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Computer control systems have been in use for a number of years in the hydrocarbon processing industry. Operators have grown accustomed to interfacing with the process through the use of keyboards and CRTs. Although user experience is substantial, little information has been collected concerning how operators actually interact with their control systems. For instance, knowing how likely operators are to alternate between available CRTs would be useful in helping to determine how many CRTs to provide. Without such knowledge, one can only rely on speculation when designing a workstation layout.

One source of information is Beville Engineering's job sampling database. The database consists of observations of oil refinery operators performing their jobs. Recently, the control board operator samples in the database were reviewed to learn how the operators utilize the available CRTs.

All operator CRT usage was rank ordered from highest to lowest for each sample and summed across all samples. The data represents the average percentage of time an operator spent operating at each of the available CRTs. The data is summed across all samples regardless of whether there were three or four CRTs available to the operator. Analysis of the data shows that operators appear to utilize one CRT roughly 70% of the time, a second CRT about 25% of the time, and a third CRT approximately 5% of the time. A fourth CRT was used only .4% of the time.

Similarly, in another analysis, all operator CRT usage was rank ordered from highest to lowest for each sample and summed across all samples. However, in this case the samples were broken into two groups, those operators with three available CRTs and those operators with four available CRTs. The operators with four available CRTs tended to distribute their CRT usage slightly more evenly among the CRTs than the operators who had three CRTs. Several possible explanations exist for the difference. One explanation is that the display designers of the four CRT systems more widely dispersed the information in the display system. Another explanation could be that the size of the consoles and their layouts encourage the operator to be more mobile.

During the sampling it was noted that it is a common practice to dedicate one CRT for alarm summary displays. This would help account for a lower usage on one of the CRTs.

In this study, on average, operators performed 94.2% of their work with two CRTs, and 99.6% of their work with three CRTs. Yet, a common practice is to configure workstations in packages of six. If a fourth CRT is used only .4% of the time, and the usage of additional CRTs decreases exponentially, what are the benefits of providing a fifth or even sixth CRT?

The data suggest that if more than three CRTs are configured into a workstation per operator, the benefits appear to be questionable. One could argue that additional CRTs could be needed for off-normal operations, such as start-ups and shutdowns. Certainly, to justify the expense, a thorough analysis should be made before ordering operating stations with more than three CRTs per operator.

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For further information, visit the links below.

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