A Guide to Resistance Measurement

Index

• 1. Introduction to resistance measurement
• 2. Applications
• 3. Resistance
• 4. Principles of resistance measurement
• 5. Methods of 4 terminal connections
• 6. Possible measurement errors
• 7. Choosing the right instrument

1. Introduction

The measurement of very large or very small quantities is always difficult, and resistance measurement is no exception. Values above 1GΩ and values below 1Ω both present measurement problems.

Cropico is a world leader in low resistance measurement, producing a comprehensive range of low resistance ohmmeters and accessories that cover most measurement applications. This handbook provides an overview of low resistance measurement techniques, explains common causes of errors and how to avoid them. It also includes useful tables of wire and cable characteristics, temperature coefficients, and various formulas to ensure the best possible choice when selecting a measuring instrument and technique.

2. Applications

Manufacturers of components

Resistors, inductors, and chokes must verify that their product meets the specified resistance tolerance for end-of-production-line and quality control testing.

Manufacturers of switches, relays & connectors

Verification that contact resistance is below pre-specified limits is required, achievable at end-of-production line testing to ensure quality control.

Cable manufacturers

Must measure the resistance of the copper wires they produce. Resistance too high means reduced current carrying capability, while resistance too low suggests the manufacturer is being too generous with cable diameter, using more copper than necessary, which can be expensive.

Installation & maintenance of power cables, switchgear & voltage tap changers

These require cable joints and switch contacts to have the lowest possible resistance to avoid excessive heating. Poor joints or contacts can fail due to this heating effect. Routine preventative maintenance with regular resistance checks ensures the best possible life performance.

Electric motor & generator manufacturers

There is a requirement to determine the maximum temperature reached under full load. The temperature coefficient of the copper winding is used for this. Resistance is first measured at ambient temperature, then after the unit runs at full load for a specified period. The change in resistance value indicates the internal motor/generator temperature. Cropico ohmmeters also measure individual coils of a motor winding to ensure no short or open circuit turns and that each coil is balanced.

The automotive industry

Requirement to measure the resistance of robot welding cables to ensure weld quality does not deteriorate, including battery lead crimp connectors, air bag detonator resistance, wiring harness resistance, and quality of crimp connectors.

Fuse manufacturers

For quality control, resistance bonding measurements on aircraft and military vehicles are necessary to ensure all equipment is electrically connected to the airframe. Tanks and other military vehicles have similar requirements. Producers and users of large electrical currents need to measure joint resistance, busbars, and connectors for electroplating.

Railway utilities

Measurement of power distribution cable joints and rail track joints, particularly for underground railways where rails are used for signalling information.

3. Resistance

Ohm's Law: V = I x R (Volts = Current x Resistance). The Ohm (Ω) is a unit of electrical resistance equal to that of a conductor in which a current of one ampere is produced by a potential of one volt across its terminals. Ohm's law, named after German physicist Georg Ohm, is a fundamental law of electricity defining the relationship between current, voltage, and resistance. When voltage is applied to a circuit with only resistive elements, current flows according to Ohm's Law.

Diagram: Ohm's Law Formula

A simple circuit diagram showing a voltage source (V), a resistor (R), and current (I) flowing through the circuit. Labels indicate: I = Electrical Current (Amperes), V = Voltage (Volts), R = Resistance (Ohms).

4. Principles of resistance measurement

Ammeter Voltmeter method

This basic method uses a battery as a voltage source, a voltmeter to measure voltage, and an ammeter to measure current. Resistance is calculated as Voltage / Current. While providing reasonable accuracy, it is not a practical solution for everyday measurement needs.

Diagram: Ammeter Voltmeter Method

A circuit diagram illustrating the Ammeter Voltmeter method. It shows a voltage source connected to a resistor (Resistance), with a voltmeter connected in parallel across the resistor and an ammeter connected in series with the resistor. The formula Resistance = Volt / Current is shown.

Kelvin Double Bridge

The Kelvin Bridge is a variation of the Wheatstone bridge used for measuring low resistances, typically from 1mΩ to 1kΩ with a resolution of 1μΩ. Limitations include the need for manual balancing, a sensitive null detector or galvanometer, and reasonably high measurement current for sufficient sensitivity.

The Kelvin Double Bridge has generally been replaced by digital ohmmeters.

DMM - Two-wire Connection

A simple digital multimeter can be used for higher resistance values (above 1Ω) where high accuracy is not required. This method forces a test current through the component and measures the voltage at its terminals. The meter calculates resistance, including the voltage drop across the connection leads. Good quality test leads have a resistance of approximately 0.02Ω per meter. Lead connection resistance can be as high as or higher than the leads themselves. This error becomes significant as measured value decreases, becoming inappropriate below 1Ω.

Diagram: Two-wire Connection Measurement

A diagram showing a DMM (Digital Multimeter) connected to a component (Rx) via test leads (R1, R2, R3, R4). The diagram illustrates that the DMM measures the total resistance, including the component (Rx) and the test leads (R1+R2) and connection resistance (R3+R4). The formula shown is DMM measures: Rx + R1 + R2 + R3 + R4.

