Milliohm and micro-ohm meters are precision test instruments capable of measuring very low resistance values in circuits, in many cases down to fractions of micro-ohms--less than one millionth of an ohm.

Standard ohm meters, like those found in multimeters, can accurately measure resistance down to perhaps a few hundred ohms. At levels below a few ohms, the intrinsic resistance of the ohmmeter, the leads, and contacts can distort the readings and affect the results. For precise measurement of such small resistances only a milliohm meter can be used.

Applications for milliohm and micro-ohm meters

There are a number of applications which rely upon the high accuracy resistance readings provided my milliohm and micro-ohm meters. Among them:

  • Switch and contact breaker resistance
  • Bus bar and cable joints
  • Aircraft frame bonds and static control circuits
  • Integrity of welded joints
  • Inter-cell connections on battery systems up to 600 V peak
  • Quality control of resistive components
  • Transformer and motor winding resistance
  • Rail and pipe bonds
  • Graphite electrodes and other composites
  • Wire and cable resistance
  • Transmitter aerial and lightning conductor bonding

Electrical Resistance

Milliohm and micro-ohm meters measure electrical resistance. Resistance is the measure of the difficulty to pass an electric current through a conductor. The SI unit of electrical resistance is the ohm. One ohm is equal to the resistance of a conductor through which a current of one ampere flows when a potential difference of one volt is applied to it.

Electrical resistance is determined by the shape and material of a conductor. In this way it is similar to mechanical friction. Just as it’s more difficult to push water through a long, narrow pipe with a rough surface than a short, wide pipe with a smooth surface; it more difficult to push charged electrons through a long, thin wire made of a poor conductor than a short, thick wire made of a good conductor.

Milliohm / Micro-ohm Meter Technology

The manner of measuring electrical resistance has changed considerably over the years. Early ohmmeters were based on a type of meter movement known as ratiometer. Modern ohmeters now have an electronic circuit that passes a constant current through the resistance, and another circuit that measures the voltage across the resistance. According to Ohm’s Law, the resistance value (R) is determined by voltage (V) divided by current (I).

Standard ohmmeters generally use a simple, two-terminal measurement technique in which the test current is forced through the test leads and the resistance (R) under test. The meter then measures the voltage across the resistance through the same set of test leads and computes the resistance value accordingly.

For high-precision measurements such as those required for milliohms or micro-ohms, two-terminal measurements are not adequate since the test lead and contact resistance are added to the measurement value causing significant errors.

To overcome these errors, milliohm and micro-ohm meters use a four-terminal measurement technique called Kelvin sensing. Of the four terminals, two are used for voltage measurement and the other for current sensing. This minimizes any voltage drop due to the resistance of the first pair of leads and their contact resistances are ignored by the meter.

Modern milliohm and micro-ohm meters are available as either benchtop or handheld models, each with its own specifications for test ranges, accuracy, sampling rate, and resolution. A wide range of optional features may also be available with certain models. Models may include an adjustable sampling rate, autoranging, data logging capabilities, selectable ranges, multiple channels, advanced software, advanced communications for transferring measurements to computers and printers, comparator functions, scaling functions, and temperature compensation. External shunts can also be used to extend the current input range.

Things to Consider When Selecting a Milliohm / Micro-ohm Meter:

  • Do you want to benchtop or handheld model?
  • What test range is needed?
  • What is a suitable accuracy?
  • Are any accessories (test leads, shunts, etc.) included or needed?
  • Is data logging needed?
  • What functions would be useful?

If you have any questions regarding milliohm or micro-ohm meters, please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.