Gas Density at pressure

[Billy666]

Designing a flowmeter which works @4bar with a flow between 0 and 25 lpm, What i am trying to understand is that why would it register the same flow reading for oxygen(higher density)than it does for air, These have been attached to a regular gas cylinder, I am assuming wrongly or rightly that this has something to do with the densities of the gasses under pressure (200 bar) and also the possibility of water (moisture) in the air cylinder but do not have anything solid to justify this, could someone explain ??

 

[jmw]

Perhaps because this is a volumetric flowmeter? You don't say what principal you are using.
Variable area meters, turbine meters, vortex meters are all volumetric or velocity types.
There aren't that many technologies that will measure mass flow directly.
This is not a problem, especially if there is a pressure regulator used because a reasonably good conversion to mass can be made based on the gas density at that pressure.




JMW

http://www.ViscoAnalyser.com

 

[Billy666]

Hello there
it is a ball and glass style of flowmeter, (see attached) the readings for oxygen and air are reading the same I assume this is down to density and the viscosity of the gas we are using under pressure, just want some form of solid justification as to why the readings are the same.

 

[jmw]

OK, variable area flowmeter sometimes called a Rotameter (Rotameter was the name disputed by a German Comapny and a GEC company in the UK but it became a generic for this type of flowmeter.)
Article here:

http://www.flowmeterdirectory.com/flowmeter_artc/flowmeter_artc_07052201.html

We need to take other factors into account.
You have constant pressure, the temperature presumably isn't varying much either so the only real difference is the density of O2 compared to air.
Temperature and pressure changes can cause significant variations in density which may well be significant.
The difference in density between O2 and air may not be that significant. The article shows a machined float type but you show and describe a ball float type.
These are mass produced to sell for a few dollars.
Accuracy is probably 2-5% FSD. It doesn't need to be any better for these types of application.

The ball type has moulded plastic tubes and floats (if it were glass you'd need a safety shield around it for gas duty) and the calibration scale is standardised.

Precision instruments use tapered glass tubes (some have double tapers) and have precision machined conical floats (usually; for viscous fluids the float may have a lip or sharp edge at the widest part) and then they are individually calibrated the float and glass tube paired together. Scales are then produced for the individual tube transfer printed onto the glass where they ceramic ink is fused into the glass tube.
These are pretty accurate, for this type of meter.
Mass produced ball float types are not and don't need to be.
The thing to ascertain is just how much difference the gas density represents and whether this would be significant or not.


JMW

http://www.ViscoAnalyser.com

 

[jmw]

Oh, and the question of how significant the density is, is asked and answered here:

http://wiki.answers.com/Q/Which_is_denser_oxygen_or_air

JMW

http://www.ViscoAnalyser.com