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Copyright Alan Rowley Associates 2001 1 Steps to an Accurate Result Select a method and validate it as suitable for the purpose envisaged. Establish that.

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Presentation on theme: "Copyright Alan Rowley Associates 2001 1 Steps to an Accurate Result Select a method and validate it as suitable for the purpose envisaged. Establish that."— Presentation transcript:

1 Copyright Alan Rowley Associates Steps to an Accurate Result Select a method and validate it as suitable for the purpose envisaged. Establish that the method gives consistent and accurate results within known limits and set up an internal quality control regime to monitor the method to ensure these limits are maintained. Document method and train staff. Test and Calibration Methods Dr Alan G Rowley Ensure all measuring equipment is traceably calibrated, ideally to SI units. Establish verification and maintenance regimes.

2 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Traceability That property of the result of a measurement whereby it can be related to appropriate standards, generally international or national standards, through an unbroken chain of comparisons. ISO Definition

3 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Traceability WHY IS THIS IMPORTANT ? If we all trace our measurements back to the same reference standard then we should all agree on the measurement result. The ‘agreement’ will not be absolute since there are measurement uncertainties within the laboratories and also at each transfer in the calibration chain.

4 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Traceability The ultimate traceability is to the relevant SI unit. The SI unit must be ‘realised’ in some way, e.g. Mass by a standard mass, length by a light wavelength.

5 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Measuring Equipment Working Standards Secondary Reference StandardsPrimary Reference Standards SI Units

6 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Measuring Equipment Testing Laboratories, Industry Calibration Laboratories Regional of National Metrology Laboratory SI Units

7 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Weights used on a daily basis, e.g. built into balances Weights used to calibrate balances in industry and laboratories Standard weight sets calibrated on mass comparators National Mass Standards Std 1Kg

8 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Traceability In chemical measurements the ultimate traceability is to the mole. In practice difficult to realise for every substance. A reasonable substitute is a ‘reference material’ of known or agreed composition. Ideally this should be a certified reference. ISO Guide 34 and ISO Guide 35.

9 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Traceability In other areas references may be ‘industry standards’. Light fastness standards, geological samples for source rock analysis, proximate analysis of foods. The key point is that everyone agrees on the ‘correct’ answer.

10 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Traceability In many cases, however, reference materials are impracticable since the reference must be:- Homogeneous and consistent with respect to the property being measured. Stable with respect to the property being measured at least over a defined timescale. Typical of the samples normally encountered, e.g. with respect to matrix.

11 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Traceability Inter-laboratory comparisons (proficiency testing) A range of laboratories all test the same or identical samples. Results are compared on the basis of an agreed ‘correct’ value. Laboratories failing to perform satisfactorily review their procedures and/or method.

12 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Traceability Inter-laboratory comparisons (proficiency testing) If the measurand is a precisely defined species of quantity then the laboratories should be able to use any validated method. Where the method defines the quantity being measured all laboratories should use the reference method or a method validated as equivalent.

13 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Calibration of Instruments Traceably calibrated references Valid and documented procedure for calibration Trained and appropriately skilled staff VALID CALIBRATION

14 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Calibration of Instruments Valid and documented procedure for calibration Trained and appropriately skilled staff VALID CALIBRATION

15 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Calibration of Instruments Valid and documented procedure for calibration Trained and appropriately skilled staff VALID CALIBRATION IN-VALID CALIBRATION

16 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Valid Calibration Purchase and maintain traceable references. Establish and document calibration procedures. Train staff to conduct calibrations. Establish Quality Control on calibration activity.

17 Copyright Alan Rowley Associates Calibration and Traceability Dr Alan G Rowley Verification Periodic checks to ensure that the calibration remains valid. Must be able to detect out of calibration condition before it affects data. Drift, malfunction. Identifies need to shorten interval between calibrations or may justify increasing it.


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