2 and 4 point measurement

Why 2-point measurement is unsuitable for material analysis

A multimeter with two probes typically serves as a cost-effective and simple device for measuring voltage, current, and resistance. However, the conventional two-point method is only of limited use for the precise evaluation of materials.

With the classic 2-point measurement (two-wire method) Additional resistances are factored into the measurement result. These include, in particular:

• Line resistances
• Contact resistances of the measuring probes
• Connector resistors

These resistances add up to the actual material resistance. The measuring device therefore does not only measure the resistance of the sample, but also the resistance of the entire measuring chain.

Influence of contact resistances in two-wire measurement

In 2-pin measurement, the contact resistances have an influence. r1 and r2 the measurement result directly. Especially with low resistances or in precise material analyses, this effect leads to significant measurement deviations.

For applications in the Material characterization, research or quality assurance Therefore, this method is only suitable to a limited extent.

Multimeter 2-pin measurement and two-wire method (As can be clearly seen, the contact resistances r1, r2 have an influence and distort the measurement.)

The four-point measurement method for precise resistivity measurements

Operating principle of the four-wire method

For measuring conductors and semiconductors in the resistance range of 10⁻⁶ Ω cm to 10⁷ Ω cm will therefore the Four-point measurement method employed.

This method is based on the Four-wire process, which eliminates the influence of contact and line resistances. The measurement is performed via two separate circuits.

Circuit 1 – Power supply

A constant current source supplies a defined current through:

• the contact resistances
• the resistance to be measured Rx

Circuit 2 – Voltage measurement

A separate voltmeter measures only the voltage across the resistor. RxThe internal resistance of the measuring device is so high that practically no current flows through this measuring circuit.

This affects the contact resistances. r1 and r2 not the measurement result.

 

 Four-point measurement with 4-pin measuring heads

Measuring instruments of the Loresta series working with special 4-pin measuring headsEliminate these:

• Line resistances
• Connector resistors
• transition resistances

The result is significantly more precise and reproducible resistance measurements.

 

Using the constant current I and the measured voltage U, the specific surface resistance ps (surface) be determined. Is the Material thickness t known, additionally the specific Volume resistance pv (volume) be determined. With 4-pole measurement, only the placement of the measuring head on the measurement object is important, as the electrodes are already firmly integrated into the measuring head.

 

Electrode arrangement in the four-point measurement method

In the four-point measurement, four needle-like electrodes placed on the surface of the sample.

For a simplified calculation of the correction factors, the electrodes are located in equidistant distance to each other.

Typical electrode arrangements are:

• collinear arrangement (in a line)
• square arrangement

The two outer electrodes (1 and 4) They conduct a defined direct current through the sample.

The two inner electrodes (2 and 3) measure the tension on the surface of the material.

Determination of surface and volume resistance

From the relationship of Current (I) and Voltage (U) The electrical resistance can be determined.

Taking correction factors into account, important material properties can be calculated from this:

• specific surface resistivity (ρs)
• specific volume resistance (ρv)

Is additionally the Material thickness t Knowing this, the volume resistance can be determined with particular precision.

This measurement method enables a Standard-compliant and reproducible characterization of the electrical properties of materials and is therefore used in research, development and quality assurance.