Like there is nothing perfect in the world we live in, measurements too - be it of any kind - are not perfect and are influenced by various factors resulting in uncertainties in measurement. For this reason, any measurement result needs to be accompanied by an uncertainty estimate to make it more meaningful. The evaluation of uncertainties is quite complex and the capability and competence of Analytical and Precision Balance Co. to have a sound management system, carry out calibrations, evaluate uncertainties, etc. are what gives Analytical and Precision Balance Co. the recognition and accreditation by IAS.
Uncertainty statements are of immense importance, more particularly when high levels of calibrations are involved. The uncertainty values help the user in evaluating his/her further processes, fine tuning them and have better control on product quality.
Written by B.N.Taranath,Staff Metrologist, International Accreditation Service.
Type A & B Uncertainties
The calculation of Type B for the balances takes into consideration the uncertainty of the reference weight. Other factors do not appear to be taken into consideration, such as eccentricity, air buoyancy, hystersis, and local acceleration of gravity. Also, the procedure needs to include the requirement for repeatability checks for Type A.
We have formulated our uncertainty software to consist of the Type A readings we were previously doing and added calculations and readings for the above-mentioned Type B uncertainties.
1. The formula for air buoyancy correction is stated in the Buoyancy Correction and Air Density Measurement Report issued by the National Physical Laboratory. After our technician enters the atmosphere conditions at the time of an uncertainty report, our software will make the adjustment from the N.I.S.T. traceable data we receive from the Metrology Lab at the Dept. of Weights and Measures for the weights standard being used in the test. The adjustment for air buoyancy is printed on the report.
2. The average acceleration of gravity was calculated using an average at 20 degrees to 40 degrees latitude and sea level to 6,800 feet above sea level. Our procedure will state that we, as a company, will not complete uncertainty reports outside these physical perimeters. This data was supplied to us from A&D Engineering - the third largest manufacturer of lab balances in the world.
3. Hystersis, as applied to scales and balances, refers to the ability of the instrument to return to its zero point after a mass has been removed from the weighing platform. The values will be input after ten weighings. Our software will record the maximum value and that value will be calculated into the uncertainty report.
4. The requirement for repeatability checks has been written into our standard procedure for calculating uncertainties. The final Type B calculation is based on the highest absolute value derived from ten required weighings using the same weight standard with the first record reading being the starting point.
5. The calculation for eccentricity will be completed after the initial ten weighings are done for repeatability and hystersis. Eccentricity as applied to a weighing instrument refers to the ability or inability of the operator to place the weight standard on the exact same location on the platform. We have written in our procedure and software three additional weighings with the same weight standard used for repeatability and hystersis. This procedure is explained as the Shift Load Test in the N.I.S.T. Handbook 112. The standard is placed on the platform in a Y shape configuration with the perimeter of the standard being as close as possible to the outside perimeter of the original placement. This is also referred to as a corner load test by some manuals. The eccentricity uncertainty will be calculated off the highest deviation from zero.
The new procedure and software was completed and validated on June 9, 2005. We have run a number of demo tests on various instruments for training purposes. One final note - the barometer used for air buoyancy calculations has a N.I.S.T./traceable calibrations certification.
Written by Mike Williams, President, Analytical & Precision Balance Co., Inc.