Clever Gas Mixer Gets Just The Right Blend For Homebrew Laser Tubes

[Lucas] over at Cranktown City on YouTube has been very busy lately, but despite current appearances, his latest project is not a welder. Rather, he built a very clever gas mixer for filling his homemade CO2 laser tubes, which only looks like a welding machine. (Video, embedded below.)

We’ve been following [Lucas] on his journey to build a laser cutter from scratch — really from scratch, as he built his own laser tube rather than rely on something off-the-shelf. Getting the right mix of gas to fill the tube has been a bit of a pain, though, since he was using a party balloon to collect carbon dioxide, helium, and nitrogen at measuring the diameter of the ballon after each addition to determine the volumetric ratio of each. His attempt at automating the process centers around a so-called AirShim, which is basically a flat inflatable bag made of sturdy material that’s used by contractors to pry, wedge, lift, and shim using air pressure.

[Lucas]’ first idea was to measure the volume of gas in the bag using displacement of water and some photosensors, but that proved both impractical and unnecessary. It turned out to be far easier to sense when the bag is filled with a simple microswitch; each filling yields a fixed volume of gas, making it easy to figure out how much of each gas has been dispensed. An Arduino controls the pump, which is a reclaimed fridge compressor, monitors the limit switch and controls the solenoid valves, and calculates the volume of gas dispensed.

Judging by the video below, the mixer works pretty well, and we’re impressed by its simplicity. We’d never seriously thought about building our own laser tube before, but seeing [Lucas] have at it makes it seem quite approachable. We’re looking forward to watching his laser project come together.

11 thoughts on “Clever Gas Mixer Gets Just The Right Blend For Homebrew Laser Tubes

  1. Very cool idea and device. But seems very much like unnecessary work to change bottle so many times. Why not add a solenoid for each bottle and get the mix done automatically.

    1. I guess that depends on how often you fill a tube.
      Is it worth buying the valves and building more control channels for something you only do occasionally ?
      This is something we geeks tend to often get wrong.

        1. same question, is it worth taking the time to design and build a more complex system that does more than what you need, while what you want to be spending you precious hobby time on isn’t building tools, but building lasers.

      1. This is a flowing gas design. It needs a constant flow of the lasing medium.

        Also, I know this isn’t in the spirit for what is being done here. But one can buy premixed CO2 laser gas in a few ratios that are optimal.

    2. Simplest option is just to get three needlevalves and flowmeters, thats how most people and companies handle their mixing since it is constant flow. It’s not like you can charge the tube and let it run, there is no catalyst to keep the co2, co2.

      Also refrigeration compressors cycle oil through the system so there is going to be a ton of oil vapor in the gas which is a very bad thing.

  2. Macgyver at its finest! If you can’t get the parts then build them, Or as I did invent the technology nearly from scratch using parts from skips, and an oblique reference in an open source paper. Now if I could just fix my 2 stage pump.

  3. It looks like the air-shim bag completely empties after each fill, or does it? When the pressure in the main tank is getting high, wouldn’t there be also be some gas remaining in the air-shim bag (pressure equilibrium between the bag and the yellow tank)? If so, would there be some mixing of gases so any residual gas in the air-shim bag will be a mixture? Will that then also mix in with the gas in the primary bottle when the ‘fill’ valve is opened, so that over time the gas in the bottles will get contaminated by the other gasses? (maybe the air-shim bag is evacuated between each fill/empty cycle?).

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