They are not always interchangable- and is application specific. in fact there are very few applications where you can do it.
Constraints in pressure and compressibility. This two, especially the second one will change the total system behaviour.
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Fantastic ideas for a Fantastic World, I make the illogical logical.They put me in cars,they put me in yer tv.They put me in stereos and those little radios you stick in your ears.They even put me in watches, they have teeny gremlins for your watches