Page 372 - Intro Predictive Maintenance

A Total-Plant Predictive Maintenance Program 363

and other performance-degrading factors that cannot be anticipated or
provided for by calibration intervals.
• Providing timely and positive correction action.
• Establishing decals, reject tags, and records for calibration labeling.
• Maintaining formal records to ensure proper controls.

Inspection Intervals. The checking interval may be in terms of time (hourly, weekly,
monthly), or based on amount of use (every 5,000 parts), or every lot. For electrical
test equipment, the power-on time may be a critical factor and can be measured
through an electrical elapsed-time indicator.

Adherence to the checking schedule makes or breaks the system. The interval should
be based on stability, purpose, and degree of usage. If initial records indicate that the
equipment remains within the required accuracy for successive calibrations, then the
intervals may be lengthened; however, if equipment requires frequent adjustment or
repair, the intervals should be shortened. Any equipment that does not have specific
calibration intervals should be (1) examined at least every six months, and (2) cali-
brated at intervals of no longer than one year.

Adjustments or assignment of calibration intervals should be done so that a minimum
of 95 percent of equipment or standards of the same type is within tolerance
when submitted for regularly scheduled recalibration. In other words, if more than
5 percent of a particular type of equipment is out of tolerance at the end of its
interval, then the interval should be reduced until less than 5 percent is defective when
checked.

Control Records. A record system should be kept on every instrument, including:

• History of use
• Accuracy
• Present location
• Calibration interval and when due
• Calibration procedures and necessary controls
• Actual values of latest calibration
• History of maintenance and repairs

Test equipment and measurement standards should be labeled to indicate the date of
last calibration, by whom it was calibrated, and when the next calibration is due (see
Figure 16–3). When the size of the equipment limits the application of labels, an iden-
tifying code should be applied to reflect the serviceability and due date for next cali-
bration. This provides a visual indication of the calibration serviceability status. Both
the headquarters calibration organization and the instrument user should maintain a
two-way check on calibration. A simple means of doing this is to create a small form
for each instrument with a calendar of weeks or months (depending on the interval
required) across the top, which can be punched and noticed to indicate the calibration
due date. An example of this type of form is shown in Figure 16–4.