Gas flow controllers for calibrating gas chromatographs

Bronkhorst supported a company specialised in gas mixers for gas analysers with a solution to prepare their own gas mixture to calibrate gas chromatographs. 

Analysing instruments like gas chromatographs must be calibrated regularly. This way they can accurately determine the chemical composition of compounds. During calibration, the measurement values of the gas chromatographs are compared with a calibration standard that has a known accuracy. The values of the device are adjusted accordingly. For gas chromatographs, such a standard is a gas mixture with an accurately known composition. 

For operators of gas chromatographs, there are two ways to generate these gas mixtures: 

1. buying cylinders with pre-mixed gas mixtures
2. prepare their own gas mixture 

Process solution

In this application gas mass flow controller of the EL-FLOW Base series are used. Up to 12 gas cylinders are connected to an equal amount of mass flow controllers. The accurately controlled gas flows enter a mixing chamber after which the mixture is fed to a gas chromatograph for calibration purposes.

The ISO 6145-7:2009 standard describes a procedure to prepare calibration gas mixtures using commercially available thermal mass flow controllers. The accuracy of the final gas composition that leaves the gas mixers is calculated by dedicated software that considers the uncertainties in the gas flows - due to the used mass flow controllers - as well as uncertainties in the gas composition of the initial gas cylinders.