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  1. #1
    Join Date
    Feb 2011
    Posts
    5

    Question Shielding gas mixing

    Hi

    I bought a Lincoln 175 pro sq wave unit a couple of years ago and have enjoyed shade tree tig and stick welding. I just bought a 350xmt CV/CC with a S64 feeder and a Bernard Q300 gun off eBay so I'm going to start exploring MIG and higher power stick welding.

    I have a large Argon bottles with a ball/flowmeter. Can I just buy a CO2 bottle and a CO2 flowmeter then T the two flowmeter outputs into one output to my XMT? Could I then custom adjust my mix ratio at will?

    Thank you

    Doug

  2. #2
    Join Date
    Oct 2004
    Location
    Edmonton, Alberta
    Posts
    7,597

    Default

    Not a good plan without a gas mix calibrator as the gasses are at somewhat different pressures. Interesting thinking but your idea simply won't work.

  3. #3
    Join Date
    Feb 2011
    Posts
    5

    Default

    Thanks Cruiser I suspected as much I guess I will find what my lws has that is close to 82/18.

    Doug

  4. #4
    Join Date
    Feb 2009
    Location
    Maryland
    Posts
    707

    Default

    Even if you could mix your own gas - how would you know what mix you have? Accurate gas analyzers are expensive. Hospital type only measure like 4% CO2, you need one that goes beyond.

    Automotive analyzers typically go to about 20% CO2. My OTC gas analyzer (pawn shop purchase) is 20% max, cost a few $1000 new, and is not guaranteed to be accurate on a argon/co2 mix. I can say my analyzer goes off the scale on C25 (as expected) and when I have it where I expect to see about 10% it infact does show 10%. It thinks the argon is O2 though. So it thinks its sniffing pure oxygen + 10% CO2.

    Just an FYI, the Smith mixers - list about $1500 are relatively cheap ones (trying to say that without choking) and are essentially a pressure equalizer (very large diaphram) and a mixer. The only real calibration on the units is the markings on the dials. Argon and CO2 have approximately the same mass so they should mix reasonably well. BTW, its a unique looking diaphram. Basically both gasses reference off each other with a fine tuning screw.

    I bought one off ebay and calibrated it with a very sensitive pressure differential meter and got the pressure differential to about .01psi between the two gases. Next I used the automotive gas analyzer (calibrated to a known mix of gas) to set the CO2 to 10%. I haven't moved it off the mark since. My excuse was the crazy price my LWS was charging for 90/10 gas in 80 cu bottles. Of course, I have since found an LWS that is cheaper - like $50.

    Bottom line, its expensive to do it right, and its expensive to even know if you did it right. Ebay helps a lot. Still hard to justify.

    BTW, I think its Praxair gases that are available in smaller bottles in all kinds of mixes. They have names like StarGold C-20 (20% CO2).
    Con Fuse!
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  5. #5
    Join Date
    Jul 2008
    Location
    Delhi, Ontario:
    Posts
    1,963

    Thumbs up C20

    I've been running C20 for a few months & I love it.
    In fact I get it for the same price as C25 !

    ....... Norm

    Sunrise Outside My Shop In Delhi, Ontario

    - Arcair- K 4000 CAC.

    - LN-25 Wire Feeder

    - Lincoln Ranger 8- Engine Drive- CC\CV:



    - Lincoln Power Mig 180C
    - Spoolgun.
    - DeWalt Chop Saw .
    - DeWalt Compressor - 13cfm, @ 100 psi.

    www.normsmobilewelding.blogspot.com

  6. #6
    Join Date
    Aug 2012
    Posts
    3

    Default

    Very interesting topic. I worked in one of the largest welding R&D labs in the early days of MIG welding. One of the engineers told me his worked showed that 22 1/2 CO2 in argon was the best gas mix for short arc MIG welding but they did just as you suggest, they used two cylinders with regulator/flowmeters to get mixed gas and the calculation was easier to set the flowmeters with 25% CO2!
    A comment on flows and pressures. Quality flowmeters operate at a minimum of 25 psi, some 50 psi to achieve "choked flow" which sets the flow by the velocity in the needle valve equal to the speed of sound- it can't flow faster. Therefore the pressure downstream of the needle valve will be whatever it takes to achieve what is read on the flow tube!
    Suggest you look up choked flow or compensated flow to see what that is all about.

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