The precision measurement of low resistance values
Bill Earlie of Cropico provides best practice advice for the accurate measurement of low electrical resistance values.
Manufacturers of electronic components have to verify that resistors, inductors, switches, relays and connectors meet certain specified tolerances and this invariably means that low resistance measurements must be taken.
The same requirement is true in power cable installations, the maintenance of switchgear, electric motor performance and vehicle or aircraft assemblies - in all cases the precision measurement of resistance values is a critical requirement for performance, quality control or safety considerations.
The ability to make accurate and reliable measurements is therefore crucial in many industrial and engineering sectors. However, with the values involved often being very low - often down to milliohms - then the key requirement is to understand and eliminate potential sources of error from the measurement process.
Four terminal measurement: Four wire or terminal measurement techniques have become the standard method of measuring low resistance and it is particularly advisable when measuring resistance below 100 ohm.
By using the so called Kelvin or Thomson configuration, the connecting lead resistance is not included in the measurement, and the need for lead balancing and nulling is eliminated. In this technique, the measuring current is passed through the unknown resistance Rx using the C1 and C2 leads.
The placing of these leads is not critical but should always be outside the P1 and P2 leads. The voltage drop across the Rx is measured across P1 and P2 and these should be placed at exactly the points to be measured.
The measuring current is simultaneously passed through an internal reference standard in the ohmmeter and the voltage drop across Rx is compared with the volt drop across this internal standard.
From the ratio of these two volt drops the resistance value of Rx is calculated and displayed. Because the same current is passed through both the standard and the Rx and the ratio is calculated, the current does not need to be a precise value, all that is required is that the current is stable for the period during which each measurement is made, typically 0.5 seconds.
Measurement errors: The most common cause of errors when making low resistance measurement is due to poor or inappropriate connection of the Rx. Connections should be clean, mechanically firm and free from oxides which can cause an insulating effect.
In terms of measurement current it is also a common misconception that the higher current the better. This was certainly true of the older digital instruments and their predecessor, the Kelvin Bridge, where high currents were needed to realise sufficient volts across the Rx for measurements to be made and particularly when Rx was 0.001Ω.
However, with today's measuring components and techniques reliable, accurate and consistent measurements of low voltage can be made with low currents. The disadvantages of using high currents are invariably the added cost, size and weight of the test instrument. With battery powered testers there is also the problem of performance periods and the reduced measuring time available.
In addition, higher test currents have the potential to heat up the Rx and therefore affect it, as well as possibly introducing thermal EMFs which can affect measurement reading accuracy. There are however, some applications where test specifications demand higher measuring currents and the argument has been made that higher test currents also test the mechanical integrity of joints. For example, if only a single strand of wire is making the connection a high test current would burn away this strand.
Poor Connections: Most causes of measurement error can be traced back to poor or inconsistent connection to the object under test. In many cases it is desirable, if not essential, to make a special jig to suit the particular component. This will ensure that the P1 P2 connections highlighted in the earlier circuit drawing will always be made at the same point on the sample.
Thermal EMF: Another source of error can be thermal EMF. When two dissimilar metals are joined together an EMF can be generated by the so called thermocouple effect. Most ohmmeters use a dc measuring system to ensure true resistance and not impedance is measured.
If the resistance to be measured is also generating an EMF, this effect will add or subtract to the EMF measured at the P1 P2 connections. Such conditions are overcome by making two measurements and reversing the current C1 C2 connections on the second measurement. The two readings are then averaged to give the correct answer; RX = (R1 + R2)/2.
This sort of compensation calculation is a standard feature on most Cropico ohmmeters, where the ability to select forward or reverse measurement current and to automatically average the two readings thus displaying the correct resistance value.
Simple precautions should also be taken when making connections to ensure that materials are used that are similar in EMF of copper. As an example, nickel plated brass connecting clips can cause very large thermal EMFs to be generated when connected to copper wires. For the best results, unplated copper or brass leads and fittings should be used.
Why measure low resistance values? There are many reasons why the resistance of material is important, including:
- Manufacturers of components such as resistors, inductors and chokes all have to verify that their product meets the specified resistance tolerance. Resistance measurements can therefore be made as part of end of production line and/or quality control testing.
- Manufacturers of switches, relays and connectors all need to verify that the contact resistance is below pre specified limits. Resistance measurements can therefore be made as part of end of production line and/or quality control testing.
- Cable manufacturers must measure the resistance of the copper wires being produced. If resistance is too high, then the current carrying capability of the cable is reduced. When resistance is too low, cable diameter is too large and the manufacturer will be using more copper than needed - needlessly increasing raw material costs.
- Installation and maintenance of power cables, switchgear and voltage tap changers require the cable joints and switch contacts to be of the lowest possible resistance to prevent the joint or contact becoming excessively hot. A poor cable joint or switch contact will soon fail due to this heating effect. Routine preventative maintenance with regular resistance checks ensures the best possible life performances.
- Electric motor and generator manufacturers need to determine the maximum temperature reached under full load. To determine this point, the temperature coefficient of the copper winding is used. The resistance is first measured with the motor/generator cold i.e. at ambient temperature.
A second measurement is then taken when the unit is run at full load for a specified period, with the change in resistance value enabling the internal motor / generator temperature to be determined. Ohmmeters are also used to measure the individual coils of a motor winding to ensure there are no short or open circuit turns and that each coil is balanced.
- Those in the automotive manufacturing industry need to measure the resistance of robot welding cables to ensure that the weld quality does not deteriorate. Digital milliohmmeters can also be used to measure battery lead crimp connectors, air bag detonator resistance, resistance of wiring harnesses and quality of crimp connectors on components.
- Fuse manufacturers need to measure resistance for quality control.
- Resistance bonding measurements on aircraft and military vehicles must be measured. It is necessary to ensure that all equipment installed in aircraft is electrically connected to the air frame, this includes galley equipment. Tanks and other military vehicles have the same requirements. Producers and users of large electrical currents all need to measure distribution joint resistance, bus bars and connectors to electrodes for electroplating
- Railway utilities including trams and underground railways use ohmmeters equipment for the measurement of power distribution cable joints. The resistance of rail track joints also need to be measured as the rails are often used for signalling.
