Gas mixing is a science that can present new challenges, even if you think you’ve seen everything. Having the right equipment alleviates many problems, since the end product can suffer if the mixing setup is subpar. It helps to understand the role of each component in a mixing system and how it can improve your process. Starting at the beginning of the process, here are some of the challenges that may arise, and suggestions to help make your gas mixing setup better.
Many processors mix gases on site, since buying a custom mixture from a gas supplier can be expensive. 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. Also, when a custom mixture is purchased it typically is delivered in a special canister that cannot be discarded, or even returned to the gas supplier. Where are you going to put all of those empty bottles? Stratification of the gases will occur if a bottle is left unused for a while, so that the denser gases will settle on the bottom and less dense gases will settle on top. Using a pure gas supply will mean not having to roll your gas bottle across the floor of your lab before you can use it.
Having an Alicat mass flow 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.
Controlling the Mix
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. Alicat mass flow controllers are calibrated for 98 gases (128 gases for the anti-corrosion S series) that can be changed on the fly. A 200:1 turndown ratio means you get more usable flow range per controller. These features will reduce the number of flow controllers required for a setup.
After the gas flow has been controlled, the gases in the stream needs to be mixed properly. If you can’t afford to lay out a bunch of right angles in your piping, a static mixing tube is recommended. Right angles and mixing tubes induce turbulence to create a homogenous mixture, and can be installed downstream of the flow controllers.
Many mixing systems have an analyzer that will provide real time updates on the mixture. If the mixture is not correct, the analyzer triggers a light or horn alarm so the technician is alerted that the mix is not within specification. Analyzers are viable options if you know which gases you are flowing, since they are typically calibrated for one specific gas. Otherwise, a mass spectrometer can determine everything in the gas sample—if you can afford it.
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, since the shield gas is protecting the weld from atmospheric gases. Pressure loss occurs because gas mixing systems aren’t on 100% of the time, and because multiple stations coming on or offline change demand. A pressure feedback system, incorporating a pressure controller, a pressure gage, or 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.
A surge tank or stabilization volume can help mitigate pressure loss. They act as a gas reservoir until you need the mixed gas. The issue of pressure control also goes back to sizing your flow controllers properly, since you want to guarantee that they can handle the maximum and minimum flow rates required to operate in all likely conditions.
While having an adequate source, precise mass flow control and good pressure sensing will provide a system that delivers gas accurately and fast, you probably will need something to control all of the elements together. In most cases, a software programmer at the mixing site will write something up to integrate everything together. Alicat has a couple of options that can make this task easier. For one, our gas mixing software Flow Vision MX, can be used with up to ten Alicat mass flow devices to control a mixing process. If the set-up incorporates other devices that aren’t compatible with Flow Vision MX, we have LabVIEW drivers on our website that can integrate Alicat devices into a LabVIEW interface to control the mixing system.
However you put your gas mixing system together, Alicat’s applications engineers are ready to help you get extraordinarily good results. Contact an Application Engineer using our online chat system, call in, send us a question on a web form, or email firstname.lastname@example.org.