Conversion from CO2 to Air flowrates

redargon

I'm not sure under which section this should go, but the application is engineering. I have a calibration problem that I'm trying to figure out:

I have a digital flowmeter that measures the flowrate of air and gives a SL/min (litres/minute at stansard conditions) value. The thing is I want to measure the flowrate of CO2 with that same device (it takes too long to acquire one that is set up for CO2). Is there a conversion factor I can use to convert SL/min (air) into SL/min (CO2)?

I tried ratios of densities and also molar masses, but I don't get a result that looks reasonable.

Any ideas?

Thanks

russ_watters

A density ratio really should work: CO2 is denser than air, so it should give readings higher than actual.

Q_Goest

Should be:

F₂ = F₁ * (S₁/S₂)^.5

Where:

F₁ = Flowrate measured by device
F₂ = Actual flow rate
S₁ = Specific gravity flow meter is calibrated for
S₂ = Specific gravity of gas going through meter

For example. You measure 100 SL/min on your meter which is calibrated for air. Specific gravity for air is 1. Specific gravity of CO2 is 1.57. Actual flow is 80.9 SL/min.

Edit for clarification:
The flow meter is calibrated for air at a specific temperature and pressure. The above assumes it's calibrated for standard conditions.

Also, the above could also be rewritten using densities for the gasses instead of specific gravity. Just replace density for S₁ and S₂ since specific gravity is nothing more than the ratio of density to air at standard conditions.

redargon

Ok, that's what I got too. I knew I was on the right path. The experimental data I have so far is telling me (comparing the digital air flow meter to a float guage CO2 flow meter) that the conversion factor is closer to 1.2, but I attribute this to the temperature and pressure. The CO2 is from a small cartridge and cools considerably as it expands, also th pressure to drive the flow is around 1 barg. Ok, I think I can find something, or at least get a close enough estimate. I should probably consider compressibility factors.

Thanks, as always, for the quick and thorough replies.

hellomoto

Is there a mistake in the calculation?

F2 = 100 * (1/1.57)^.5 = 79.80

so F2 does not equal 80.9?