HIGH-ACCURACY BLEND CONTROL 603 N z z I- z x

604 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS CONCLUSION The introduction of very accurate turbine flow transmitters provided the impetus for the evolution from mechanical to digital blending systems. Error- free electronics and pacing reinforced their superiority over analog systems for these types of applications. Today, well over 500 digital in-line blending systems are installed and operating in this country with many more world- wide. Although direct computer control might seem to be the next likely step, future trends appear to be toward supervisory computer control of digital blending instrumentation rather than its replacement by a computer. For instance, many major refineries already have supervisory control of blending operations. As was the case with the initial acceptance of digital blending itself, this philosophy will ultimately carry over to other industries as well. APPENDIX Further Comments on Measurements Digital measurement signals are necessary for digital instrumentat:on how- ever, this is not to imply that the primary flow element must be a digital device. The signal to. the comparators must be digital, and to accomplish this while using an analog primary flow element, an additional device must be inserted into the loop to convert the flow signal into a pulse signal. This is an extra step, and it will result in higher cost and decreased overall ac- curacy. There are times when less accuracy has to be tolerated because an analog primary flow element is absolutely necessary. Highly abrasive fluids contain- ing solids (slurties), as well as aerated fluids may dictate the use of an analog primary flow element. Using an analog primary flow element may limit the flow range over which a control system will sustain a stated accuracy, although it is true that for a limited range, an analog device can be as accurate. There are two reasons for the emphasis on flow range. Dit•erent product mixes require dit•erent flow rates of the individual components. Therefore, rangeability of the flow de- vice is critical. Suppose a given blend consists of 80% of one component, and fi•at another blend is only 10% of that same component. That means that the flow clement for that component must have 8:1 rangeability. This is beyond the capability of most analog primary flow elements available for industrial use. So equipment flexibility, then, is one reason that flow measurement range is important. A second reason has to do with the blending instrumentation. In another section of this paper, an important feature of automatic in-line blending was discussed, namely the pacing capability. This feature allows the control sys-