Inductance standards and decades provide highly accurate and digitally variable standard values of inductance for calibration, comparison and testing. Inductance opposes a change in an electrical circuit and is usually measured in henries (H). Inductance standards usually consist of a single, fixed value component or several components (e.g., several fixed value resistors) that are not in the same circuit. Continuously variable devices often include a potentiometer. Inductance standards with more than one component have multiple posts or a rotary switch for selecting different resistance values. Most inductance decades consist of several components in the same circuit. The standard resistance value is selected by varying the number of resistors in the series or parallel circuit. Transfer, primary, and laboratory grade inductance standards are used to compare or transfer accuracies from the National Institute of Standards and Technology (NIST) or other nationally certified organizations to working decades. Typically, these working devices are robust or rugged units for production or in-line testing and calibration.
Specifications for inductance standards and decades include inductance range, inductance accuracy, self-resonant frequency (SRF), inductance resolution, quality factor, temperature coefficient, operating temperature, and stability. Inductance measures a circuit element’s opposition to changes in current flow. Inductance accuracy or tolerance is the allowed amount of variation from the nominal value as specified by the manufacturer. Self-resonant frequency (SRF) is the frequency at which an inductor’s distributed capacitance resonates with the inductance and cancels the capacitance. At this frequency, the inductor acts as purely resistive, high impedance element. Inductance resolution is the digital value represented by one bit in the display of a digital measure. Quality factor or Q value measures the relative losses in a capacitor and is defined as the ratio of capacitive resistance to the equivalent serial resistance (ESR). Temperature coefficient, the rate at which the nominal resistance value changes as a function of temperature, is expressed in parts per million per degree Celsius (ppm/C). Operating temperature is a full-required range. Stability is measured over a year-long period.
Inductance standards and decades are available with a variety of features. Some inductance standards and decades are suitable for alternating current (AC) or direct current (DC) calibration and testing applications. Other devices are designed for use in larger systems or products, or can perform precision measurements. Rack mounted, stand-alone, and benchtop inductance standards and decades are commonly available. Low zero or residual decades have low values of zero, offset or residual impedance resistance, inductance or capacitance. Variable decades provide a precise and continuously variable value such as a variable capacitor or potentiometer. Inductance standards and decades with a computer interface for programming, control, or data acquisition are also available.
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