Calibration certificates for test and measuring equipment are important documents and contain valuable information about the calibration of the equipment and its measurement results.
While most companies regularly have equipment calibrated by outside calibration laboratories, many of them do not properly review or use the information from the calibration certificates to their fullest benefit.
The quality procedures of every organization that sends equipment out for calibration should have a process for the review of the calibration certificate before using the equipment. This will ensure that the proper information is received and that the calibration results show that the equipment will perform as intended.
Calibration Certificate Items to Review
Although calibration certificates may contain different information or formats from different calibration laboratories or for different calibration services, most certificates should contain the following elements that should be reviewed.
The first thing that should be checked is to make sure that the certificate lists the correct information for the specific instrument. The item’s manufacturer, model, description and serial number must all be correct. While most calibration laboratories have processes to review the certificates before they are released to ensure that the correct information is provided on their calibration certificates, mistakes can happen. If any of this information is incorrect, it will invalidate the traceability of the certificate.
Statements such as pass/fail or in-tolerance/out-of-tolerance need to be reviewed to determine the initial calibration status of the equipment.
In many international standards, there are clauses that state that if an item is found to be out-of-tolerance an investigation needs to be conducted on the equipment or processes that the item was used on since the last calibration to determine if the out-of-tolerance condition had any adverse effect of the measurements made with that item. If so, appropriate action may need to be taken, such as product inspections or possibly even product recalls.
This is a listing of the calibration standards used to perform the calibration of the equipment. This information should include the description of the calibration standards along with their appropriate calibration due dates.
This is important because it provides the information to prove that the calibration standards were compared with national or international standards. The traceability of the certificate cannot be determined without this information.
There is usually a ‘Remarks’ section on the calibration certificate that is used to record specific issues on the performance of the equipment that were found during the calibration process that may impact the use of the equipment. This section may include information on ‘limited calibrations’ where the calibration did not cover the full range of capabilities for the equipment or the item failed to meet the required instrument accuracy and new specifications were assigned.
Accredited calibration certificates require additional information that needs to be reviewed. If any of these items are missing then the calibration cannot be considered accredited.
The calibration certificate needs to have the calibration provider’s accreditation logo prominently displayed. This logo identifies the organization that accredited the laboratory and will contain the accreditation certificate number of the calibration provider.
Every measurement should have its corresponding measurement uncertainty value provided. This number is important to determine the true error associated with the reported measurement value.
The actual calibration measurement results need to be provided for all accredited calibrations. The data should include the ‘As Found’ and ‘As Left’ data along with specific identification for items that were found outside of the calibration specifications.
Specific measurement values should be reviewed to determine if the instrument was out of tolerance when it was received by the calibration laboratory or if the measurement data is near their tolerance limits. It can also be used to evaluate the instrument’s performance and determine long-term trends by comparing the current values to the values from previous calibrations.
If it is found that certain measurements are trending toward their tolerance limits or they are frequently out-of-tolerance, adjustments can be made to the equipment’s calibration interval to schedule calibrations before the equipment falls out of tolerance. This will prevent critical measurement errors which will save additional time and money.