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40 October 2018 Tablets & Capsules truly random in their occurrence and/or duration, as shown in Figure 5, you must use a divide-and-conquer strategy of methodically eliminating all potential causes. Is the feeder being buffeted by rogue air currents, which can occur in a high-laminar-flow room or when the air pres- sure changes constantly within an isolator? Do the distur- bances occur when the rest of the process is shut down? Is something odd going on inside the feeder, such as random changes in material characteristics causing occasional ratholes? Are the feeder's process connections secure and not causing additional stress on the feeder and scale? One by one, you must nominate, assess, and eliminate possible causes until you identify the culprit or culprits. Scenario 4: Disturbances correlated with refill. An often-underappreciated requirement of LIW feeding is the need to return to acceptable weighing conditions as soon as possible after completing a refill, allowing the feeder to resume gravimetric operation. A refill is a major disturbance to the feeder's weight and requires a settling time after the refill to allow the feeder's scale system to stabilize and begin to collect the correct weight-loss data, as shown in Figure 6. Several external process factors can contribute to dis- turbances following feeder refill. Although a required ele- ment in a fully automated LIW feeding system, the refill system itself is not weighed and is considered to be part of the process environment. Any unintended post-refill leakage from the refill device, such as less-than-complete shutoff, can corrupt the feeder's weight measurement Scenario 1: An isolated, short duration weight dis- turbance. A passing plant worker bumping the feeder, or some other form of momentary incident, can cause an isolated, short-duration weight disturbance, as shown in Figure 3. Most LIW feeders can recognize a brief distur- bance and quickly determine whether to ignore it or compensate for any resulting excess or shortfall in dis- charge depending on the disturbance's duration and severity. While a disturbance of any kind will tend to reduce a feeder's overall measured accuracy, the harm inflicted by isolated, infrequent disturbances is typically not significant. Scenario 2: Disturbances of regular occurrence and duration. When you require the feeder to perform in a more disturbance-prone process environment, the cumu- lative effect of ongoing disturbances can degrade the feeder's overall performance, as shown in Figure 4. If your feeder's weight trending displays such a disturbance pat- tern, the likely cause is shock or vibration transmitted to the feeder from some nearby piece of equipment, such as a continuous mixer or the support platform itself. If multi- ple sources of regular disturbance are at play, the distur- bance's pattern may appear random, as in the next sce- nario, but closer inspection should reveal the pattern's composite character. The frequency and duration of reg- ular disturbances can direct you to the offending equip- ment, but the particular remedy depends on the situation. Scenario 3: Disturbances of random occurrence and duration. If you find, upon analysis, that disturbances are Figure 4 Disturbances of regular occurrence and duration Figure 3 Isolated, short duration weight disturbance Isolated, Short Duration Scenario 1 Figure 5 Disturbances of random occurrence and duration Random Occurrence and Duration Scenario 3 Figure 6 Disturbances correlated with refill Correlated With Refill Scenario 4 Regular Occurrence and Duration Scenario 2