Tham khảo tài liệu 'the gauge block handbook episode 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The standard deviation square root of the variance a measure of the short term variation of the comparison process is recorded for each calibration in the MAP file along with other pertinent data from the calibration. These data are later used to determine the value of one control parameter the short term variation as described in the next section. Control Parameter for Repeatability There are a number of ways to monitor the comparator process. The simplest is to compare the variance of each calibration to an accepted level based on the history. This accepted level average variance from calibration history is characteristic of the short term repeatability of the comparator process and is called the within variance. In our system the within variance is used for two purposes. One is as a control point for the calibration as discussed below. The second is that being a measure of short term variation of the entire calibration process it is used to calculate the process uncertainty a part of the total uncertainty reported to the customer. In our system the variance for each calibration is recorded in the MAP file. Each year the last three years of data are analyzed to update the process control parameters used for the next year. Figure shows a typical MAP file data plot. YEAR Figure . Variation of the short term standard deviation with time for the inch size. The most prominent characteristic of the graph is the continuing reduction of the standard deviation since 1981. The installation of new comparators during 1987 and the replacement of one of the old master sets with a new chrome carbide set in 1989 have resulted in significant reductions in the 57 process uncertainty. When the current process parameter ow is calculated only the data since the last process change new master set in 1989 will be used. If the calibration variance was independent of the block length only one value of the control parameter would be needed and the average of the last .
