Welding CO2: Find Cylinders, Exchanges, and Refills That Actually Work
If you’re MIG welding mild steel, CO2 is the cheapest shielding gas path and it’s everywhere. This page helps you pick a sane cylinder setup, avoid fitting confusion, and find suppliers who do CO2 exchange or refill without wasting a trip.
Quick start
The easiest setup
- Start with exchange if you want fast, simple, and predictable.
- Tank size: 20 lb is the sweet spot for most hobby + light shop use. 5–10 lb is portable. 50 lb is “shop life.”
- Regulator: CO2 regulator with the correct inlet fitting. If you already own one, confirm it matches your tank connection.
- Flow: Most MIG setups live in a typical shielding gas flow range, but wind, nozzle condition, and leaks matter more than obsessing over one number.
- Reality: pure CO2 can be spattery compared to argon/CO2 mixes, but it’s cheap and common.
What CO2 does in welding (and when it’s the right choice)
Why people use 100% CO2
Pure CO2 is a common MIG shielding gas for mild steel because it’s widely available and typically the cheapest option. The tradeoff is often more spatter and a harsher arc compared to argon/CO2 mixes.
- Good “value gas” for steel MIG
- Easy to find at welding suppliers and industrial gas shops
- Expect more cleanup vs mixed gas in many setups
When you should consider mixed gas instead
If you’re chasing smoother arc behavior and less spatter, a common answer is an argon/CO2 mix (often referred to as “75/25”). Your supplier can tell you what they carry and what cylinder sizes are available.
Industrial suppliers vs “random places”
For welding, the best leads are usually welding supply shops and industrial gas suppliers because they already handle cylinders, exchanges, and certification rules daily.
Gear & fittings (the part that trips people up)
Core gear
- CO2 cylinder: 5, 10, 20, and 50 lb are common.
- CO2 regulator: stable output, not mystery-metal.
- Gas hose: rated for the pressure and the job.
- Flow device: regulator flowmeter or separate flowmeter (depends on your setup).
- Spare consumables: nozzle, tip, diffuser, liner basics (because clogs are inevitable).
Fittings reality check
Cylinder connections are standardized, and CO2 commonly uses the CGA-320 connection in many contexts. The important part is: match your regulator inlet to the cylinder valve. If you’re not sure, ask the supplier what connection their CO2 cylinders use.
Lease vs own (quick translation)
- Exchange: you swap your cylinder for a full one (fastest).
- Customer-owned refill: they fill your exact cylinder (may require it to be in-date).
- Lease: you pay to use the supplier’s cylinder long-term (common in industrial shops).
Dial-in (get stable shielding, stop fighting the arc)
A practical dial-in checklist
- Check for leaks at the cylinder valve, regulator, and hose connections (bubbles beat guessing).
- Keep the nozzle clean so gas can actually flow around the weld puddle.
- Set your gas flow to a sane starting point, then adjust if you see porosity or turbulence.
- Shield from wind if you’re outside. Wind will clown you every time.
- Confirm polarity and wire for your process (wrong basics = endless suffering).
If you keep getting porosity
- Confirm you actually have shielding gas flowing (sounds obvious, but… humans).
- Check nozzle for spatter blockage and replace consumables if needed.
- Check for drafts and shorten stickout if you’re way out in the wind.
- Look for leaks. Tiny leaks cause big headaches.
Troubleshooting (symptom → likely cause → fix)
| Symptom | Likely cause | Fix |
|---|---|---|
| Porosity / pinholes | Low/blocked shielding gas, drafts, leaks | Check flow, clean nozzle, leak test fittings, shield from wind |
| Excessive spatter | Settings out of range, dirty consumables, CO2 character | Adjust voltage/wire speed, replace tips/nozzle, consider mixed gas if aesthetics matter |
| Arc feels harsh / unstable | Wrong parameters, liner/tip issues, gas turbulence | Check consumables, confirm flow, re-check machine settings |
| Gas bottle empties “too fast” | Leak | Leak test cylinder valve + regulator + hose connections |
| Regulator won’t hold steady | Regulator wear/failure | Repair/replace regulator (don’t trust the $19 special) |
| Supplier won’t refill your cylinder | Policy: exchange-only or customer-owned restrictions | Ask for exchange options or ask what’s required for customer-owned refill |
Refill vs exchange for welding (what’s best?)
