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

Question

Our laboratory operates at an altitude well in excess of 2000 meters above sea level. 
 While standard pressure at sea level is ~14 PSI. Our pressure is ~11.2 PSI. This is enough of a difference to affect the flow rates of the instruments. 
 
 When a volumetric flowmeter such as the ADM 1000 is used to measure flow, the flow reads approximatley 20% higher than the setpoint. A mass flowmeter reading matches the setpoint. This makes sense according to P1V1=P2V2 (at the same number of moles and temperature (mass flow), volume will change inversely to pressure). 
 
 This presents a problem when setting up FID setpoints or constant flow conditions, since the displayed setpoints are incorrect for the conditions. While we can manually make the adjustments, it makes training and troubleshooting more difficult than it needs to be.

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alfausett · Answer

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

alfausett · Answer

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

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

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