Mixing gases with versatile flow control
Many applications require the use of custom gas mixtures. Pre-mixed gases are often purchased from compressed gas suppliers, however it is also a viable solution to purchase pure gases and blend them yourself. Whether buying gases pre-mixed or mixing your own gas, there are several challenges to overcome. Here we discuss common gas mixing challenges and solutions.
Challenges of gas mixing
Custom gas mixtures purchased from suppliers tend to be expensive and are typically delivered in special canisters that cannot be discarded or returned to the gas supplier. Additionally, stratification of the gases will occur if a bottle is left unused for a while. Having a supply of the pure gases reduces costs, while allowing the user to adjust the concentration of a constituent for a different end result. Many processors therefore mix gases on site.
Dynamic gas mixtures
In a dynamic system, your mixture may not always be 75% Gas 1 + 25% Gas 2. What do you do if your mixture changes to 25% Gas 1 + 75% Gas 2 the next day, and then 50% Gas 2 + 50% Gas 3 the next? Careful consideration of your potential mixes is very important when selecting flow controllers for a mixing setup.
In some real-time mixing applications, continuous pressure is important. For example, in welding, pressure loss in the shield gas can result in a bad weld. Pressure loss occurs because gas mixing systems aren’t on 100% of the time, and because multiple stations coming on or offline change demand.
An adequate source, flow control, and good pressure sensing all ensures a system delivers gas quickly and accurately. Setting up a system to control all the system components simultaneously typically requires a software programmer at the mixing site to write something up and integrate everything together.
Mixing gases with flexible mass flow controllers
Alicat mass flow controllers possess several features that make them an optimal solution for custom gas mixing systems:
- Easily switch between 98+ gases and 20 custom-defined gas mixtures on the fly | A mixer initially set up to blend ethane, methane, and hydrogen may be quickly reconfigured to blend nitrogen, argon, and helium. Having a controller for each gas in your process provides fast and accurate control of your mixture. If you need to measure a mixture, whether pre-mixed or mixed on site, the mixed gas recipe can be programmed into the mass flow controller using the COMPOSER firmware on the Alicat.
- One device can be used to control pressure or mass flow | Using the pressure control loop on an Alicat mass flow controller will responsively increase or decrease flow rates based on pressure readings at each station.
- Wide 10,000:1 controllable range for each gas & mixture | A 10 SLPM mass flow controller will flow accurately down to 1 SCCM. This makes it easier to ensure your device can handle the maximum and minimum flow rates required to operate in all likely conditions. Controllers configured for 1 SLPM full scale of air will remain at 1 SLPM full-scale, whether flowing H2 or SF6.
- Multi-drop RS-232 or RS-485 interface | Read and set all of the mass flow controllers through a common RS-232 or RS-485 serial interface.
- Easy system integration using BB9 multi-drop box and FlowVision™ gas blending software | Simplify system integration and control up to 10 devices at once. LabVIEW drivers are also available to integrate Alicat devices into a LabVIEW interface for control of the mixing system.
Note that the arrangement of components shown in the figure above is typical of gas mixing systems flowing to pressures near or above atmospheric pressure. If the application requires exit pressures in the vacuum range, the controllers need to be configured with the control valves between the measurement head and the vacuum chamber.
It is important to include a turbulent mixing tube in the flow path downstream of the mixed components, as this reduces gas separation and stratification. These low-cost components are available from third-party sources in a variety of materials.