ILAC P-14 addresses resolution as part of the Expanded Uncertainty in section 6.4 which states, “Contributions to the uncertainty stated on the calibration certificate shall include relevant short-term contributions during calibration and contributions that can reasonably be attributed to the customer’s device. The calibration process error, therefore, includes temporary and non-correctable influences incurred during the calibration such as repeatability, resolution, error in the measurement source, operator error, error in correction factors, environmental influences, etc.” Therefore, the estimate includes all components of the error that have an influence on the measurement results of the calibration which would also include the influences of the item being calibrated, with the exception of the bias of the M&TE. The value of this uncertainty, the estimate should reflect the results that are reasonably expected from the use of the approved procedure to calibrate the particular type of M&TE. ANSI/NCSL Z540.3 Handbook states, “For the denominator, the 95 % expanded uncertainty of the measurement process used for calibration per the calibration procedure is to be used to calculate TUR. We typically deal with the resolution of the UUT in the denominator, as shown in figure 1 above. This calculation of TUR is crucial as it is the basis for making statements of conformity required by ISO/IEC 17025:2017section 7.8.6.1 & 2.įigure 1 The typical minimum contributors to calculate TUR at the time of calibration TUR is defined in ANSI/NCSLI Z540.3-2006 and ILAC G8, as “The ratio of the span of the tolerance of a measurement quantity subject to calibration, to twice the 95 % expanded uncertainty of the measurement process used for calibration”. Morehouse reports the resolution of the Unit Under Test on our calibration certificates as well as uses the resolution of in the divisor for calculating TUR and reporting Expanded Uncertainty. However, we are first going to briefly discuss why calculating the resolution of the Unit Under Test properly is essential. We are hoping to simplify how to calculate the resolution of the Unit Under Test (UUT) so this can be a simple guidance document. Per Section 4.14 of JCGM 200:2012, Resolution is the “smallest change in a quantity being measured that causes a perceptible change in the corresponding indication.” Over the years, this simple definition has become a topic of confusion amongst the metrology community.
How to Calculate the Resolution of a Load Cell