why does inlet pressure is taken in considering when purchasing rotameters?

rotameters are variable area flow meter not variable pressure. then why input pressure make difference in flow. please reply

 

Rotameters are just like any other flow restriction, the more pressure you put on them the more flow you get. However if you are talking about the type with a needle valve on the inlet, e.g., a purge meter you are right the inlet pressure is not so important.

Bear in mind your glass must be able to withstand the full line pressure and also if the pressure is very low it may not be enough to lift a heavy float.

If you are measuring gas flow the pressure effects the density.
In my experience rotameters for measuring process gas flow are one of the trickiest instruments to specify.

Regards
Roy

 

For one thing, there's usually a minimum inlet pressure necessary to overcome the pressure drop and still keep the flow moving...

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