T.O. 33B-1-14-84c. Determine the meter or scope readings corresponding to each of the intermediate standards and recordthe conductivity value.d. Note each of the values on a graph with meter or scope readings on the vertical axis and conductivityvalues on the horizontal axis.e. Construct a smooth curve through all the points. The curve should increase or decrease smoothlythroughout the range with no minimum or maximum values. This curve is then be used to measureconductivity with the specific instrument and probe.4.7.7.5 CalibrationForSeparationOfMixedAlloys.To calibrate the general purpose instruments for separating two groups of materials with different conductivities, theinstrument is set to obtain readings at one end of the scale for one group of material, and the other end of the scale forthe second group of material. Lift-off is usually set on a specimen representing the group with the lower value ofconductivity or permeability.4.7.7.6 CalibrationCheck.Calibration or standardization should be checked approximately every 10 minutes during continual use and wheneverabnormal values are obtained. Whenever an instrument is found to be out of calibration, all measurements performedsince the previous calibration verification should be rechecked.4.7.7.7 Acceptance/RejectionCriteria.Acceptance/rejection criteria can be found in the applicable T.O. or material specification. Acceptable conductivityranges for many aluminum alloys are shown in Table 4-7.SECTION VIIITHICKNESS MEASUREMENT4.8 SPECIFICAPPLICATION—THICKNESSMEASUREMENT.4.8.1 CriteriaforApplication.4.8.1.1 TypesOfMeasurements.In general, three types of thickness measurement may be performed by eddy current techniques. The total thickness ofthin metallic products such as foils, strip and sheet, may be determined when the thickness dimension is less than theeffective depth of penetration of eddy currents in the material. A second category of thickness measurement includesthe measurement of metallic plating or coating on a conductive or magnetic base. Subcategories of plating and coatingmeasurements can be established on the basis of the relative conductivity or permeability of the plating and the basemetal on which it is plated. Typical subcategories of plating measurements include the following:a. Low conductivity plating on high conductivity base.b. High conductivity plating on low conductivity base.c. Low permeability plating on a high permeability base.d. High permeability plating on a low permeability base.The terms high and low are relative and are not meant to indicate specific values. The third category of measurementis the determination of nonconductive coating thickness on a metallic base. This application can also be extended tomeasure the total thickness of thin nonconductive materials that are accessible from both sides.
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