Measuring of airflow using flowmeters

[ryanwolff90]

Hi All,

Our machine uses compressed dry air to power certain sub-systems (pressurized systems such as isolation, or hydraulics) and as cooling air for hot items. If I measure the airflow coming from the regulator at the wall to the machine by installing a flow meter in line, I am measuring the actual flow rate at the line pressure set by the regulator, correct? And I would need to calculate the standard CFM by normalizing the pressure and temperature down to STP, 14.7psi and 68F? I guess I am confused because the King's flow meter I bought says SCFM on it. I didn't think measuring flow with a flow meter would give me SCFM, only actual CFM at pressure, unless I was measuring the airflow going out to the atmosphere.

Thanks!
Ryan W.

 

[zdas04]

Virtually all gas meters are pressure and temperature compensated to give you SCF (there are a lot of definitions of SCF, most that use °F us 60 not 68). For air the calc is simply P[sub]act[/sub]*T[sub]std[/sub]/P[sub]std[/sub]/T[sub]act[/sub] not terribly difficult if you have any electronic capability at all.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[ryanwolff90]

Hi David,

Thanks for your prompt reply. So what you are saying is the flow meter I have is already compensated for my airflow being at 100PSIG? Or would I need to convert using that equation you provided?

Thanks,
Ryan W.

 

[zdas04]

Read the literature that came with the meter. I'm not familiar with that particular meter.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

Kings seem to make lots of meters. If yours is digital it might incorporate pressure and temperature correction to some standard method and some definition of standard. You can normally change this via an interface or via a set up function but unless you post a link to the meter you have we're all guessing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[moltenmetal]

King makes rotameters, and to correct a rotameter for pressure you take the square root of the ratio of the two densities. The scfm scale on a rotameter is only accurate at one discharge pressure.

 

[bimr]

Rotameter manufacturers usually have a chart to convert from actual to standard conditions.

 

[ryanwolff90]

Sorry for the delay in response. It has been one crazy week at work.

The meter we use is the 7520 & 7530 Series - Acrylic Tube Flowmeter series (

http://www.kinginstrumentco.com/7520_7530/7520_7530.html).

These have a float inside of a "tube" and as the air flow past it, it raises to the flow rate of the air. Exactly what moltenmetal called it, a rotameter.

Apparently the website gives an equation for converting from Actual CFM to SCFM: ACFM Multiply by [[(Operating PSIG+14.7)(530)]÷ [(14.7)(Operating °F+460)]]. If we are at room temp, this equation reduces to ACFM Multiply by (Operating PSIG+14.7) ÷ 14.7. Is there supposed to be a square root in this equation?

I am confused because when I called kings, they said there isn't a big difference between ACFM and SCFM. But according to this equation, there is an 8X difference at 100PSI... They also mentioned that I should be more worried about back pressure than ACFM vs SCFM. That just adds more confusion for me. If I measure the pressure in the line and it is at 100PSI, is that technically the back pressure?

 

[moltenmetal]

The equation they gave you is to convert ACFM to SCFM and vice versa. It is NOT the correction factor for correcting the READING of a rotameter when it is operating at a pressure different than the scale is calibrated for. That correction factor is related to the ratio of the square root of the ratio of the densities of the fluids at the two different conditions.

http://www.mathesongas.com/pdfs/flowchart/Rotameter-Pressure-Correction-Factors.pdf

Or, if you want a more technical treatment,

https://web.njit.edu/~barat/ChE496_Spring2011/gas_rotameters.pdf

 

[ryanwolff90]

moltenmetal,

Thanks for linking me to that article. I love seeing the theory behind the charts. So basically what we are saying is that rotameters are calibrated for air at a specific pressure. If we use it at a different pressure, we need to adjust for that. This will still leave the measurement in actual cubic feet per minute, hours, or whatever. We will still need to use the equation i provided to bring it to STP conditions?

Ryan W.

 

[moltenmetal]

If you go through that chem eng lab paper, you'll find the equation that relates the mass flows of different gases of different densities giving the same rotameter reading (i.e. air at two different temperature/pressure combinations), which is proportional to the square root of the density ratio of the two gases. Once you know the mass flow the meter is actually measuring, it's easy to convert back to volumetric flow units using PV=nRT at any set of conditions you wish. SCFM is just the mass flow, reported as a volumetric flow at a particular set of "standard" conditions.