How to calculate the combined uncertainty if I took weight of 0.120 g ±0.001 and dissolved it to 50 ml ±0.1 and then I transferred 1 ml with micropipette ± (0.63% at 200 µl) to make another dilution at 50 ml ±0.1
The best source for this is:
Introduction
JCGM 100 series – Guides to the expression of uncertainty in measurement (GUM series)
https://www.iso.org/sites/JCGM/GUM/JCGM100/C045315e-html/C045315e_FILES/MAIN_C045315e/07_e.html
Two people measuring the same product with the same ruler on different days would probably get different results. This could be because of factors such as a change in the room temperature (important for a metal ruler) or different eyesight capabilities. The two measurements might be equivalent or not, depending upon their individual uncertainties. This concept of uncertainty is a measure of the quality of a measurement and can be vital in many cases. The JCGM/100 series of documents establishes general rules for evaluating and expressing uncertainty in measurement that can be followed at various levels of accuracy and in many fields — from the shop floor to fundamental research. Therefore, the principles of these Guides are intended to be applicable to a broad spectrum of measurements, including those required for:
- maintaining quality control and quality assurance in production;
- complying with and enforcing laws and regulations;
- conducting basic research, and applied research and development, in science and engineering;
- calibrating standards and instruments and performing tests throughout a national measurement system in order to achieve traceability to national standards;
- developing, maintaining, and comparing international and national physical reference standards, including reference materials
- other metrology-related aspects
The series consists of the following parts:
- JCGM 100 – Evaluation of measurement data – Guide to the expression of uncertainty in measurement (ISO/IEC Guide 98-3)
- JCGM 101 – Evaluation of measurement data – Supplement 1 to the "Guide to the expression of uncertainty in measurement" – Propagation of distributions using a Monte Carlo method (ISO/IEC Guide 98-3-1)
- JCGM 104 – Evaluation of measurement data – An introduction to the "Guide to the expression of uncertainty in measurement" (ISO/IEC Guide 98-1)
(For the moment, only JCGM 100 is available in HTML ; PDF versions of JCGM 100, 101 and 104 are available here)
The following parts are under preparation:
- JCGM 102 – Evaluation of measurement data – Supplement 2 to the "Guide to the expression of uncertainty in measurement" – Models with any number of output quantities (ISO/IEC Guide 98-3-2)
- JCGM 103 – Evaluation of measurement data – Supplement 3 to the "Guide to the expression of uncertainty in measurement" – Modelling (ISO/IEC Guide 98-3-3)
- JCGM 105 – Evaluation of measurement data – Concepts and basic principles (ISO/IEC Guide 98-2)
- JCGM 106 – Evaluation of measurement data – The role of measurement uncertainty in conformity assessment (ISO/IEC Guide 98-4)
- JCGM 107 – Evaluation of measurement data – Applications of the least-squares method (ISO/IEC Guide 98-5)
Dear Raed Abouzeid,
The combined standard uncertainty for this procedure is simply calculated by root-sum-squaring the uncertainty components estimated in the relative form for the sequential operations you described. To do this you need first to convert the error limits ±delta (usually taken from a specification) to the value of standard uncertainty: uc = delta/k, with the coefficient k depending on the selected probability distribution. In most cases, where knowledge about possible values of the quantity within the limits is lacking, a uniform (rectangular) probability distribution is assumed, while in the case of volumetric flask, with the specified nominal capacity (50 ml), the use of a triangular distribution is more correct. Calculation is given in the file attached.
Note that specification of the volumetric error for the "micropipette" should be given for the actual volume (1 ml) that is transferred, not for the volume 200 ul.
Best regards,
Rouvim Kadis
Also note that an additional contribution to the combined uncertainty, caused by the possible temperature variation, should be taken into account in the estimation of volumetric uncertainty.
- Badges
- Science topic