, conductivity can also provide valuable information about material condition, composition, and processing.
Depending on the material and application, conductivity measurements can help evaluate:
- Aging condition of aluminum
- Strength-related properties of aluminum
- Alloy composition and consistency
- Material condition after heat treatment
- Density after a sintering process
- Material sorting and verification
However, accurate conductivity measurement depends on more than simply placing a probe on a component. Temperature, probe distance, material thickness, geometry, and magnetic properties can all influence the result.
Understanding these factors can help operators improve measurement accuracy and obtain more repeatable results.
What Can Affect Electrical Conductivity Measurements?
Six important factors can influence the accuracy of an eddy current conductivity measurement:
- Temperature
- Probe lift-off
- Material thickness
- Curved surfaces
- Edge effects
- Magnetic permeability
The influence of each factor depends on the material, component geometry, measuring frequency, probe, and inspection setup.
1. How Does Temperature Affect Conductivity Measurements?
Electrical conductivity changes with temperature.
For most metals, conductivity decreases as temperature increases. This means that measuring the same component at two different temperatures can produce different conductivity values even though the material itself has not changed.
For example, the conductivity of copper is approximately 58.0 MS/m at 20 °C and 56.9 MS/m at 25 °C. This represents a change of approximately 0.4% per degree Celsius.
Conductivity values are commonly referenced to 20 °C. When the component temperature differs from this reference temperature, the measured value can be converted using the material's temperature coefficient.
According to DIN 50994, the relationship can be expressed as:
σ(20 °C) = σ(T) × [1 + α × (T − 20 °C)]
Where:
- σ(20 °C) is the conductivity corrected to 20 °C
- σ(T) is the measured conductivity at the current temperature
- α is the material's temperature coefficient
- T is the temperature of the test object
Because temperature coefficients vary by material, the correct coefficient should be used for the material being measured.
How Can Temperature Effects Be Reduced?
Temperature can be compensated for directly or indirectly.
With direct temperature compensation, the temperature of the test object is measured and the conductivity value is corrected using the appropriate temperature coefficient.
With indirect temperature compensation, conductivity standards are brought to the same temperature as the test object and used for calibration.
Temperature compensation is particularly important when comparing measurements taken under different environmental or production conditions.
2. How Does Probe Lift-Off Affect Conductivity Measurements?
The distance between the conductivity probe and the surface of the test object is commonly called lift-off.
Changes in this distance can influence measurement accuracy because the electromagnetic interaction between the probe and the material becomes weaker as the distance increases.
Lift-off may occur because of:
- Paint or protective coatings
- Small gaps between the probe and component
- Surface contamination
- Uneven surfaces
- Inconsistent probe positioning
Modern eddy current conductivity instruments can detect and compensate for a certain amount of lift-off. This allows conductivity measurements to be performed through some thin, non-conductive coatings without removing them.
However, the allowable lift-off depends on the probe, measuring frequency, material, and instrument being used.
For repeatable measurements, the probe should be positioned as consistently as possible.
3. How Does Material Thickness Affect Conductivity Measurements?
Material thickness becomes important when measuring thin components.
Eddy current conductivity measurement relies on electromagnetic fields penetrating the material. If the material is too thin for the selected measuring frequency, the measurement can be influenced by the material thickness or by conductive material located behind the test object.
A minimum material thickness is therefore required for reliable conductivity measurement.
According to DIN 50994, the critical minimum thickness can be evaluated using the relationship:
dₛₐₜ = 3 × δ₀
where δ₀ represents the standard penetration depth.
Penetration depth depends primarily on:
- Measuring frequency
- Electrical conductivity
- Probe characteristics
How Does Frequency Affect Minimum Material Thickness?
Higher measuring frequencies result in a lower penetration depth. This is commonly known as the skin effect.
Because of this relationship, selecting a higher frequency can make it possible to measure thinner materials without the electromagnetic field being significantly influenced by material behind the test object.
However, the appropriate frequency still depends on the material and application.
How Can You Check Whether a Material Is Thick Enough?
