Anita Joshi Manufacturing medical devices is a highly complex process, and calibration requirements according to ISO 13485 mean high precision and close monitoring. The manufacture of medical devices makes use of various types of equipment and measuring instruments ranging from simple temperature sensors, weighing balances, pressure gauges, micrometers for measuring dimensions, to more complex ones, including digital instruments.
The importance of correct measurements from these instruments cannot be understated, since it can lead to a faulty product. Accuracy of all instruments decays with usage and wear and tear of the instruments. Factors such as electrical and mechanical shocks or environmental conditions like temperature and humidity may affect the accuracy of measurement. Therefore, a corrective procedure is required to maintain accuracy.
What is calibration?
Calibration of instruments is one of the primary processes used to maintain instrument accuracy. The process of calibration involves configuring an instrument to provide sample measurement results within an acceptable range. This activity requires that a comparison is made between a known reference measurement (the standard equipment), and the measurement using your instrument (test instrument). As a rule of thumb, the accuracy of the standard should be ten times the accuracy of the measuring device being tested. However, accuracy ratio of 3:1 is acceptable by most certification bodies.
The instrument used as a reference should be traceable to an instrument that is calibrated to your country’s National Standard; for example, the UK National Standard (UKAS) or to the US National Institute of Standards and Technology (NIST). When no such standards exist, the basis used for calibration or verification should be recorded. Calibration is usually followed by an adjustment made to the test instrument so that the output matches with that of the standard.
Two important parameters that should be defined for every calibration process are equipment tolerance and operating range of that instrument. Calibration tolerance is defined as the maximum allowable deviation between a standard of known accuracy and your test instrument. If your equipment exceeds the specified tolerance inaccuracies, it is usually adjusted.
What do you achieve by calibrating measuring and monitoring equipment?
- Restoring the accuracy of the instrument
- Adjusting or repairing an instrument which is out of calibration
- Minimizing uncertainty or error
- Ensuring the reliability and consistency of the instrument
- Keeping measurements within specification limits
- Building trust, confidence and reliability in measurements
- Establishing traceability of the measurement to a National / International Standard, which is a mandatory requirement for most standards.
To summarize, calibration quantifies and controls errors and uncertainties within measurement processes and brings them to an acceptable level.
Types of Calibration Programs
Most companies have calibration programs that are either in-house or performed externally through a third-party calibration service provider.
- In-house calibrations are sometimes done on a daily basis, or every time the instrument must comply with a national or international standard. A documented procedure is used and records of these regular calibrations are maintained.
- Additionally, it is a common practice to get the instrument calibrated at defined intervals by a third-party calibration service provider who provides a calibration certificate from an accredited laboratory.
What determines the frequency of calibration?
The frequency of calibration is influenced by several factors:
- In-house or external calibration program
- Usage of the instrument
- Behavior of the instrument – frequent out-of-tolerance results
- Accuracy and precision requirements
- Environmental conditions
- Overall calibration program and policy
- Instrument manufacturer recommended calibration interval
- Unscheduled calibration due to accidental dropping, or mishandling that leads to non-conforming results
Practical tips for a calibration program
Here is a list of practical tips for a calibration program:
- All instruments used in the manufacturing, testing and related processes must be calibrated at all times during the life cycle of the instrument
- Design and document an SOP for calibration
- Conduct calibration training
- Create a master list of all equipment and instruments needing calibration, including details of equipment ID, make, location, etc.
- Define frequency or the intervals of calibration – weekly, monthly, quarterly, bi-annually, annually
- Define calibration range which covers the operational range of the instrument
- Design a Calibration Plan with dates and timelines for performing calibration
- Implement the program
- Monitor and maintain all records of calibration and verification, making them easily available at point of use
- Plan what is to be done in case of deviations
- Affix calibration status labels which identify date and due date of calibration, providing a control to ensure that only calibrated instruments are used
- After reviewing them carefully, store your Calibration certificates, with the process owner approving and signing them
- Once calibrated, do not adjust the instrument, as adjustments may invalidate the measurement result
- Protect equipment used in measuring and monitoring from damage and deterioration during handling, maintenance and storage
Compliance to ISO 13485 requirements
A well-designed calibration program as described above helps you maintain the accuracy of your instruments. It also helps you achieve compliance with the calibration requirements set out in ISO 13485:2016; Clause 7.6 (Control of monitoring and measuring equipment).
Learn more about requirements in ISO 13485 in this free download: Clause-by-clause explanation of ISO 13485:2016.
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