Rotameter Correction Factors
Rotameter Correction Factors
(OP)
I have a rotameter measurement from a compressed air system. The rotameter is calibrated for air at STP, so I'm trying to get a real SCFM value by correcting for the compressed air pressure and temperature. In looking for correction equations I've found 3 different methods so far that can result in significantly different conversions.
The instrument manufacturers that I've seen all use the form with the temperature and pressure ratios under a square root:
Omega: http://www.omega.com/green/pdf/techref_sect_b.pdf (3rd page)
Matheson correction tables are based on the square root version: http://www.mathesongas.com/pdfs/flowchart/Rotamete..., http://www.mathesongas.com/pdfs/flowchart/Rotamete...
Cole-Parmer: http://www.coleparmer.com/TechLibraryArticle/813
King Instrument (Gas Correction Factor on the bottom right): http://www.kinginstrumentco.com/correctionformulae...
ASTM D3195 (2004) "Standard Practice for Rotameter Calibration" uses a different conversion factor:
Lastly, the discussion here seems to indicate that you only need to use the ideal gas law to convert to actual SCFM, if I'm reading it correctly.
http://www.eng-tips.com/viewthread.cfm?qid=137523
Anybody have any insight?
The instrument manufacturers that I've seen all use the form with the temperature and pressure ratios under a square root:
Qs = Qi * sqrt((Ts*Pa)/(Ta*Ps))
Where Qs = true SCFMQi = indicated flowrate ('SCFM')
Ts = calibration temp (R)
Ta = actual temp (R)
Pa = actual pressure (psia)
Ps = calibration pressure (psia)
Omega: http://www.omega.com/green/pdf/techref_sect_b.pdf (3rd page)
Matheson correction tables are based on the square root version: http://www.mathesongas.com/pdfs/flowchart/Rotamete..., http://www.mathesongas.com/pdfs/flowchart/Rotamete...
Cole-Parmer: http://www.coleparmer.com/TechLibraryArticle/813
King Instrument (Gas Correction Factor on the bottom right): http://www.kinginstrumentco.com/correctionformulae...
ASTM D3195 (2004) "Standard Practice for Rotameter Calibration" uses a different conversion factor:
Qs = Qi * Ts*Pa/(Ta*Ps)*sqrt(Ts/Ta)
Lastly, the discussion here seems to indicate that you only need to use the ideal gas law to convert to actual SCFM, if I'm reading it correctly.
http://www.eng-tips.com/viewthread.cfm?qid=137523
Qs = Qi*(Ts*Pa)/(Ta*Ps)
I think that the 3rd method assumes the rotameter's reading is a true ACFM, which I don't think is the case? Anybody have any insight?





RE: Rotameter Correction Factors
For converting actual CFM of air to SCFM of air you use Qs = Qa x (Pa/Ps) x (530/(Ta + 460)), also see the Omega page http://www.omega.com/green/pdf/rotameter_faq.pdf
Dejan IVANOVIC
Process Engineer, MSChE