UoM is defined as a parameter associated with the result of a measurement that characterizes the dispersion of the values that can reasonably be

Transcription

1 M Grundlingh

2 Introduction UoM is defined as a parameter associated with the result of a measurement that characterizes the dispersion of the values that can reasonably be attributed to the measurand. The word uncertainty means doubtthus one can define UoM as the doubt about the validity of the result or the exactness of the result R W followed an approach to determine UoM by combining the different approaches from available literature.

3 Introduction Implementation of UoMwas done in 5 steps Step1: Specification and modeling the measurement Step2: Identification of Uncertainty sources Step3: Evaluate standard uncertainty of the uncertainty components to Type or Type Step4: Determine the individual, relative combined and expanded standard uncertainty Step5: Reporting the uncertainty

4 1.Specification and modeling the measurement Give a clear statement of what is being measured, e.g Measurand Total Coliform bacteria/100ml Matrix: ny water sample including surface, ground water, effluent water and drinking water Method: Membrane filtration and confirmation test

5 Specification and modeling the measurement The model can be an analytical expression or a simple expression. TC confirmed = TC isolated / volume filtered * Percentage confirmed Where: TC isolated = number of typical colonies isolated on M-Endo agar LES volume filtered = volume analysed Percentage confirmed = part of the total number of isolated typical colonies confirmed as TC TC confirmed = number of confirmed TC/100ml

6 Specification and modeling the measurement rief flow chart of the method to ensure that all steps are covered for UoM Clean all working areas Media preparation ph meter alances Pipette verification QC on dehydrated media Prepare media QC of prepared media Receival of samples and scanning of samples on Labware Prepare the filter units Place samples next to corresponding plates Filter the sample according to method Sterility and QC Inoculate the positive and negative controls with specified reference cultures on corresponding plates Sterility test of media plates and funnels used Read and record temperature of incubator Incubate inverted plates at 35±1 C for hours. Do not stack higher than 6 plates

7 2.Identification of Uncertainty sources Classification of sources of uncertainty So urce Type Include in Reason for not including budget Yes/no 1 W ork instruction to clean work area No W ork instruction m ust be compiled 2 ph meter Calibration specification verification standard Yes Yes Yes 3 Conductivity Yes Sp ecification Verification standard 4 alan ce Yes 5 Pipettes Yes 6 Preparation of media Volum e of water Recovery of m edia Verification m ust still be done ( future project) No No No Verification m ust still be done( future project) 7 Sample homogeneity 8 Volum e of sam ple Dilution 9 Sterile filter - No QC step 10 Vacuum contribution 11 Incubator Tem perature Yes Calibration Time No Verification m ust still be done ( future project) No Verification m ust still be done ( future project)

8 Identification of Uncertainty sources Structure the process by means of a cause and analysis effect Sampling method Sample Confirmations mixed count Type of water * media media # Transport temp ph test volume (>over grown) small volumes/dilutions Calculation of final result TC, EC Light (M-endo) Temp Time Daily verify ph meter alance External cal Level Isolation Incubation Counting on isolation media * Isolation media

9 3.Evaluate standard uncertainty of the uncertainty components to Type or Type The type of uncertainty and the probability of distributions for all sources of uncertainty included in the budget. Source Type Probability distribution 2 ph meter Calibration specification verification standard 3 Conductivity Specification Rectangular Rectangular Normal Verification standard Rectangular Normal 4 alance Rectangular 5 Pipettes Rectangular 11 Incubator Temperature Calibration Time 18 Reproducibility ( real data) 20 Temperature Fridge (prepared media) Room (dehydrated media) Triangular Rectangular Rectangular Normal Normal Normal 22 Waterbaths Rectangular 23 utoclaves Rectangular

10 4.Determine the individual, relative combined and expanded standard uncertainty Determine the combined standard uncertainty, relative standard uncertainty and the expanded uncertainty. ccording to the EurachemGuide (2000), for all models involving a product and a quotient (e.g. y = p q r.. or y = p/q.) the combined standard uncertainty is calculated by expressing the uncertainty parameters as relative standard uncertainties as follows: u c n ( y) = [ u( xi) / xi] 1 2 where: u(xi)/xietc are the uncertainties in the parameters, expressed as relative standard deviations.

11 Determine the individual, relative combined and expanded standard uncertainty Relative uncertainty Symbol Description Type Expected Value Probability Distribution Uncertainty Estimate (±) (i.e. semirange for Type ) Divisor relative uncertai nty Standard uncertainty Sensitivity units coefficient Standard uncertainty: u(x i ) Uncertainty contributor Degrees Reliability of Significance Freedom u(x i ) c i u(y i ) v i u(y i ) 2 u(y i ) 4 u(y i ) 4 2 M Rectangular C % % E E-09 9.E-10 3 M Rectangular C % % E E-09 8.E-10 4 M Rectangular C % % E E-09 9.E-10 5 M Rectangular C % % E E-09 9.E-10 6 M Rectangular C % % E E-09 9.E-10 7 M Rectangular C % % E E-09 9.E-10 8 M Rectangular C % % E E-09 9.E-10 9 M Rectangular C % % E E-09 9.E M Rectangular C % % E E-09 9.E M Rectangular C % % E E-09 9.E M Rectangular C % % E E-09 9.E Thermometer refer to thermometer uncertainty budget % % E+01 Combined standard uncertainty: u c (y) u c (y) E+01 v eff 2.0 Level of Confiden 95% LOC v i Expanded uncertainty: U Coverage facto 1.96 U Coverage factor : k

12 Determine the individual, relative combined and expanded standard uncertainty

13 Imported uncertainty budget

14 5.Reporting the uncertainty Results must be rounded up. The coverage factor must be recorded The level of confidence must be stipulated The expanded uncertainty is ±11 at a 95% level of confidence with a coverage factor of 1.96.

15 Conclusions Not the perfect approach y calculating UoM: Gives the opportunity to improve areas within the method with huge contributors to the uncertainty Critically analyze all steps of the method to eliminate or reduce the uncertainty contributors Vision to improve and reduce the expanded uncertainty over time Gives an indication on where the possible problems within the method could be.

16 Thank you

17 REFERENCES Doiron,T ; Stoup, J Uncertainty and Dimensional Calibrations. Journal of Research of the National Institute of Standards and Technology Volume 102 (6). E-04/ (rev 2) ccreditation for Microbiological Laboratories. ISO/IEC 17025: General requirements for the competence of testing and calibration laboratories. De eer, W.H.J; Statistical method validation for Test laboratories. S 8496:2007 ritish Standard: Water quality Enumeration of microorganisms in water samples Guidance on the estimation of variation results with particular reference to the contribution of uncertainty of measurement. Eurachem/Citac Guide CG4 (2000). Quantifying uncertainty in nalytical Measurement. Second Edition.