Re: flow rate unit

Posted by Richard O'Leary ( on 23:23:42 28/03/05

In Reply to: Re: flow rate unit posted by Gordon England

Imagine you have a cylinder of 1 litre, which is closed by means of a moveable piston of egligible weight.
This cylinder contains 1 litre of air at ambient pressure, approx. 1 bar. The weight of this volume of air at 0 °C is 1.293 g, this is the mass. When we move the piston half way to the bottom of the cylinder, then the contained volume of air is only 1⁄2 litre, the pressure is approx. 2 bar, but the mass is unchanged, 1.293 g; nothing has been added, or left out.

Following this example, mass flow should actually be expressed in units of weight such as g/h, mg/s, etc.
Most users, however, think and work in units of volume. No problem, provided conditions are agreed upon, under which the mass is converted to volume.
A temperature of 0 °C and a pressure of 1,013 bar are selected, and these reference conditions are indicated by the underlying letter 'n' in the unit of volume used.
The direct thermal mass flow measurement method is always based on these reference conditions unless otherwise requested. There are cases, for instance, where the reference conditions are based on 20 °C instead of 0 °C. If this difference is not considered, then there is an error of 7%!

It is worth noting that there is commonly a reversal between EUROPE & USA for the meanings of 's' (standard) and 'n' (normal).

Europe: 'n' = Volume at 0 (zero) Degrees Celsius and 1013.25 mBar
Europe 's' = Volume at 20 (twenty) Degrees Celsius and 1013.25 mBarp

There is a very useful (and free to use) online tool with which conversions between different reference conditions can be calculated.p
It is called FLUIDAT.

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