TABLE 1: Examples of possible measurement errors

To measure true DC, resistance ohmmeters typically use 4-wire measurement. DC current is passed through the Rx and the ohmmeter's internal standard. The voltage across the Rx and the internal standard is measured, and the ratio of the two readings calculates resistance. The current only needs to be steady for milliseconds for both readings. This method requires two measurement circuits and usually a sensitive voltage measurement.

Diagram: 4-wire Measurement Principle

A circuit diagram illustrating the 4-wire measurement principle. It shows the Device Under Test (Rx) with current flowing through it. Two voltage measurements are indicated: one across Rx and one across the ohmmeter's internal standard (Rstd). The diagram shows current flowing through both Rx and Rstd.

Alternatively, a constant current source can be used to pass current through Rx. The voltage drop across Rx is then measured and resistance calculated. This method requires one measurement circuit, but the current generator must be stable under all measurement conditions.

Diagram: Constant Current Source Measurement

A circuit diagram showing a constant current source connected to the Device Under Test (Rx). A voltmeter is shown measuring the voltage across Rx.

Four wire connection

The four-wire (Kelvin) method is preferred for resistance values below 100Ω. All Seaward milliohmmeters and microhmmeters use this method, employing 4 separate wires: 2 for current (source/current leads) and 2 for voltage sensing (sense/potential leads). A small current flows in the sense leads, which is negligible. The voltage drop across the ohmmeter's sense terminals is virtually the same as the voltage drop across Rx, producing accurate and consistent results for resistances below 100Ω.

Diagram: Four-wire Connection Detail

A diagram illustrating the placement of four test leads (C, P, P1, C1) on a sample cable. It details the position of current (C, C1) and potential (P, P1) leads, emphasizing that potential lead positions are critical and must be inside the current leads, exactly at the measurement points. Current lead positions are less critical but must be outside the potential leads.

This is the best connection type for measurement with 4 separate wires. Current wires (C and C1) must be placed outside the potential wires (P and P1), though their exact placement is not critical. Potential wires must be connected exactly at the points to be measured between. While this provides the best results, it's often not practical. Cropico offers practical measurement solutions.

5. Methods of 4 terminal connections

Kelvin clips

Kelvin clips are similar to alligator clips but with insulated jaws. The current lead connects to one jaw, and the potential lead to the other. They offer a practical solution for making four-terminal connections to wires, busbars, and plates.

Diagram: Kelvin Clips Connection

Illustrations showing the connection of Kelvin clips to a sample cable, indicating the position of current (C, C1) and potential (P, P1) connections.

Duplex Handspikes

Handspikes are practical for sheet material, busbars, and areas with difficult access. Each handspike has two sprung spikes: one for current connection and one for potential/sense connection.

Diagram: Duplex Handspikes

A diagram showing duplex handspikes being used to measure busbar joint resistance, illustrating current and potential spikes.

Stacked Lead connection

This method is used when it's the only practical solution for connecting to Rx, with the current lead pushed into the back of the potential lead. This can cause small errors as the measurement point is where the potential lead connects to the current lead. It's a compromise solution for awkward-to-reach samples.

Diagram: Stacked Lead Connection

A diagram illustrating the connection of stacked leads to a sample, showing the position of current (C, C1) and potential (P, P2) leads and the measurement point.

Cable clamps

For measuring cables during manufacture and quality control, consistent measuring conditions are essential. A 1-meter cable sample is typically used. Cropico offers various cable clamps that accommodate most cable sizes. The cable is placed in the clamp, with ends in current terminals. Potential connection points are usually knife-edge contacts exactly 1 meter apart.

Diagram: Cable Clamp Connection

A diagram showing current clamp connections (distance not critical) and potential knife-edge connections 1 meter apart on a cable.

Jigs and fixtures

When measuring components like resistors, fuses, switch contacts, or rivets, using a test jig to hold the component is crucial for ensuring consistent measurement conditions (e.g., lead position). Jigs are often specially designed for specific applications.

6. Possible measurement errors

Dirty connections

Ensure devices are clean and free from oxides and dirt. High resistance connections cause reading errors and can prevent measurements. Some coatings and oxides (e.g., anodising) are good insulators and require cleaning at connection points. Cropico ohmmeters have a lead error warning for high resistance connections.

Resistance of leads too high

While the four-terminal method is theoretically unaffected by lead length, the resistance of the leads themselves must be considered. Potential leads are less critical (up to 1kΩ), but current leads are critical. High resistance in current leads reduces the voltage across the Device Under Test (DUT), leading to inaccurate readings. Cropico ohmmeters check compliance voltage and provide a warning if it's too low. For long leads, use thicker cables to reduce resistance.

Measurement Noise

Noise can be a problem in low voltage measurements, often created within test leads exposed to changing magnetic fields or movement. To minimize noise, keep leads short, still, and ideally shielded. Cropico's ohmmeter circuits are designed to minimize these effects. Thermal EMF is a significant source of error in low resistance measurements. It's generated when two dissimilar metals join, forming a thermocouple junction. The EMF depends on the materials and temperature difference between junctions.

