The preparation of accurate calibration gas standards depends on the precise regulation of gas flows by devices known as mass flow controllers (MFCs). Ensuring MFCs are performing accurately requires careful evaluation of each process mass flow’s gas conversion factors (known as K-factors or GCF). Here, we take a look at why gas conversion factors are important, and explain how to calculate GCF for your process gasses.
The Importance of Calibration Gas Standards
Calibration gas standards are carefully prepared gas mixtures in which the concentrations of certain target gasses are precisely known. Virtually all equipment that measures gas concentration must be routinely calibrated using calibration gas standards in order to ensure accurate operation: this includes mass spectrometers, gas chromatographs, gas and chemical sensors, auto emissions analyzers and handheld detectors.
Calibration gas standards are prepared using mass flow controllers (MFCs): devices capable of measuring the mass flow rate of fluids with extremely high accuracy and automatically compensating gas flow using a valve. By measuring the mass flow rate of gasses (as opposed to the volume flow rate), MFCs controllers provide an accurate measure of the amount of gas flowing through a tube that is independent of fluctuations in temperature and pressure.1
Mass flow controllers typically use thermal techniques to quantify flow rate: measuring the temperature differential as gas flows into and out of a small tube. However, gasses have unique specific heat capacities, meaning that the amount of energy associated with a given change in temperature is different for a given gas.2 This in turn means that mass flow controllers must themselves be calibrated in order to accurately measure the mass flow rate of a given process gas.
Calibrating Mass Flow Controllers Using Mass Flow Gas Conversion Factors
When calibrating an MFC, it’s best to use the process gas that is to be analyzed. However, this isn’t always possible. When the process gas can’t be used for calibration, a substitute gas can be used – but this entails the use of a mass flow conversion factor, denoted K, in order to ensure that the flow through the MFC is accurate with respect to the process gas.
Essentially, the gas conversion factor expresses the ratio of flow rates of different gases required to produce a given output voltage from an MFC. All GCF are calculated relative to nitrogen, which by convention has a mass flow conversion factor equal to one.
The accuracy of any MFC depends on the proper evaluation of mass flow conversion factors for each process gas that will be used with the blender. Inaccurate mass flow conversion factors mean inaccurate MFCs, which means inaccurate gas mixing, which means inaccurate reference standards.
Calculating Mass Flow Conversion Factors for Your Process Gases
Ideally, mass flow conversion factors should be calculated empirically – though companies often use a combination of empirical and theoretical calculation of mass flow conversion factors depending on the process gas.
Mass flow conversion factor for a given gas is defined as the ratio of the actual gas flow rate to the equivalent nitrogen flow rate.
So, to calculate the mass flow conversion factor for a given gas, simply divide the gas flow rate of that gas by the gas flow rate of nitrogen that produces an identical output voltage from the MFC. In cases where the mass flow controller is calibrated in an alternate gas, the ration used must be the ratio of the actual gas being used to the calibration gas.
Ensuring mass flow conversion factors have been accurately obtained for all process gases is a vital part of developing precision gas mixing systems for production of calibration gas standards.
At Environics, we’re experts in gas mixing. Our technology is based on extremely precise control of thermal mass flow controllers, offering superior gas mixing to meet and exceed even the most exacting requirements. To find out more, get in touch with us today.
References and Further Reading
- Bramford, T. Using Mass Flow Controller-Based Dynamic Gas Blenders to Produce Accurate Calibration Gas Standards. https://www.environics.com/wp-content/uploads/2020/05/bamford-2007.pdf.
- Kim, S. J. & Jang, S. P. Experimental and numerical analysis of heat transfer phenomena in a sensor tube of a mass ¯ow controller. International Journal of Heat and Mass Transfer 14 (2001).