The clause (6.4.5)
states that:
The equipment used for
measurement shall be capable of achieving the measurement accuracy and/or
measurement uncertainty required to provide a valid result.
Capability: The
equipment must be technically capable of achieving the required level of
accuracy or uncertainty. This involves factors like its measurement range,
resolution, and calibration capabilities.
Accuracy and Uncertainty: These
are two different ways to express the reliability of a measurement. Accuracy
refers to how close a measurement is to the true value, while uncertainty
indicates the range of possible values within which the true value is likely to
lie.
Valid Result: The
ultimate goal is to ensure the equipment produces meaningful and reliable
results. If the equipment's capabilities don't meet the required accuracy or
uncertainty, the results could be misleading or inaccurate, jeopardizing the
validity of the entire testing or calibration process.
Key considerations for
laboratories:
Selecting equipment:
Carefully consider the required accuracy or uncertainty before purchasing or
using any equipment. Consult technical specifications, manufacturer
recommendations, and relevant standards.
Calibration and maintenance:
Regularly calibrate equipment to ensure it maintains its accuracy over time.
Follow proper maintenance procedures as recommended by the manufacturer.
Documentation:
Maintain records of all equipment, including calibration certificates,
maintenance logs, and performance checks. This demonstrates compliance with ISO
17025 and helps identify potential issues early on.
Clause 6.4.6
emphasizes that measuring equipment should be calibrated under specific
conditions, such as when the accuracy or uncertainty of the equipment could
impact the validity of reported results or when establishing metrological
traceability is required. It recognizes the importance of ensuring the accuracy
of equipment involved in direct measurements, corrections, and calculations to
maintain the reliability of laboratory results.
Types of Equipment for which calibration is
required:
- Direct
Measurement Equipment: Equipment used
for the direct measurement of the measurand (the quantity being measured), such
as a balance used for mass measurement.
- Correction
Equipment: Equipment used to make
corrections to the measured value, for example, equipment used for temperature
measurements where corrections may be necessary.
- Calculation
Equipment: Equipment used to obtain a
measurement result calculated from multiple quantities. Calibration is
necessary to ensure the accuracy of calculations based on multiple inputs.
Impact on Result Validity
Calibration is necessary when
the measurement accuracy or uncertainty directly affects the validity of the
reported results. This means:
Inaccurate measurements: If
the equipment deviates from the true value by an amount that could
significantly impact the interpretation of the test results, it needs
calibration. For example, an inaccurate balance in measuring food samples could
lead to incorrect assessments of nutrient content or contamination levels.
Excessive uncertainty: Even if
the average measurement is close to the true value, a high level of uncertainty
indicates a wider range of possible values. This can make it difficult to draw
definitive conclusions from the test results. For example, high uncertainty in
measuring pesticide residues might raise concerns even if the average level
falls below safety limits.
Metrological Traceability
Calibration is required when
it's crucial to establish the metrological traceability of the reported results.
This means ensuring a clear and unbroken chain of comparisons back to
international standards:
Linking measurements to
international units: All measurements should ultimately be traceable to
internationally recognized units like meters, kilograms, or degrees Celsius.
Calibration ensures that the equipment's measurements align with these
standards.
Demonstrating measurement
integrity: Traceability provides documentation and evidence that the equipment
is functioning correctly and producing reliable results, essential for
maintaining laboratory accreditation and demonstrating compliance with
regulations.
Examples of Equipment
Requiring Calibration:
Direct measurement:
Balances for weighing samples, thermometers for measuring temperature, pH
meters for acidity levels.
Corrections:
Thermometers used to correct for ambient temperature during food analysis,
barometers for pressure corrections in specific tests.
Calculated results:
Spectrometers for analyzing chemical composition, where final results depend on
accurate measurements of multiple parameters.
Clause 6.4.7 emphasize that the laboratory needs to create a plan (calibration program) to regularly check and adjust its measuring equipment. The plan should be flexible and updated as needed to make sure the equipment is working correctly.
A laboratory that tests the
concentration of chemicals in water samples. Their measurements rely on various
instruments like pipettes, balances, and pH meters. According to the clause:
Establish a Calibration
Programme: They need a documented programme outlining which
instruments require calibration, how often they need it (based on manufacturer
recommendations, usage frequency, or risk assessment), and who is responsible
for performing the calibrations.
Review and Adjust: This
programme shouldn't be static. The laboratory should regularly review its
effectiveness (e.g., annually) and make adjustments as needed. For example, if
a specific instrument shows unusual drift or instability, they might tighten
its calibration frequency.
Maintain Confidence: The
ultimate goal is confidence in the calibration status of every instrument. This
means having documented records of calibrations, including dates, calibration
data, and any adjustments made. It also involves ensuring calibration is
performed by competent personnel using accredited calibration providers or
traceable reference materials.
Benefits of a Robust
Calibration Programme:
Accurate and reliable results:
Ensures measurements and test data are trustworthy, minimizing errors and
increasing customer confidence.
Compliance with regulations: Many
industries and regulations require laboratories to have documented calibration
programmes.
Reduced risk:
Detects and corrects potential issues with equipment before they impact
results, minimizing errors and liability.
Improved efficiency:
Optimized calibration schedules prevent unnecessary calibrations and ensure
instruments are always in good working order.
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