Conductivity Reference Standards

T.O. 33B-1-1 4-82 lowering the conductivity of the metal. Lower temperatures reduce thermal oscillation of the atoms resulting in an increased electrical conductivity. The conductivity of standards is usually determined at a specific temperature; 68°F (20°C) is most commonly used. Typical conductivity values and allowable conductivity ranges are also established at approximately this temperature. If all instrument calibration and conductivity measurements could be performed at this temperature, errors in conductivity measurement related to temperature variation would not occur and/or temperature compensation would not be required. In field applications, testing temperatures can conceivably be anywhere in the range of 0°F to 120°F. Unless precautions are taken in selection of standards, calibration of the instrument, and testing, errors will occur in the measured conductivity values. Two ways in which erroneous readings may be obtained are: a. Difference in temperature between standards and test part. b. Difference in temperature at which conductivity of the standard was originally established and the temperature at which instrument calibration and conductivity measurements are performed. To prevent errors from differences in temperature between standard and test part, the instrument and standards should be allowed to stabilize at the test part temperature before calibration and conductivity measurements are performed. In no instance should measurements be taken if part and standards temperature differ by more than 10°F. Even though standards and test part are at the same temperature, error in determining conductivity values occurs when the measuring temperature differs from the temperature at which the conductivity of the standards was originally established. The magnitude of the error becomes larger as this difference in temperature increases. 4.7.6 Conductivity Reference Standards. 4.7.6.1 Number of Standards Required. For calibration of eddy current conductivity meters, at least two calibration blocks with accurately determined conductivity values must be available. When using general purpose instruments, the number of standards may vary from two to several depending on the inspection purpose and the accuracy required. 4.7.6.2 Conductivity Range. The conductivity range of the standards must be within the range of the instrument and cover the range of conductivity values to be measured. Preferably, the calibration blocks should have the same change in resistivity with temperature as the test parts 4.7.6.3 Size Of conductivity Standdards. For convenience of transportation and storage, conductivity standards are usually kept relatively small. Standards must have sufficient size to prevent edge effects or thickness from having a bearing on conductivity readings. These requirements can be satisfied by requiring length and width to be 1 inch greater than the probe diameter and the thickness greater than 3.5 times the standard depth of penetration at the test instrument frequency. Standards should be flat, have a surface finish of 63 RMS or better, and be free of any coatings. 4.7.6.4 Accuracy Of Standards. Standards used for calibrating instruments immediately prior to measuring conductivity should be accurate within ±0.5% IACS of the nominal value. A second set of standards accurate within 0.35% IACS should be periodically made available for checking the performance of instruments and field calibration standards. Standards should be traceable to the National Bureau of Standards. Standards are available from manufacturers of eddy current conductivity instruments. 4.7.6.5 Stability Of Standards. Many standards, particularly those of aluminum alloys, are subject to metallurgical changes if exposed to excessively high temperature, and thermal shock from sudden changes in temperature. Surfaces of standards can also corrode if exposed to moisture or other hostile environments. Damage caused by rough handling can also lead to error in conductivity readings. For these reasons standards should be transported and stored in dry, clean, protected areas not subject to excessive temperatures.