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Volume vs Quantity vs Pressure

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Old   March 1, 2003, 03:24
Default Volume vs Quantity vs Pressure
L Jones
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Hello, I have very little experience in fluid dynamics. I am an electronics technician, who often works on equipment containing air and fluid pressure as a source of periperal processes powering, in pharmacutical manufacturing. (My actual job is to do the programming for the computers and PLCs which control the processes). One of the processes is to squirt glue at a particular point in a machine's travel to seal a box of medicine. The mechanics have been complaning that too much volume of glue is being squirted; causing the boxes to have visable glue seep out of the seam of the glued flap. I suggested they reduce the size of the hole on the device which squirts the glue. They argued that reducing the size of the sqirt hole whould result in the same amount of glue .. only at a higher pressure. They compared it to holding one's finger over the end of a garden hose .. the water sqirts a greater distance.. but the same volume comes out. I think that is not necessarilly the case. In electronics, I would compare this to OHM's Law. In theory, as one increases the load placed upon an electrical source, the current draw will increase to infinity. In practice, there are no sources of infinite current. In other words, one can only draw as much current as the souce is able to supply. I put my finger over my garden hose. When I covered just a fraction of the end of the hose, it was true that the water pressure increased - the water shot further - and the volume per time interval was the same as with the hose end unrestricted .. but when I covered a large percentage of the hose end .. the out-flow pressure and volume fell off sharply. I equate this to the fact that city water has a finite amount of available pressure - just as all electrical sorces have a finite amount of current available.

Question: Am I thinking correct?

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Old   March 1, 2003, 07:16
Default Re: Volume vs Quantity vs Pressure
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Yes, you are thinking O.K. The similarity you made with the Ohm's law is correct when your tank (source) has a constant pressure (voltage): the bigger the load you place (smaller the hole you make) the lower the flow-rate (Volume/second) you have.

However, if the glue is pumped with a kind of positive displacement pump (similar to constant current source in electronics), since they are able to pump mostly the same flow-rate regardless of the load, what the mechanics told you would be true.

Hope this helps.
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