Calibration Standards and Reference Sources

Calibration standards and reference sources provide standard values for calibrating instruments and comparing materials and components. Gage blocks, balls, and rings provide dimensional standards for calibration. For electronic components, there are highly accurate standard values for capacitance, inductance, resistance, voltage, and current. Inspection and non-destructive testing (NDT) also use calibration standards and reference sources. 

Gage blocks, a common form of calibration standards and reference sources, are available in several shapes for dimensional calibration. Rectangular gage blocks have holding bars and one or more centrally located holes for mounting. Square gage balls do not have central holes, but are held together with external gage block holders. Wear gage blocks are used on the ends of the gage block stack that contacts the parts or variable gages being evaluated. Gage balls are used to gage spherical cavities and tapered holes, and to set up machine tools. Ring gages are production-quality, functional gages for comparing hole and slot dimensions or locations to specific tolerances.

There are several tolerance grades for gage blocks. Grade 0.5 (AAA), or Laboratory Master Grade, products provide the highest degree of accuracy and are used in precision or master calibration gaging operations in temperature-controlled metrology or standards laboratories. Grade 1 (AA) and Grade 2 (A1 / A+) products also provide a high degree of accuracy and are used in precision gage checking or setting operations. Grade 3 (A), or Inspection Grade, products provide sufficient accuracy for use in gaging, setting, and general inspection operations in production or tool room areas.   

Calibration standards and reference sources for electronic components take several forms. Standards consist of a single, fixed value component or several components not in the same circuit. Decade boxes, or substituters, consist of several components (e.g., resistors) within the same circuit. Voltage dividers use a very accurate potentiometer and a series of fixed resistors to provide a highly accurate ratio of voltage and current. Other calibration standards and reference sources for electronic components simulate resistance temperature detectors (RTDs) and thermocouples.

For inspection and non-destructive testing (NDT), calibration standards and reference sources include flaw, conductivity, densitometer, and radiographic standards, as well as standards for hardness and thickness. Flaw standards such as Quantitative Quality Indicators (QQIs) used in magnetic particle inspection include artificial flaws to verify readings. Conductivity standards are used to calibrate conductivity testers, while densitometer standards are used with black and white or color images with known patterns and area density variations. Radiographic film standards apply to X-ray reference radiographs and contrast shims. Metals of a known hardness and heat treatment are usually calibrated to standards from the American Society of Testing and Materials (ASTM). Blocks or stepped down blocks of a known material and thickness are used for calibration of thickness or flaw detection instruments such as ultrasonic and Eddy current units.

• Temperature Calibration; Depths of Immersion
  Of all the sources of errors and uncertainties in thermal calibration by far the largest source of error and least understood effect is that of immersion of unit under test, and the reference...
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