Rotameter Correction Factors

[jamf]

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:

Qs = Qi * sqrt((Ts*Pa)/(Ta*Ps))​

Where Qs = true SCFM

Qi = 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/Rotameter-Pressure-Correction-Factors.pdf,

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

Cole-Parmer:

http://www.coleparmer.com/TechLibraryArticle/813

King Instrument (Gas Correction Factor on the bottom right):

http://www.kinginstrumentco.com/correctionformulae/correctionformulae.html

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?

 

[EmmanuelTop]

The square root correction factors are used when the instrument was calibrated by using other gas than the actual process gas. Interchanging between various fluids is shown here (including Excel spreadsheets):

http://www.tokyokeiso.co.jp/english/technology/tech_flow/f-0007/index.html

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