T.O. 33B-1-1
4-84
c. Determine the meter or scope readings corresponding to each of the intermediate standards and record
the conductivity value.
d. Note each of the values on a graph with meter or scope readings on the vertical axis and conductivity
values on the horizontal axis.
e. Construct a smooth curve through all the points. The curve should increase or decrease smoothly
throughout the range with no minimum or maximum values. This curve is then be used to measure
conductivity with the specific instrument and probe.
4.7.7.5
Calibration For Separation Of Mixed Alloys.
To calibrate the general purpose instruments for separating two groups of materials with different conductivities, the
instrument is set to obtain readings at one end of the scale for one group of material, and the other end of the scale for
the second group of material. Lift-off is usually set on a specimen representing the group with the lower value of
conductivity or permeability.
4.7.7.6
Calibration Check.
Calibration or standardization should be checked approximately every 10 minutes during continual use and whenever
abnormal values are obtained. Whenever an instrument is found to be out of calibration, all measurements performed
since the previous calibration verification should be rechecked.
4.7.7.7
Acceptance/Rejection Criteria.
Acceptance/rejection criteria can be found in the applicable T.O. or material specification. Acceptable conductivity
ranges for many aluminum alloys are shown in Table 4-7.
SECTION VIII
THICKNESS MEASUREMENT
4.8
SPECIFIC APPLICATION — THICKNESS MEASUREMENT.
4.8.1
Criteria for Application.
4.8.1.1
Types Of Measurements.
In general, three types of thickness measurement may be performed by eddy current techniques. The total thickness of
thin metallic products such as foils, strip and sheet, may be determined when the thickness dimension is less than the
effective depth of penetration of eddy currents in the material. A second category of thickness measurement includes
the measurement of metallic plating or coating on a conductive or magnetic base. Subcategories of plating and coating
measurements can be established on the basis of the relative conductivity or permeability of the plating and the base
metal 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 measurement
is the determination of nonconductive coating thickness on a metallic base. This application can also be extended to
measure the total thickness of thin nonconductive materials that are accessible from both sides.
