Milliohm meters are capable of measuring very small resistances. Devices which use a four-wire Kelvin probe system pass a test current through the probe so that the voltage is sensed by the center pin. This test method eliminates contact resistance and minimizes the possibility of errors caused by the resistance of the leads. By definition, resistance or impedance is a measure of the degree to which a material or device opposes the flow of current. This value is equal to the voltage drop across the element divided by the current through the element. As their name suggests, milliohm meters measure resistance in milliohms, a unit of measure which equals 1/1000 of an ohm.
Milliohm meters carry specifications for bandwidth, sampling rate, maximum channels, and operating temperature, as well as display type and display digits. Bandwidth is the frequency range for AC current or voltage to be measured. Sampling rate is the frequency that a digital meter tests an analog signal and converts it to a digital value. Maximum channels are the total number of channels for the device. Operating temperature is the full required range of temperatures over which milliohm meters operate. There are two choices for display type: analog and digital. Analog devices usually display values with a needle. Digital devices have an electronic display. For milliohm meters, the number of display digits ranges from 3 to 7 or more. Devices that can display a half-digit are also available.
Milliohm meters are available in several different form factors. Benchtop meters are designed to sit atop a bench or table. Free-standing devices have a full case or cabinet and an integral interface. Milliohm meters that function as clamp meters measure current through wires that are connected to the circuit. Rack-mounted devices include hardware such as rail guides, flanges and tabs. Handheld milliohm meters are relatively lightweight and can be operated while held in the hand. Devices with a computer-board form factor are printed circuit boards (PCBs) that plug into motherboards or backplanes. Milliohm meters with other form factors are also available.
Standards, options and features are important parameters to consider when specifying milliohm meters. Some products have an adjustable sampling rate, alarm lights, auto-ranging capabilities, and integral application software. Others feature data acquisition, data storage or logging, decibel reading, and external triggering capabilities. External shunts can be used to extend the current input range. Integrating functions allow the active power and current to be integrated. Mirrored scales facilitate readings to a given accuracy and help operators avoid parallax errors. Range switches can be used to select the range of units to measure. Milliohm meters with overload protection, filters, scaling functions, and temperature compensation are also available.