Dwyer Rotameter validation

[flukeynub]

I'm looking for advice on validating the flow of non-process N2 through two parallel Dwyer VFA-27-SSV rotameters. The supply to the rotameters has pressure regulation set to 80 psi. The N2 is used for post vacuum pump dilution.

Currently, each rotameter reads 20 lpm (40 lpm total) and then T's into an electronic laminar flow meter (calibrated for N2) that reads 90 lpm.

I know there is a slight error using an air calibrated rotameter for N2, but it shouldn't be a factor of 2 difference.

 

[KiwiMace]

Check for standard flow vs actual flow readings between your rotameters and the electronic meter. A substantial pressure difference or pressure compensation in the electronic flowmeter could be the cause for this kind of error.

 

[flukeynub]

Ah yes, KiwiMace you are correct. The rotameters are ACFM and the laminar flow meter is SCFM. Shouldn't I be able to use ideal gas law to correlate a relationship between SCFM and ACFM (at our operating conditions) so that I do not even need to use an expensive electronic flow meter in every situation?

 

[KiwiMace]

Like this - in english units but you get the idea - basic ratios to the standard conditions in absolute units.

SCFH = ACFH x (14.7+ psig)/14.7 x (460 + stdtemp)/(degF+460)

ACFH = SCFH x 14.7/(14.7+psig) x (degF+460)/(460+stdtemp)

 

[flukeynub]

Sweet, that's what I'm using now except in SI. Thank you!