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Australian Government: National Measurement InstituteAustralian Government: National Measurement Institute
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Impedance and Resistance Capabilities

NMI's primary standard of resistance at dc is derived from the calculable capacitor, in terms of which a group of 1 Ω resistors is maintained as the working standard. Capacitance and inductance are derived from NMI's calculable capacitor and frequency standard. Calibration services are available for resistors, capacitors, inductors and ratio transformers.

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Before consigning an instrument for calibration always consult us to discuss your requirements. For further information contact calibration.coordinator@measurement.gov.au.

Resistors

NMI has facilities for dc tests on resistors over a range of temperatures from 0 to 60 °C and currents up to 2000 A. The necessity for temperature and load tests depends upon the type of resistor and its use, and, in general, they need be done once only. The results of such tests should be submitted with the resistor.

Resistors with nominal values of 100 Ω or less should be fitted with current and potential terminals (4-terminal resistors). Resistors with nominal values greater than 10 MΩ should be enclosed within a conducting case separately connected to its own terminal (3-terminal resistors).

Precision Resistors Rated at Currents up to 100 A and with values up to 1 GΩ

New resistors are normally measured at a range of currents and/or temperatures specified by the customer. Recalibrations are normally made at one temperature and at negligible power dissipation only. Uncertainties range upwards from 0.13 µΩ/Ω depending on the resistance value and its stability.

High-current Resistors Rated at Greater than 100 A

Calibration services are provided for high-current resistors up to a maximum of 2000 A. These resistors may be oil filled, with or without water cooling and/or mechanical stirring, or may be of the open-frame type (current shunts). Measurements are usually made at an ambient temperature of 20 °C. The measured value of a resistor of this type is often dependent upon its mounting position, and upon the nature and location of the current-carrying leads or bus-bars. The value of an oil-filled resistor (at appreciable power dissipation) will also depend upon whether or not the mechanical-stirring device is operating. Full information about mounting and current leads, and operation or otherwise of stirring devices should be given. Any conducting plates used to connect a resistor to bus-bars should be forwarded with the resistor.

High-value Resistors with Values of 1 GΩ or Greater

These resistors are usually of the carbon-film or metal-film type and may have values which are highly dependent upon the voltage applied to them. Measurements are normally made at an ambient temperature of 20 °C with the resistor in a screened enclosure. In special circumstances, tests can be made at temperatures other than 20 °C. Uncertainties range upwards from 5 μΩ/Ω depending on the resistance value and its stability.

Capacitors

Tests for capacitance and power factor can be made at various frequencies. The preferred frequency for the most precise test is 1592 Hz and, unless otherwise specified, tests are made at this frequency. Fees for tests at other frequencies, or at other than room temperature (nominally 20°C), will be quoted on application.

Three-terminal and two-terminal pair gas-dielectric capacitors with small temperature coefficients can be measured with an uncertainty of 0.1 µF/F if their stability is proven. Mica dielectric capacitors and other types are measured according to their performance, usually to about 10 µF/F.

A precision capacitor should be completely enclosed in a metal screen which may be connected permanently to one terminal or provided with a separate terminal. In the latter case the method of connection to the screen should be stated.

The preferred method of connection to the capacitor is by (two) co-axial connectors.

Inductors

Tests can be made on inductors over a range of frequencies. The preferred frequency for the most precise test is 1000 Hz and, unless otherwise specified, tests will be made at this frequency. Fees for tests at other frequencies or at other than room temperature (nominally 20 °C) will be quoted on application. In the case of shielded inductors the method of connection to the shield should be stated.

Mutual Inductors

The mutual inductance and phase defect can both be measured. Owing to capacitance effects between primary and secondary windings, the effective mutual inductance and the phase angle are dependent on the method of connection. If a particular connection is required it must be specified.

Self Inductors

The self inductance and the resistance of the equivalent series circuit can both be measured. Inductors of 1 mH or less should be provided with a short circuiting link so that the most precise results can be obtained.

Ratio Transformers (Inductive Voltage Dividers)

Tests on ratio transformers may be made in the frequency range 40 Hz to 10 kHz and up to 1000 V at selected frequencies.

The standard test for a multi-dial inductive voltage divider measures approximately 40 dial settings at a frequency of 1000 Hz and at room temperature (nominally 20 °C). The measured dial settings are chosen to include those dial settings for which the errors are usually largest. The report quotes the largest error measured. Fees for tests at other frequencies will be quoted on application.

Alternatively, the errors for a number of specified outputs or dial settings may be measured and reported.