Preserving evidence that monitoring and measuring resources are suitable for the purpose (7.1.5.1). What documented information is appropriate to be able to fulfill this requirement?
The recording of the results of calibration and verification of devices indicating their accuracy or integrity. It is important to record the results of calibration and verification to determine whether the devices were within the prescribed limits at the last use. This work also allows us to monitor the trend and degree of drift that must be taken into account.
Calibration and verification of measuring equipment (7.1.5.2a)
Measurement traceability means that there is an unbroken chain of measurements from the measuring instrument that is used up to national or international measurement standards. It is a requirement when it is indicated in a contract, in a regulation to which reference is made, or in the organization's policies. Calibration applies to all measuring instruments used that must provide evidence of conformity in any of the product realization and release phases.
There are two systems used to maintain the accuracy and integrity of measuring equipment:
- A calibration system
- A verification system
Calibration is a process that allows a physical signal to be compared with a reference signal of known quantity, while verification is performed by establishing the correctness of a quantity. Some variables could be calibrated (such as, for example, length or capacity), but it is also possible to verify attributes such as shape and function (for example, the presence of a substance is verified, not calibrated). Therefore, depending on the equipment, it may be necessary to calibrate, verify, or both.
Demonstrating that measuring equipment is calibrated or verified against traceable measurement standards when measurement traceability is a requirement can be done in this way:
- Presenting evidence that measurement traceability is a requirement of interested parties;
- Presenting a description of the measuring equipment calibration or verification process and, in particular:
- The criteria for determining which equipment requires calibration or verification;
- The criteria for determining calibration intervals;
- How measurement traceability is ensured
- Selecting a representative sample of equipment used for measurement purposes and providing evidence that its calibration is controlled by this process;
- Retrieving the records for the sampled equipment that provide evidence to trace the validity of the measurement results according to national or international measurement standards
Recording the basis for calibration (7.1.5.2a)
For physical and chemical measurements based on fundamental units of measurement (meter, kilogram, second, ampere, etc.), there are national or international standards, but for other measures, there are no national or international standards against which to calibrate a measuring instrument. Each sector has developed a series of measures through which the quality of its products and services is measured and, as a result, has developed standards that represent agreed definitions of the measures. Without a solid basis for comparison, the effort of the measurement process is wasted.
Demonstration of the basis used for calibration or verification can be obtained by:
- Identifying those measurement standards used for which there are no national or international references;
- Presenting evidence of the process by which such standards have been validated;
- Selecting a representative sample of such standards and evidence that they have been validated according to this process
Indicating the calibration status (7.1.5.2b)
The calibration status is given by the position occupied by a measuring device with respect to the period that must elapse between one calibration and another. If the calibration due date is in the future, the calibrated device can be used, but if the current date is later than the calibration due date, the device may be inaccurate and must not be used until verified. However, even devices that are dropped or have been damaged may not be reliable, even when the calibration date is scheduled for the future.
The requirement only applies to physical devices subject to wear, drift, or variations in use over time. While a robust calibration system should ensure the absence of invalid measuring instruments, it must always be remembered that system failures are a possibility. Since the consequences of failure are greater than the effort required to verify the validity of the devices before use, it is prudent to provide means to check the calibration status.
To demonstrate that the status of measuring equipment is identified, you can:
- Presenting evidence of the process by which the status of calibrated or verified measuring equipment is identified;
- Selecting a representative sample of measuring instruments and evidence that their calibration or verification status has been clearly identified in accordance with process requirements
Safeguarding monitoring and measuring instruments (7.1.5.2c)
Each measuring and monitoring device has a range within which accuracy and precision remain stable: using the device outside of this range means obtaining readings that may not be correct. Once a device has been calibrated or verified, safety measures must be in place to prevent unauthorized or unintentional adjustment, damage, and deterioration.
Measuring instruments are very sensitive to vibration, contamination, shock, and tampering, and it is therefore necessary to protect them to preserve their integrity during use, maintenance, and periods of non-use. To demonstrate that adequate safeguards exist to prevent the invalidation of calibration, you can:
- Presenting evidence of the process by which it is demonstrated that the calibration status is safeguarded;
- Selecting a representative sample of measuring instruments and providing evidence that:
- The devices used to prevent inadvertent adjustments or tampering are intact;
- When applicable, protection is provided for devices in use;
- The equipment is not damaged;
- The equipment, when not in use, is stored in a protective container
Determining the impact due to any defective instruments and solving the problem (7.1.5.2)
This, for some organizations, is perhaps the most difficult requirement to satisfy. In fact, it is not always possible or practical to trace the product that has been declared acceptable by an instrument that has subsequently proved to be defective.
If a measurement has been made with an instrument that has subsequently been found to be inaccurate, it must be suspected that the measurement is not valid and, therefore, an assessment is necessary to determine how to act. In most cases, the instrument used is calibrated to an order of magnitude greater than the required accuracy, so, if found out of tolerance, it may not mean that the product measured is not compliant.
To demonstrate that appropriate measures have been taken to verify the validity of the results of measurements made by an instrument that then had to be recalibrated, it is necessary to:
- Presenting evidence of the process initiated when it was found that the measuring equipment used may not have been suitable for the intended purpose;
- Retrieve the records relating to the problem and prove that the prescribed process was followed