Measuring of airflow using flowmeters
Measuring of airflow using flowmeters
(OP)
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.
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.





RE: Measuring of airflow using flowmeters
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
RE: Measuring of airflow using flowmeters
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.
RE: Measuring of airflow using flowmeters
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
RE: Measuring of airflow using flowmeters
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Measuring of airflow using flowmeters
RE: Measuring of airflow using flowmeters
RE: Measuring of airflow using flowmeters
The meter we use is the 7520 & 7530 Series - Acrylic Tube Flowmeter series (http://www.kinginstrumentco.com/7520_7530/7520_753...). 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?
RE: Measuring of airflow using flowmeters
http://www.mathesongas.com/pdfs/flowchart/Rotamete...
Or, if you want a more technical treatment,
https://web.njit.edu/~barat/ChE496_Spring2011/gas_...
RE: Measuring of airflow using flowmeters
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.
RE: Measuring of airflow using flowmeters