Exchange: fastest, most common
You hand them your empty cylinder, they hand you a full one. For most welders, this is the “stop thinking and keep working” option.
- Usually the simplest walk-in process
- Great for standard sizes
- No waiting for your specific cylinder to be filled
Refill: good when you have a customer-owned cylinder
Refilling means they fill your exact cylinder. This can be cheaper or preferable if you’re attached to your cylinder, but many shops have policies around customer-owned refills.
- May require cylinder certification/hydro to be current
- May be slower depending on the shop
- Varies wildly by region and supplier type
Lease: common in industrial shops
If you’re buying gas frequently, some suppliers will push lease programs. Leasing can be convenient, but it’s not required for most people. Ask for all options (exchange, refill, lease) and pick what fits your actual usage.
Call script (so you get a real answer in 30 seconds)
Don’t ask “Do you do CO2?” That gets you the human equivalent of a shrug. Ask like this:
- If they say “exchange only,” that’s still a win. Ask what sizes they stock.
- If you actually want mixed gas, ask: “Do you carry 75/25 (argon/CO2) as well?”
- Ask for walk-in hours and whether you need to bring the cylinder in the vehicle or can drop it at the counter.
Safety notes (short, serious, and actually relevant)
- Secure cylinders upright. Don’t let a tank tip and smash a valve.
- Ventilation matters. CO2 can displace oxygen in enclosed spaces.
- Don’t bake cylinders in heat. Keep them out of direct sun and hot vehicles for long periods.
- Use correct fittings. Forcing threads is how leaks and failures happen.
- Transport smart. Keep it stable in the vehicle so it can’t roll like a steel bowling ball.
Welding CO2 FAQ (quick answers that prevent wasted trips)
Can I MIG weld with 100% CO2?
Yes. 100% CO2 is a common shielding gas for MIG on mild steel, especially when cost matters. Expect a “hotter” arc and typically more spatter than mixed gas. Many hobbyists use it successfully once settings and technique are dialed in.
Is CO2 the same as C25 (75/25 argon/CO2)?
No. C25 is a mix and behaves differently, usually with less spatter and a smoother arc. CO2 is still valid, just different. If you switch gases, your settings (and results) will change.
What fitting does a CO2 cylinder use for welding setups?
Most CO2 cylinders use a CGA-320 valve. Many regulators are CGA-580 (argon/argon-mix) and will not fit CO2. If your regulator doesn’t physically match the tank valve, don’t force it. Get the correct regulator for the cylinder.
Why do some places refuse to refill my cylinder?
Common reasons: it’s out of hydro test date, the cylinder isn’t in their approved program, it’s not a standard size/type, or they simply don’t do customer-owned fills (exchange only). Asking “Do you do customer-owned refills?” up front saves trips.
What should I ask a supplier so they don’t give me a useless answer?
Ask: “Do you do CO2 exchanges or customer-owned refills for a [size] cylinder? If refill, do you require it to be in hydro?” That’s the difference between a real answer and a shrug.
I swapped cylinders and now my weld looks worse. Why?
Swapping cylinders can change your setup more than you think: different pressure behavior, minor flow changes, or a leak from reconnecting. Re-check flow rate, inspect for leaks (especially at the regulator connection), and confirm you’re using the right gas for the job.
What’s the simplest “good enough” cylinder plan for hobby MIG?
Exchange a standard cylinder size locally and stop over-optimizing. Exchanges are fast, consistent, and reduce the “will they fill this?” drama. If you later want to minimize costs, move to customer-owned refills once you know who will actually do it.
Find CO2 near you (welding/industrial)
Use these links to open the directory already filtered for welding/industrial CO2. If you’re new, start with exchange. If you already own a cylinder and want it filled, check refill.
Next steps
Tank sizing
Pick a cylinder size that fits your usage and storage. Portable vs shop life.
Refill vs exchange
Cost, convenience, and policy differences by supplier type.
Supplier types
Welding shops, industrial gas suppliers, beverage gas, paintball fills.