Resistance standards and decades provide highly accurate, standard values of resistance for testing, comparing, and calibrating electrical and electronic components and products. Measured in ohms (W), resistance is the degree to which a body opposes the passage of electrical current. Resistance 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. Resistance standards with multiple components often have multiple posts or a rotary switch for selecting different resistance values. Decades, decade boxes, or decade substituters are digitally variable and usually consist of multiple components in a single circuit. The standard resistance value is selected by varying the number of resistors in the series or parallel circuit. RTD simulators provide resistance values for the calibration and testing of resistance temperature detectors (RTD) at specified temperatures. Resistance standards and decades that include rheostats or potentiometers are also available.
There are several grades of resistance standards and decades. Devices that can be traced to the National Institute of Technology Standards (NIST) are compared to a master standard that has been checked against a NIST standard. Transfer, primary or laboratory grade resistance standards and decades are used to compare or transfer the accuracies from NIST standards to working standards, sources, simulators and calibrators. Working standards are robust or rugged units for production or in-line testing and calibration. Reference or working resistance standards are compared or verified by transfer or laboratory standards and then used for calibration, testing and inspection of production resistors as well as other components, meters and instruments.
Performance specifications for resistance standards and decades include the number of decades per decade box, resistance range, resistance accuracy, resolution resistance, temperature coefficient, operating temperature, and stability. Resistance accuracy or tolerance is the allowable amount of variation from the nominal value as specified by the manufacturer. Resistance resolution is a digital value represented by one bit. 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.
Resistance standards and decades are available with a variety of features. Some resistance 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 resistance 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. Resistance standards and decades with a computer interface for programming, control or data acquisition are also available.
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