When accuracy really counts, you should make four-wire resistance measurements. Sometimes referred to as the Kelvin method, the four-wire resistance method is often used in automated test applications where long cable lengths, numerous connections, or switches exist between the digital multimeter (DMM) and the device under test (DUT).
This article will discuss the benefits of making four-wire resistance measurements, how VTI Instruments EX1200 DMMs from AMETEK Programmable Power stand out as high-accuracy resistance measurement tools, and how to cope with common errors that may occur when making these measurements.
Discover the benefits of four-wire resistance measurements
Four-wire resistance measurements allow you to:
- Make resistance measurements down into the milliohm (mΩ) range. This may be important when testing connectors, cables that will handle high currents, and cables used in high-reliability applications.
- Compensate for errors caused by test lead and test fixture resistance.
- Make high-current resistance measurements, which may allow you to make more precise measurements.
Improve accuracy with EX1200 DMMs
VTI EX1200 6-½ digit DMMs — specifically the EX1200-2165 and the EX1200-2365 — enable you to make high-accuracy four-wire resistance measurements. Figure 1 shows you how to connect a resistance to a VTI EX1200 DMM to make a four-wire measurement.
Figure 1: How to connect a resistance to a VTI EX1200 DMM
Thanks to its ability to run four-wire resistance measurements, the VTI EX1200 DMM can subtract the resistance of the test lead wires from the final result, effectively making the test lead wires 0 Ω. With this function, you can measure down below 0.1 Ω as you will see no lead wire resistance. Alternatively, you can measure a component some distance away from the DMM or through multiple connections and relays, assuming all lead wires are routed through the same quantity of connections and relay contacts.
Since all DMMs produce a test current to measure voltage drop across a resistance, this can produce a heating effect — especially on a low-resistance DUT. Because a higher test current creates a greater voltage drop, this larger response provides better accuracy and an improved signal-to-noise ratio. However, a test current that’s too large can create a heating effect, which can cause the DUT to change resistance and create measurement instability. Thus, balance is needed to provide enough of a test current to provide good accuracy without creating any self-heating effect.
The VTI EX1200 DMM enables the end user to program the test current to a range that best suits his/her test. This flexibility combined with the DMM’s four-wire measurement capability maximizes test accuracy. Table 1 shows the available test currents for the range and resolution selected.
Table 1: Available test currents for specific ranges and resolutions
Minimize common errors
Four-wire resistance measurements are prone to many of the same errors as high-accuracy DC voltage measurements. In addition, when making four-wire resistance measurements, you may encounter the following errors:
- Self-heating errors: When current passes through the resistance under test, the resistor temperature will increase and can cause a self-heating error. This is unavoidable. When the DUT is heated, the temperature of the DUT increases, changing the DUT’s resistance value. The magnitude of this error depends on the DUT’s temperature coefficient and thus cannot be specified. To minimize this error, apply the best practice of minimizing the time that the current is applied to the DUT.
- Settling time errors: In some cases, the test lead resistance combined with the DUT capacitance is large enough so that the resistor-capacitor (RC) time constant becomes significant. Settling times due to this RC time constant can be quite long, particularly when measuring resistances greater than 100 kΩ. Certain types of precision resistors, for example, have a large, lumped capacitance tied across its terminals to arrest noise. In these cases, taking a resistance reading without calculating the setting time will yield an inaccurate result.
The VTI EX1200 DMM’s Auto Delay feature will help you avoid RC time constant errors. When you enable Auto Delay, the VTI EX1200 DMM will wait until the input has settled before making the measurement. In some cases, however, you may have to set the delay time manually.
- Insulation resistance and surface cleanliness errors: Moisture-absorbing insulation and “dirty” surface films in test cables and fixtures can increase leakage currents, which can cause measurement errors when measuring resistances of 1 MΩ or greater. To reduce errors of this type, maintain a “clean” high-resistance system.
Selecting the proper cable insulation can also reduce errors. Nylon and polyvinyl chloride (PVC) are relatively poor insulators (109 Ω) when compared with polytetrafluoroethylene (Teflon®) (1013 Ω). Leakage from nylon or PVC insulators can easily contribute a 0.1% error when measuring a 1 MΩ resistance in humid conditions. Physically touching the DUT while the resistance is being measured should be avoided, as the body acts as a leakage path for the test current. Connecting a second instrument across a test resistance will change the resistance measurement being made by the VTI EX1200 DMM. Instead of just the test resistance, the DMM will be measuring the parallel combination of the test resistance and the impedance of the second test instrument.Learn more
For more information on the VTI EX1200 instrument family, contact one of our sales representatives by visiting www.powerandtest.com/sales-reps/programmable-power-sales. You can also email us at email@example.com or call us toll-free at 800-733-5427 or 858-450-0085.