jamf
Mechanical
- Apr 16, 2015
- 1
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: (3rd page)
Matheson correction tables are based on the square root version: Cole-Parmer: King Instrument (Gas Correction Factor on the bottom right):
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.
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: (3rd page)
Matheson correction tables are based on the square root version: Cole-Parmer: King Instrument (Gas Correction Factor on the bottom right):
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.
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?