A practical check can be performed by placing a conductivity reference standard behind the test object.
Measure the component normally and then repeat the measurement with the standard positioned behind it.
If the measured conductivity changes significantly when the standard is added, the test object's thickness may be influencing the measurement.
This provides a simple way to determine whether the selected frequency and material thickness are suitable for the application.
4. How Do Curved Surfaces Affect Conductivity Measurements?
Component geometry can also influence conductivity measurements.
A probe placed on a curved surface interacts with the material differently than it does on a flat surface. The smaller the radius of curvature, the greater the potential influence on the measured value.
This can become important when inspecting components such as:
- Tubes
- Rods
- Shafts
- Cylindrical aerospace components
- Other curved or rounded parts
How Can Curvature Effects Be Reduced?
One option is to calibrate the conductivity instrument using reference standards that have a similar material and geometry to the actual component.
With instruments that support curvature compensation, the operator may also be able to enter the component's radius or diameter. The instrument can then compensate for the known relationship between curvature, measuring frequency, and conductivity.
Proper and repeatable probe positioning is also important. A positioning or lowering aid can help keep the probe consistently aligned with the measurement surface.
5. Do Edges Affect Conductivity Measurements?
Measurements taken close to the edge of a component can produce inaccurate results if the probe's electromagnetic field extends beyond the available material.
This is known as the edge effect.
The influence of edges depends heavily on probe design.
An electromagnetically shielded conductivity probe can reduce the influence of surrounding material and magnetic fields. This makes it possible to perform measurements closer to component edges and in areas where access is limited.
Depending on the probe and application, this can help with measurements on:
- Small-diameter rods
- Component end faces
- Bores
- Milled recesses
- Areas close to edges
Using the appropriate probe for the component geometry can therefore be an important part of obtaining reliable conductivity measurements.
6. How Does Magnetic Permeability Affect Conductivity Measurements?
Eddy current conductivity measurement is intended primarily for non-ferromagnetic materials.
If the test material is magnetizable, its magnetic permeability can influence the electromagnetic response detected by the probe.
Even relatively small variations in permeability can create significant errors in a conductivity measurement.
Before relying on the measured conductivity value, it is therefore important to confirm that the material is appropriate for eddy current conductivity testing.
If magnetic properties are present, a different measurement method or additional material evaluation may be required.
How Can You Improve Conductivity Measurement Accuracy?
If conductivity measurements are inconsistent or different from expected values, start by checking the basic measurement conditions.
Ask the following questions:
- Is the component temperature being measured or compensated for?
- Is the probe positioned consistently against the surface?
- Is there a coating or gap creating excessive lift-off?
- Is the material thick enough for the selected measuring frequency?
- Is the measurement being taken on a curved surface?
- Is the probe too close to an edge?
- Could the material have magnetic properties?
- Was the instrument properly calibrated using appropriate reference standards?
Checking these factors can help determine whether a variation represents an actual material difference or is being introduced by the measurement setup.
What Is the SIGMATEST Conductivity Meter?
The FOERSTER SIGMATEST is a portable eddy current instrument designed for non-destructive electrical conductivity measurement of non-ferromagnetic metals.
Conductivity testing with SIGMATEST can be used for applications including:
- Material verification and sorting
- Quality control during manufacturing
- Evaluation of aluminum alloys
- Detection of heat-related material changes
- Inspection of material combinations
- In-process manufacturing inspection
- Aerospace maintenance and inspection
- Verification of incoming materials
The instrument is designed to provide fast and repeatable conductivity measurements while helping operators compensate for several of the factors that can otherwise influence measurement accuracy.
How Do You Choose the Right Conductivity Measurement Setup?
Reliable conductivity measurement depends on both the instrument and the inspection conditions.
Temperature, lift-off, thickness, curvature, edges, and permeability should all be considered when developing a measurement procedure. The appropriate probe and measuring frequency should also be selected based on the material and component geometry.
Taking these factors into account helps improve repeatability and makes it easier to determine whether a change in conductivity represents a real change in the material.
Having trouble getting consistent conductivity measurements?
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