AAOM Handbook

connected elements. The capacity of these storage elements, coupled with the maximum throughput rate of the process, help us to determine how quickly a significant change in the process can occur and hence how frequently we need to look at a measure - the sampling interval. For example, if you must maintain the concentration of a solution within specification limits, and the accuracy of the measuring device is 5%, you will probably wish to be able to identify any 5% change in concentration. The minimum time interval in which this can occur is the determined by the volume of the solution and the maximum rate at which a chemical can be added. If we take a measurement more frequently than the minimum time in which a 5% change can occur then we will always detect the change. If we take the measure much less frequently than the minimum time in which a 5% change can occur then variation that occurs between sampling intervals will not be seen. Selecting the sampling interval for a measure is a trade-off between the maximum rate of variation that you will identify and the effort required to collect the measurement. The Nyquist criteria states that the maximum frequency of variation that can be represented by the data has a period of twice the sampling rate. Stated another way – you must sample at less than half the time interval in which the change you wish to detect can occur. Select a sampling interval after considering the above information. NB  If the sampling interval is equal to or greater than the Nyquist rate then reconstructing a graph by connecting the instantaneous values at the sampling interval represents the relevant performance of the process between samples.  If the sampling interval is set to less than the Nyquist rate the maximum frequency of variation cannot be observed, and a graph reconstructed by joining the instantaneous values at the sampling intervals does not represent the relevant performance of the process between samples. 7. Construct a measurement system that conditions the data (filters out irrelevant points), before capturing and recording data at the specified sampling interval and accuracy. Note – it is critical that the sample intervals be strictly adhered to or the data will not accurately represent the true variation. Note – the record is based on the value of the measurement at the sampling interval. 8. All purpose and theory measures shall be reported in both a Control Chart and Capability Histogram.