×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Rotameter Correction Factors

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:
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/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

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/tec...

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources