Thanks for your reply.
I was in the lab and discovered what you said. The main reason for my surprising observation is a discrepancy between the flow rate measured by the GC and the flow rate measured by the bubble flow meter (I
do not have an external digital flowmeter, not this project on a tight budget, so just using a simple glass bubble flow meter).
The detector PPC flow modules are automatic pneumatics, however, the carrier gas flow module (carrier to packed column) is manual pneumatics. I disconnected my column from the injection sampling valve and attached my bubble flow meter hose. Then measured the actual flow rate measured by meter and flow rate shown on GC display. There was a large discrepancy.
The main reason is:
1- I selected ArCH4 from the drop-down menu rather than Helium. I am using pure Argon only. ArCH4 and Ar are obviously different size molecules so will give different flow rate/pressure differences on the PPC module. I know that for carrier gas. For the detector reference gas, it must be the same reason because I am using Argon for that as well.
2- The options on my GC are only He, H2, N2, ArCH4. I am using CH4 for my experiments but I don't want to use it as a carrier or detector reference. It just goes into the reactor.
I think the solution is to try and use Helium if possible for detector reference. Share with someone nearby. Or I can stick with ArCH4 and keep using the calibration curve to check the right flow rate. I will keep using the calibration curve. Helium cylinders are expensive and there is a shortage of helium sadly.
Which brings us back to rule number 1 which you mentioned: never trust GC pneumatics on their own, always use an external device, in my case, it is my lovely bubble flow meter.
As you said, pressure setpoints on the GC simply act as a regulator guage, just show the outlet pressure like a regulator. Thank God for that!, my cheap single stage argon regulator only shows outlet flowrate in L/min and cylinder contents in bar/psi.
Thank you for sharing your experience, greatly appreciated!
All the best!