Calculate GC capillary column flow or required inlet pressure from column dimensions, carrier gas, oven temperature, and atmospheric, vacuum, or custom outlet.
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Agilent Flow Rate Formula
This ideal, isothermal capillary-GC model estimates reference carrier-gas flow at 25ยฐC and 1 atm from absolute inlet and outlet pressures:
Variables:
- Qแตฃ is reference flow in mยณ/s; multiply by 60,000,000 for mL/min. Tแตฃ = 298.15 K and Pแตฃ = 101325 Pa.
- d is the open column diameter in meters, after subtracting twice the stationary-phase film thickness. L is column length in meters.
- Pแตข and Pโ are absolute pressures in Pa; ฮผ is gas viscosity in Paยทs and T is uniform column temperature in K.
Use the pressure-squared difference and effective open diameter in the formula. The reverse mode solves for inlet pressure. Gauge pressures are relative to 1 atm; vacuum uses the analytical zero-outlet-pressure limit.
What is Reference Flow in GC?
In gas chromatography (GC), volumetric flow depends on pressure and temperature. This calculator normalizes flow to 25ยฐC and 1 atm, so its reference flow is not the local volume flow at a hot or vacuum outlet. It assumes steady laminar ideal-gas flow in a uniform circular bore. The Blumberg gas-viscosity correlation is used only within its 250โ750 K fitted range. This independent estimate is not an Agilent instrument calibration or a safety limit.
How to Calculate Flow Rate?
Use the following steps for a capillary-GC estimate; confirm the instrument, gas and column operating limits separately:
- Select reference flow or required inlet pressure, then select helium, hydrogen or nitrogen.
- Enter column temperature, length and inner diameter. Enter film thickness if a film correction is needed; blank means zero.
- Select atmospheric, vacuum or custom absolute outlet pressure, then enter inlet gauge pressure or target reference flow.
- Select Calculate. Review the reference flow or inlet pressure, average linear velocity and hold-up estimate; expandable details show assumptions.
Example Problem:
Use the following variables as an example problem to test your knowledge.
For helium at 0ยฐC, column length 30 m, open diameter 0.25 mm and no film correction, use atmospheric outlet pressure.
At 1 atm inlet gauge pressure (2 atm absolute), reference flow is approximately 1.7075 mL/min. The viscosity and flow model are described in GasChromatographySimulator.jl documentation.
