Can you switch a 7890A from displaying standard flow rates to actual flow rates?

Hi dprobst, 

The 7890A GC has atmospheric pressure correction built into the electronics.  I think it is also worth mentioning that the system isn't controlling flow through the FID, it is controlling pressure across a fixed restriction and calculating flow based on the inputs.  

You can view the atmospheric pressure sensor by following this sequence using the GC keypad:

Service Mode/Diagnostics/Electronics/Pneumatics - Atmospheric Pressure sensor.   

I would suggest checking the atmospheric pressure sensor on the GC and make sure the reading makes sense for your location.  The system is rated for altitudes up to ~4600m, so you should be within the operable range. 

Are the readings off on all gases measured through the FID?  Air, Hydrogen, Makeup and Carrier?     

Best,

Abbey

Yes, it is off on all gasses,  always by roughly 20% (occasionally nearly 30% or as low as 15%, depending on the instrument).   Air, Hydrogen, Makeup (N2), carrier (H2 or He) on every instrument (12 GC and 3 GC/MS, which haven't been evaluated).  It's also verified on makeup and carrier gasses on ECD's as well.  I didn't bother checking other detectors, figuring at that point it was universal.  The scale of the flow deviation and the deviation from sea level at this altitude are suspiciously similar.

I have checked the ambient pressure on all the instruments.  They are all 11.2 +- 0.1 PSI.  

I understand the the control is pressure-based and the flow calculations are based on that.  The mass flow readings coincide with the setpoints, when adjusted for standard conditions.  However, past that constriction the atmospheric pressure is only 80% of that at sea level, resulting in an approximately 20% expansion of the volume, resulting in a higher actual flow.  In the above photo, with a flow set at 37.5 mL/min on the panel, you can see that the SCCM is correct (calculated from mass flow), but the *volumetric* flow is 28% high.

I  concluded that, even though the ambient pressure may be used to calculate the neccessary constriction to achieve the proper flow at standard conditions,  the instrument displays it at standard conditions, not ambient conditions.  It would be more useful for us if it were to display actual CCM.  It's probably never been brought up, because 99+% of your instruments are likely used within 1000 feet of sea level and there's no appreciable difference between SCCM and CCM.

To give an example, if we set the FID flows to the recommended 30 mL/min fuel, 400ml/min air, and 30 mL/min makeup (460 mL/min), we would ACTUALLY see 36 mL/min fuel, 480 mL/min air, and 36 mL/min makup (552 mL/min).  This makes the flame harder to ignite, and more challenging to keep lit, particularly with megabore columns at high flows.

Hi dprobst, 

Thanks for the detailed response!  I got the chance to dig into some math that I hadn't visited in a while!

To answer your initial question, it is not possible to display actual flows on the FID gas parameters because the module does not contain flow readers.  These detector flow outputs are calculated to be in mass flow, as they are related to a specific gas.  

You are mostly correct in your deduction.  The atmospheric P sensor is used to correct the mass flow, but at different altitudes, the volumetric flow will be different because the outlet pressure varies.   Volumetric flow through a tube is dependent on initial and exit pressures, with gas viscosity, temperature and restriction parameters remaining constant.  Having a greater pressure differential than typical of STP will result in a higher volumetric flow, which we have seen here.  The method set points/displays are kept consistent and the adjustments are handled in the background processors to maintain consistency in results should a weather system pass through in the middle of a sequence.   

Do you have any flexibility to adjust your flow setpoints?  If you decrease the flow setpoints by 20%, do your FIDs light easier?  I would also recommend decreasing your makeup flow if you are running higher column flows.  I target ~20-30mL/min combined makeup and column flow, but that setting will depend on your local guidance and system requirements.  

For what it is worth, I maintained several GC labs at an elevation of ~5400'.  We used primarily FIDs, FPDs and MSDs, and the only adjustments I made with ignition issues were to reduce the makeup gas and bump up the hydrogen to make our flames a little richer.  Once the detectors lit and stabilized, I would return the flow to method settings and all ran well.    

Excellent discussion.  Thanks for taking the time to share your observations with the Community!  

Abbey

Hi dprobst, 

Thanks for the detailed response!  I got the chance to dig into some math that I hadn't visited in a while!

To answer your initial question, it is not possible to display actual flows on the FID gas parameters because the module does not contain flow readers.  These detector flow outputs are calculated to be in mass flow, as they are related to a specific gas.  

You are mostly correct in your deduction.  The atmospheric P sensor is used to correct the mass flow, but at different altitudes, the volumetric flow will be different because the outlet pressure varies.   Volumetric flow through a tube is dependent on initial and exit pressures, with gas viscosity, temperature and restriction parameters remaining constant.  Having a greater pressure differential than typical of STP will result in a higher volumetric flow, which we have seen here.  The method set points/displays are kept consistent and the adjustments are handled in the background processors to maintain consistency in results should a weather system pass through in the middle of a sequence.   

Do you have any flexibility to adjust your flow setpoints?  If you decrease the flow setpoints by 20%, do your FIDs light easier?  I would also recommend decreasing your makeup flow if you are running higher column flows.  I target ~20-30mL/min combined makeup and column flow, but that setting will depend on your local guidance and system requirements.  

For what it is worth, I maintained several GC labs at an elevation of ~5400'.  We used primarily FIDs, FPDs and MSDs, and the only adjustments I made with ignition issues were to reduce the makeup gas and bump up the hydrogen to make our flames a little richer.  Once the detectors lit and stabilized, I would return the flow to method settings and all ran well.    

Excellent discussion.  Thanks for taking the time to share your observations with the Community!  

Abbey

This was the answer I was expecting (never hurts to hope, though).  Thank you for explaining everything.  My fluid dynamics isn't rusty so much as made of wet packing paper.  I've adjusted everything down so the ACTUAL flows sum to the recommended 460 mL/min.  I still needed to tweak the ignitor coils out a tiny bit to get consistent ignition, but they are staying lit... even with a couple of aqueous injections, so I'm happy.  I will blatantly plagiarize your answer to use in training, because it makes a lot more sense.