M1L-STD-1537C
4.1.3 Instrument accuracy.
Direct and indirect reading instruments shall be capable of determining the conductivity of aluminum alloys as a percentage of the 1nternational Annealed Copper Standard (%1ACS) with an accuracy of +1.0 %1ACS or better through electrically non-conductive films and coatings at least 0.003 inch (0.076 mm) thick.
4.2 Standards
4.2.1 Laboratory conductivity standards.
The contractor or calibrating facility shall maintain three or more certified aluminum alloy laboratory conductivity standards. One laboratory conductivity standard shall be between 25 and 32 %1ACS, another between 32 and 38 %1ACS and the third between 38 and 72 %1ACS. Laboratory conductivity standards shall be certified by comparing eddy current measurement with primary conductivity standards. Using the comparison methods, the values of these standards shall be certified to be within ±0.35 %1ACS or ±1 percent of their stated value whichever is less, at 20°C (68°F). Calibration shall be performed in accordance with ASTM
8193 procedure in a system in accordance with ANS1 Z540-1. 1ntervals shall be based on
objective evidence that accuracy as the lesser of ±0.35 %1ACS or ±1 percent of value is constantly maintained. Recalibration of laboratory conductivity standards shall not exceed 12 months.
4.2.2 Instrument conductivity standards.
At least two instrument conductivity standards are required. These standards are similar to the laboratory conductivity standard (see 4.2.1), with the exception that these standards shall
always accompany the conductivity instrument. Also, these standards are calibrated by comparison with laboratory conductivity standards. Using the comparison method, instrument
conductivity standards shall be certified to ±0.85 %1ACS accuracy. The high and low value instrument standards shall have a minimum difference of 10 %1ACS. At least one standard shall cover the 25 to 32 %1ACS range and one standard shall cover the 38 to 62 %1ACS range. Calibration intervals shall be such that they coincide with the eddy current instrument recalibration cycle (see 4.3.2) or as supported by objective evidence that the required accuracy is consistently maintained.
4.3 Calibration and standardization.
4.3.1 Standardization.
1nstruments shall be standardized before using as required by following the manufacturer's instructions. Allow approximately 20 minutes warm-up time for all types except solid state which may take five minutes. Use the instrument conductivity standards to calibrate the eddy current instrument using the standards nearest to the conductivity range being measured and adjust as required.
4.3.1.1 Standardization frequency.
During continuous operation, calibration of the conductivity instrument shall be conducted at 15 minute intervals.
4.3.1.2 Lift-off effect.
Lift-off effect is determined by comparison of readings taken when placing the probe on a bare standard followed by placing a 0.003 inch (0.076 mm) thick nonconducting flat shim between
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