Helpful guidance for assessing internal calibration activities

For many service centers, specific customer requirements regarding control of measuring & test equipment will be much more specific than as listed below. Customer requirements should always be evaluated first as they are prerequisite for doing business.

When calibrations are performed internally, calibration procedures should be used that contain adequate information for the calibration of the measuring & test equipment (M&TE). 

1. Calibration procedures can be developed by the service center, the M&TE manufacturer or a third party. They can also be developed using material compiled from any of these sources. 
2. Calibration procedures should contain sufficient information for qualified personnel to perform the calibration. 
3. Calibration procedures should be approved and controlled.

Typical contents of adequate calibration procedures are listed below.

1. Equipment description (e.g. type, model, specifications) 
2. Measurement standards and auxiliary equipment required (e.g. gage blocks, power supplies) 
3. Preliminary operations required (e.g. safety, handling, cleaning, operational checks) 
4. Calibration process (e.g. detailed set of instructions, tolerance limits) 
5. Calibration results (e.g. records of calibration including control, retention) 
6. Closing operations (e.g. labeling, tamper prevention)
7. Storage & handling (e.g. requirements for maintaining fitness for use) 

It is common practice for many calibration procedures to require Test Accuracy Ratios (TAR) of 4:1 or better between the standard used and the M&TE being calibrated. That is, M&TE with an accuracy of 4% would be calibrated by using a standard whose accuracy is 1% or better. Additionally, it is reasonable to adjust the required accuracy of M&TE to meet the needs of the application. For example, if 10% accuracy is sufficient for an application, the standard used for calibration may have 2.5% or better accuracy. Whether manufacturer specified accuracy or an alternative accuracy is used, the conditions of the calibration will be documented through a label on the M&TE that is linked to a calibration certificate on file at the service center that references the applicable calibration procedure.

For additional help and tips regarding equipment calibration, you may find this white paper helpful: "Minimizing Calibration Costs for Measuring and Test Equipment" by Mike Howell, EASA Technical Support Specialist.