SYSTEMS OF MESUAREMENT ¿what is a system of mesuarement?

SYSTEM OF MESUAREMENT A system of measurement is a set of units which can be used to specify anything which can be measured and were historically important, regulated and defined because of trade and internal commerce. Scientifically, when later analyzed, some quantities are designated as fundamental units meaning all other needed units can be derived from them, whereas in the early and most historic eras, the units were given by fiat (see statutory law) by the ruling entities and were not necessarily well inter-related or self-consistent.

PROPUSE If measurements are used to guide decisions, then it follows logically that the more error there is in the measurements, the more error there will be in the decisions based on those measurements. The purpose of Measurement System Analysis is to qualify a measurement system for use by quantifying its accuracy, precision, and stability. An example from industry serves to illustrate the importance of measurement system quality: A manufacturer of building products was struggling to improve process yields, which had a significant impact on product cost. Experience indicated that there were several process and environmental characteristics that influenced the process yield. Data were collected on each of the variables believed to be significant, followed by regression and correlation analysis to quantify the relationships in statistical terms.

REQUIRMENTS Following are general requirements of all capable measurement systems: Statistical stability over time. Variability small compared to the process variability. Variability small compared to the specification limits (tolerance). The resolution, or discrimination of the measurement device must be small relative to the smaller of either the specification tolerance or the process spread (variation). As a rule of thumb, the measurement system should have resolution of at least 1/10th the smaller of either the specification tolerance or the process spread. If the resolution is not fine enough, process variability will not be recognized by the measurement system, thus blunting its effectiveness.

Measurement Systems Analysis Fundamentals Determine the number of appraisers, number of sample parts, and the number of repeat readings. Larger numbers of parts and repeat readings give results with a higher confidence level, but the numbers should be balanced against the time, cost, and disruption involved. Use appraisers who normally perform the measurement and who are familiar with the equipment and procedures. Make sure there is a set, documented measurement procedure that is followed by all appraisers. Select the sample parts to represent the entire process spread. This is a critical point. If the process spread is not fully represented, the degree of measurement error may be overstated. If applicable, mark the exact measurement location on each part to minimize the impact of within-part variation (e.g. out-of-round). Ensure that the measurement device has adequate discrimination/resolution, as discussed in the Requirements section. Parts should be numbered, and the measurements should be taken in random order so that the appraisers do not know the number assigned to each part or any previous measurement value for that part. A third party should record the measurements, the appraiser, the trial number, and the number for each part on a table.

MESUAREMENT’S IMAGE