Diagram: Thermal EMF Effect

Illustrations showing two scenarios of thermal EMF. In the first, a copper-nickel junction with a 10°C difference generates EMF. In the second, a copper-copper junction with a 10°C difference generates no EMF.

This thermocouple effect introduces errors. Cropico microhmmeters and milliohmmeters use an automatic average mode (switched DC or average method) to compensate. A measurement is taken with current in one direction, then reversed. The displayed value is the average, canceling out thermal EMF. This method is best for resistive loads, not inductive samples like motors or transformers, where inductance saturation might affect readings.

Diagram: Voltage Measurement with Thermal EMF

A diagram showing voltage measurements (V1, V2) across a DUT with current and thermal EMF. The formula Vx = (V1 + Vemf) + (V2 - Vemf) is presented, defining Vx, V1, V2, and Vemf.

Wrong Test Current

Consider the effect of measurement current on the DUT. Devices with small mass or high temperature coefficients (like thin copper wire) require minimum current to avoid heating. A single pulse of current may be appropriate. For thermal EMF influences, the switched current method is suitable. Cropico DO5000 series ohmmeters offer selectable currents (1% to 100%) and a single pulse mode.

7. Choosing the right instrument

Temperature influences

Resistance is affected by temperature. For accurate measurements, control the environment or use Automatic Temperature Compensation (ATC). ATC uses a temperature probe to sense ambient temperature and correct the reading to a reference temperature (e.g., 20°C). Copper has a temperature coefficient of +0.393% per °C, and Aluminium is +0.4100% per °C.

Graph: Temperature Coefficient of Copper and Aluminium

A graph showing resistance (Ω) on the y-axis and temperature (°C) on the x-axis. Two lines represent Aluminium (4100 ppm/°C) and Copper (3930 ppm/°C), showing increasing resistance with temperature.

TABLE 2: Typical Instrument specification chart

Range: The maximum reading possible at that setting.

Resolution: The smallest number (digit) displayed for that range.

Measurement Current: The nominal current used by that range.

Accuracy: Uncertainty of the measurement over the ambient temperature range 15°C to 25°C.

Temperature Coefficient: The additional possible error below ambient temperature of 15°C and above 25°C.

When selecting an instrument, consider:

• Accuracy: The closeness of agreement between measured and true values, expressed as a percentage of reading plus a percentage of full scale. Accuracy statements should include the applicable temperature range and duration. Note that some manufacturers' high accuracy statements are valid only for short periods (30 or 90 days), whereas Cropico ohmmeters specify accuracy for a full year.

Resolution

Resolution is the smallest increment a measuring instrument displays. High accuracy requires suitably high resolution, but high resolution alone doesn't guarantee high accuracy.

Example: Measuring 1Ω with 0.01% (±0.0001Ω) accuracy requires a minimum resolution of 100μΩ (1.0001Ω).

A high-resolution display might show 1.0001Ω for a 1Ω measurement with 1% accuracy, but only the first few digits (1.0100) are meaningful, showing fluctuations. Select resolution to ensure a comfortable reading.

Measurement Scale length

Digital instruments often have a maximum display count of 1999 (or 30 digits). For a 1Ω measurement, a 1999 scale gives a reading of 1.000 with 0.001Ω resolution. A 2Ω measurement requires a higher range (19.99Ω full scale) for a reading of 2.000 with 0.01Ω resolution. Cropico ohmmeters offer scale lengths up to 6000 count, providing better resolution.

Table: Scale Length and Display Readings

Range Selection

Range selection can be manual or automatic. Automatic selection is useful when the resistance value is unknown, but it takes longer as the instrument finds the correct range. For measuring similar samples, manual selection is better. Automatic range selection should not be used for inductive samples (motors, transformers) as it can interrupt the measuring current and affect readings.

Temperature coefficient

The temperature coefficient of a measuring instrument affects accuracy. Instruments are calibrated at 20°C or 23°C. The coefficient indicates how accuracy changes with ambient temperature variations.

Current Magnitude and Mode

Selecting the appropriate measuring current is important. High currents can heat thin wires, altering resistance. Copper wire's temperature coefficient is 4% per °C; a 10Ω wire at 30°C (10°C rise) increases by 0.4Ω to 10.4Ω. Some applications benefit from higher currents.

The measurement current mode is also important. For thin wires, a short pulse minimizes heating. Switched DC mode can eliminate thermal EMF errors, but is unsuitable for motor windings or transformers where continuous current is needed for inductance saturation. Automatic Temperature Compensation is useful for materials with high temperature coefficients.

Measurement speed

Measurement speed is usually not critical, with most ohmmeters providing about 1 reading per second. For automated processes like component selection or production line testing, faster speeds (up to 50 measurements per second) may be desirable, requiring remote control via computer or PLC interfaces.

Remote connections

For remote connection, IEEE-488, RS232, or PLC interfaces are suitable. IEEE-488 is a parallel port with faster transmission than RS232 but limited to 20-meter cable distances. RS232 is a serial port using 3 lines for data transmission. PLC interfaces allow basic remote control by a Programmable Logic Controller.

Environmental

Consider the operating environment: portability, ruggedness for site conditions, and required temperature and humidity ranges.