Running a 90/10 N2/He Mix
For running a 90% nitrogen, 10% helium mixture, we recommend you use Terra Part #2702-00, with a 2-20 SCFH N2 flowmeter and a 5-50 SCFH N2 flowmeter. Terra does not supply flowmeters calibrated for helium. Rather, we supply a correction factor to adjust for the differences in specific gravity. To properly read helium in an N2 flowmeter, the N2 reading is multiplied by 2.65. For example, with the N2 flowmeter set at 2 SCFH, the volumetric flow rate of helium would be 5.3 SCFH. To create a 90/10 N2/He mix, connect your helium line to the 2-20 SCFH N2 flowmeter. Connect the nitrogen line to the 5-50 SCFH N2 flowmeter. Set the 2-20 SCFH N2 flowmeter to 2 SCFH. This corresponds to 5.3 SCFH of helium. Set the 5-50 SCFH N2 flowmeter to 45. This will give a proper 90/10 mixture. The error on the 10% mixture would be ± 1%. If more accuracy is required, call a Terra Sales Associate and ask about a custom trace gas mixer.
The correction factor, K, is used to find the difference in reading a flowmeter using a different gas than the one it was calibrated for. The equation below shows the relationship between two volumetric flow rates and K.
Where Q1 is the actual volumetric flow rate of the gas you’re measuring and Q2 is the volumetric flow rate that you read from the flowmeter. K is defined by the following equation.
SG1 represents the specific gravity of the gas you’re measuring and SG2 is the specific gravity of the gas the flowmeter is calibrated to measure. These two simple equations can be used to correct for any difference between the flowmeter’s calibration gas and the gas you’re measuring.