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Old April 22nd, 2005, 11:55 AM   #5
Ron Beaufort
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Join Date: Jul 2002
Location: Charleston, SC
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in Figure 5 we’ve closed the drain valve completely ... and we’ve manually opened the fill valve just 1% ... since we have SOME water coming in but NO water going out, naturally the level in the tank begins to rise ... and the resulting shape of the trace is different from anything that I’ve shown before ... this one is a straight line








... now keep in mind that the tank we’re using for our example is a straight-sided vessel and that we’re maintaining a constant rate of fill ... but under these conditions, basic common sense and life’s experiences are enough to convince us that, sooner or later, the tank will become full and then overflow ...



the main idea to come away with from this example is that the current level of the tank does NOT affect how fast the level continues to rise ... and so the characteristics of the “level control” system are different from those of our “pressure tank” example ... in the “pressure” example, the current amount of pressure in the tank DID indeed affect how fast the pressure could continue to rise ... and in our original “oven” example, the current temperature of the oven DID indeed affect how fast the temperature of the oven could continue to increase ... (remember that the DIFFERENCE between the oven and the surrounding air came into play in that example) ...



and so at this point we’ve looked at a few different types of processes and seen quite different responses from each one of them ... no wonder our friend Ayman admits to being a little bit confused ... I can assure you that he is not the only student that I’ve ever met who finds all of this somewhat perplexing ... the basic idea is that different types of processes have different characteristics ... and when it comes to automatically controlling them, “one-size-fits-all” does NOT apply ...
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