Why do I get different air flow rates in this system?
January 11, 2024 11:57 PM   Subscribe

Can you help me understand a system with a pump, a gas analyzer, and three different flow meters reading giving three different figures for flow rate?

I work at a research station and have recently started working with a system that analyzes air samples and don't quite understand what's going on with the pump and analyzer and flow rate.

We have a sample line in that goes through a flow meter (rotameter) then to a diaphragm pump, then a second flow meter (digital), then through an analyzer (Li7200), then finally through a final flow meter (rotameter again).

The flow rate through the analyzer should be between 1-5 L/min, and we measure 2 L/min at the digital flow meter immediately before the analyzer. We also measure 7 L/min at the flow meter before the pump, and 4 L/min after the analyzer.

I guess the first question is - does this make sense? We don't know the air pressure at each of these points, so is the explanation just that we have different air pressures, and therefore different flow rates for the same volume of air? (as it is a closed system and we're not adding or removing air at any point). I.e., increased air pressure after the pump, then a drop in air pressure after the analyzer? Is that a reasonable explanation for the figures we're seeing? Or is there something else going on?

Thanks for the help!
posted by twirlypen to Science & Nature (5 answers total)
 
Where is the air coming from before the first flow meter, and where does it go after the last one? I have no experience with the equipment you’re using, and the airflow I’ve worked with is in a very different context, with vastly higher flow rates and velocities, but the numbers you cite seem improbable unless the diaphragm pump is quite powerful, the analyzer is quite restrictive, and the air pressures before and after the processes you’ve described are very different from each other. For the same airflow to be 7 L/min at one point and 2 L/min elsewhere implies a ~3.5x difference in pressure, which is a lot. Can you swap the positions of the flow meters to see whether the readings change?
posted by jon1270 at 2:07 AM on January 12, 2024 [2 favorites]


Have you carefully checked for any leaks using sight, sound, maybe soapy water? Have you checked for obstructions in the analyzer? Does the data coming off the analyzer pass sanity checks? Can you test it with a sample of known properties?

This is also outside my field, but the people I know work with this stuff are constantly wading through the specs and manuals of the devices, sometimes calling the vendors for support. I'm thinking the analyzer in particular would be highly documented with respect to what it does to flow when operating properly, and also come with some level of support.
posted by SaltySalticid at 7:13 AM on January 12, 2024


Without knowing the pressure rotameters are useless, so I'd discount them immediately. What kind of flowmeter is the other one? Is it measuring volumetric flow (like a differential pressure type) or mass flow (like a thermal or coriolis meter)? Are there any calibration records you can look at?

I'd also look at the pump - assume it has speed control? Are you running it at a speed where you'd expect the right amount of flow?
posted by Jobst at 9:31 AM on January 12, 2024 [1 favorite]


I will also add the caveat that this is outside of my field, but a diaphragm pump will, I imagine, create pulses of air rather than smooth, continuous airflow. As such, the erratic flow of air may be throwing the flow meters off, and also there may also be resonant nodes of high pressure, low pressure zones in the pipework.
A different style of pump might be the hot ticket.
posted by Larry David Syndrome at 10:45 AM on January 12, 2024


What sort of calibration comes with the rotameters? If you swapped them, do they both have the same sort of reading? What altitude are you at? Does the analyzer Have enough resistance to generate a back pressure?
posted by nickggully at 2:31 PM on January 12, 2024


« Older A flight and a show   |   Kind Bay Area dentist recommendations? Newer »
This thread is closed to new comments.