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“One accurate measurement is worth a thousand expert opinions”. This quote from Grace Murray Hopper is undeniably relevant to our modern scientific days and applies to Cold Chain monitoring.
Many measurement instruments are used everyday to capture information about temperature-sensitive products and are trusted by cold chain and quality assurance experts to ensure that product quality is maintained throughout the supply chain. From a variety of measurement instruments and sensors, ranging from temperature to relative humidity, differential pressure, shock, tilt, light, and air quality, cold chain surveillance can become intricate. The confidence in every measure must therefore be guaranteed by means of calibration to facilitate cold chain data gathering and process of analysis.
Since measurement instruments are calibrated at regular intervals during their lifetime, multiple calibration certificates can be found for a single apparatus. Each certificate provides important information such as the instrument measurement quality, its compliance with specified requirements (i.e., accuracy and traceability), and its inherent measurement uncertainty. All of this information can be used during cold chain monitoring to evaluate performances, detect drifts, investigate excursions, and determine the level of trust cold chain and quality assurance experts can have in the measurements. Understanding calibration certificates in its entirety and ensuring the content is clear, concise, complete, and compliant is essential.
Here’s a procedure on how to evaluate and utilize certificates:
1. Verify calibration certificates for completeness, clarity and compliance
A complete calibration certificate complies with the ISO 17025:2017 requirements and all the recommended practices and guidelines from the NCSLI, the ANSI and the BIPM. It should include:
The complete address of the calibration laboratory and instrument owner
A description of the Unit Under Test (i.e., make, model, serial number, asset number)
A description of the calibration references (i.e., make, model, serial number, asset number, calibration certificate number, calibration due date)
A calibration certificate unique identification number
The calibration technician’s name and the performance/certificate issuance date
A procedure number and environmental conditions such as temperature and relative humidity
The As Found and As Left calibration results, the measurement tolerances and unit of measure
The expanded measurement uncertainties and the coverage factor (typically K=2 for 95% coverage).
A statement of compliance and traceability
The authorization name, date and signature
It is recommended to carefully review calibration certificates and ensure they include all the information listed above. Missing information should be reported to the calibration laboratory with a request for corrections.
*Note: Calibration certificates with only a PASS/FAIL statement are not considered valid calibration certificates (i.e., calculated error between the reference value and the As Found results from the unit under test without any expanded measurement uncertainties).
2. Utilize Calibration Certificates in Cold Chain monitoring applications
After a complete and successful verification, calibration certificates can be utilized for handling situations such as:
Detected excursions where investigation must be performed
Identified marginal values where product risk must be evaluated
In the first situation, the excursion’s impact on the product must be evaluated and quantified to confirm if it can still be delivered to the end-user. This is usually achieved by comparing product specifications and stability data with measurement captured during transportation and the length time at which the product was exposed to out-of-specification conditions. During an investigation, calibration certificates are commonly requested to confirm if measurement instruments can be trusted, but rarely utilized as supporting data. For example, the expanded measurement uncertainties stated on the certificate can be used to understand where the true value can actually be and help reduce the probability of falsely accepting or rejecting the excursion.
As shown below in figure 1 b), marginal values (near specified limits but within tolerance) have a significant probability of being out-of-tolerance. Whereas in Figure 1 c) shows an out-of-tolerance value that has a probability of being within tolerance. In situations such as figure 1 c), the probability of conformance should be calculated to determine the likelihood of rejecting a conforming value.
Figure 1.
For the second situation where marginal values are identified and product risk must be evaluated, using the guard-banding approach can help deciding if the marginal value should be treated as conforming or non-conforming. Guard-banding is a technic used to reduce the specified limits or tolerance based on the measurement uncertainty and therefore reduce the likelihood of accepting non-conforming values. The guard-banding formula, based on ANSI/NCSL Z540.3, Method 5, is simple and goes as follow:
A = L – U, where A is the acceptance limit, L is the specified limit (upper or lower), and U is the expanded uncertainty stated in the calibration report.
*Note: For a higher degree of trust in the calculation, calibrations should have a test uncertainty ratio of 4:1 or higher. This information should be provided by the calibration laboratory upon request.
In figure 2, the conformance zone is the area where limits are defined using guard-bands. Although this reduces the range at which measurements can be accepted, it provides higher confidence in conforming results and helps reduce false acceptance. The Uncertainty Zone is the area where there is a risk of false acceptance or false rejection. Any measurements recorded beyond this zone should be considered as non-conforming or out-of-tolerance without a doubt.
Figure 2.
Both of these scenarios are common occurrences and it is the duty of cold chain and quality assurance experts to mitigate product risks or make an educated decision about temperature-sensitive products that were exposed to out-of-specification conditions. Products that are robust and very stable as opposed to products that are sensitive to environmental conditions should be treated differently. Needless to say, complete and compliant calibration certificates can be very useful to truly understand the measurement trueness and support quality assurance and cold chain professionals through their daily assessments.