AAOM Handbook

INTERNAL

3. The physical boundary conditions for measurements shall be as defined in the Business Structure flow sheets. The data boundaries shall be based on the boundaries to the process delivering its purpose - see examples in Appendix 1. 4. Measurements can contain ‘noise’, i.e. values that are not a true representation of the performance. Noise may arise from various sources but is identifiable as changes that are either not physically possible for the process, or changes that are too rapid for the process to actually deliver. Noise is not a relevant component of the measure and should be excluded from it. Examples of measured values that are not possible include; a bin level that is either negative or higher than the physical bin, a speed for a transfer mechanism that is beyond its capacity. With regard to rate of change, each area of a process will have a maximum throughput rate limited by the physical design and capacities of the equipment, as for the transfer mechanism just discussed. This limit will affect the rate of change that can occur in all of the quantities that we may need to measure, whether they be product level in a tank, chemical concentration in a solution, or temperature in a furnace. For example, the maximum rate of level change in a tank will be determined by the tank volume and geometry and the maximum rate of either the inflows or outflows. The maximum rate of change of the concentration of a solution will be determined by the volume of the solution and the maximum rate at which a chemical can be added. The maximum rate of change of temperature in a furnace is determined by the maximum difference between the heat injection and thermal losses. A conditioning filter must be specified, based on the above criteria, and applied to the measure so that impossible values are recognised and rates of change that are irrelevant are removed. 5. The measurement accuracy will determine the minimum level of variation that can be identified (referred to as the quantisation level). Any variation that is less than this will not be seen. Selecting the quantisation level for measuring performance is a trade off between the detail needed to determine that performance is within specifications and the effort/cost of achieving that accuracy. In most instances today we have inexpensive and highly accurate technology that can provide a much higher level of resolution than is necessary to help us understand the capability of a process. In practice, a Capability Histogram that is based on 15 to 20 quantisation steps of the full range of variation typically provides enough detail. The quantisation level can generally therefore be set to the minimum level of variation that you wish to observe. 6. Storage capacity within, or between, processes provides a natural filter that smooths variation (noise) within a process, and buffers connected processes against the effects of variation occurring in one of the

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