Gas flow rate is measured in CFH (cubic feet per hour) and controls how much shielding gas reaches the weld zone. Set it too low and atmospheric contamination causes porosity. Set it too high and turbulence sucks air into the gas stream. Either way, you get bad welds and waste gas.
The Turbulence Problem
Most flow rate mistakes lean toward too much gas, not too little. It’s a natural instinct: more protection should mean more gas. But gas flow through a nozzle or cup obeys the same physics as water through a garden hose. Turn it up too high and smooth (laminar) flow becomes chaotic (turbulent).
Laminar flow produces a stable, dome-shaped gas shield over the weld puddle. The gas moves in parallel layers with no mixing. Turbulent flow creates eddies and vortices that pull surrounding air into the gas column. That air contains nitrogen, oxygen, and hydrogen, all of which contaminate the weld.
The transition from laminar to turbulent flow depends on gas velocity, nozzle diameter, and gas density. For most MIG nozzles and TIG cups, turbulence starts between 35-50 CFH. For gas-lens TIG setups, the transition point is even lower because the laminar flow is more fragile.
MIG Flow Rate Chart
| Nozzle Size | Indoor, Still Air | Indoor, Mild Drafts | Outdoor with Screens |
|---|---|---|---|
| 3/8" (small) | 15-20 CFH | 20-25 CFH | 25-30 CFH |
| 1/2" (standard) | 20-25 CFH | 25-35 CFH | 35-45 CFH |
| 5/8" (large) | 25-35 CFH | 35-45 CFH | 40-55 CFH |
| 3/4" (heavy duty) | 30-40 CFH | 40-50 CFH | 45-60 CFH |
Larger nozzles have a bigger opening, so the gas velocity for any given CFH is lower. That means they can handle higher flow rates before turbulence sets in. If you’re fighting drafts, upsizing the nozzle and running moderate flow is often better than cranking up the flow on a small nozzle.
TIG Flow Rate Chart
TIG cups are numbered by their inside diameter in sixteenths of an inch. A #7 cup has a 7/16" opening. A #12 cup has a 12/16" (3/4") opening.
| Cup Size | Standard Collet Body | Gas Lens |
|---|---|---|
| #4 (1/4") | 8-12 CFH | 6-10 CFH |
| #5 (5/16") | 10-15 CFH | 8-12 CFH |
| #6 (3/8") | 12-18 CFH | 10-14 CFH |
| #7 (7/16") | 15-20 CFH | 10-15 CFH |
| #8 (1/2") | 18-25 CFH | 12-18 CFH |
| #10 (5/8") | 20-30 CFH | 15-20 CFH |
| #12 (3/4") | 25-35 CFH | 18-25 CFH |
Gas lens advantage: A gas lens is a mesh screen that straightens the gas flow into a smooth laminar column. It lets you run 30-40% less gas than a standard collet body while getting equal or better coverage. The laminar flow also extends further from the cup, which means you can run longer tungsten stickout for better joint access.
For most TIG welding, a gas lens with a #7 or #8 cup at 12-18 CFH provides excellent coverage. Save the large #10 and #12 cups for applications where you need extended gas coverage, like walking-the-cup pipe welding or welding titanium where an extended trailing shield is critical.
Flow Rate by Gas Type
Different shielding gases have different densities, which affects their shielding effectiveness at a given flow rate.
| Gas | Density vs. Air | Flow Rate Adjustment | Reason |
|---|---|---|---|
| 100% Argon | 1.38x heavier | Baseline | Dense gas blankets the weld zone effectively |
| 75/25 Ar/CO2 | ~1.40x heavier | Same as argon | Similar density to pure argon |
| 100% CO2 | 1.52x heavier | Slightly higher (+5 CFH) | Active gas, more volume needed for consistent arc |
| 75/25 Ar/He | ~1.10x heavier | +30-40% over argon | Helium is light and dissipates fast |
| 50/50 Ar/He | ~0.83x lighter | +40-60% over argon | Even lighter, needs more volume |
| Tri-mix (90 He/7.5 Ar/2.5 CO2) | ~0.30x lighter | +50-70% over argon | Mostly helium, very light |
Helium-rich blends require significantly higher flow rates. The light helium atoms rise and dissipate quickly, providing less natural blanketing effect than the heavier argon. This is one reason helium blends cost more to use: you’re buying a more expensive gas and using more of it per hour.
Setting Flow Rate Correctly
Dynamic vs. Static Reading
Set your flow rate with gas actually flowing through the system. Pull the MIG trigger or open the TIG torch valve, then adjust.
Why this matters: Most ball-in-tube flowmeters read differently under static conditions (no flow, regulator set to a pressure) versus dynamic conditions (gas flowing through the gun/torch). The flow restriction of the gas hose, solenoid, and gun/torch creates backpressure that changes the actual flow rate. Setting the ball height with no flow gives you an inaccurate reading.
Flowmeter Calibration
Flowmeters are calibrated for a specific gas at a specific temperature and pressure. An argon-calibrated flowmeter reads correctly for argon and Ar/CO2 blends (close enough). But it reads inaccurately for helium-containing blends and pure CO2.
If you’re running helium blends through an argon-calibrated flowmeter, the indicated flow rate will be lower than actual flow. You may need to set the indicated reading 20-30% higher than your target to get the real flow rate where you want it. Or invest in a flowmeter calibrated for your specific gas blend.
Checking Coverage Quality
The best test isn’t the flowmeter reading. It’s the weld itself. Run a test bead on scrap at your set flow rate. Examine the bead for:
- Porosity (scattered pinholes): Insufficient gas coverage. Increase flow by 5 CFH and retest.
- Worm tracking (surface porosity trails): Gas turbulence or contamination. Try reducing flow.
- Clean, porosity-free bead: Flow rate is adequate. Don’t increase further.
- Gray or oxidized surface on TIG welds: Insufficient coverage at the puddle’s trailing edge. Increase flow or add a trailing shield.
Position-Specific Adjustments
Welding position affects gas coverage because gravity influences the gas column.
Flat position: Gas falls naturally over the weld pool. Standard flow rates work well. This is the easiest position for consistent coverage.
Horizontal position: Gas tends to roll off the upper side of the joint. Increase flow by 5-10 CFH over flat-position settings, or angle the gun/torch slightly upward.
Vertical position: Gas rises past the weld zone quickly. Increase flow by 5-10 CFH. On vertical-up welding, the arc and heat rise, and the gas follows. Coverage is generally adequate without major adjustment.
Overhead position: Gas falls away from the weld pool due to gravity. This is the hardest position for gas coverage. Increase flow by 10-15 CFH over flat-position settings. Keep the nozzle or cup close to the work to minimize the distance gas must travel against gravity.
Common Flow Rate Mistakes
Running 50+ CFH “just to be safe.” This guarantees turbulence on any standard nozzle or cup. You’re wasting gas and creating porosity at the same time. Scale back to 25-30 CFH and watch the weld quality improve.
Not adjusting for drafts. A ceiling fan, open bay door, or HVAC vent can disrupt gas coverage at normal flow rates. If you can’t eliminate the draft, increase flow by 5-10 CFH and add a wind screen around the work area. If the draft is persistent and strong, switch to self-shielded flux-core wire for MIG work.
Ignoring pre-flow and post-flow. Pre-flow establishes the gas shield before arc ignition. Post-flow protects the cooling puddle (and the tungsten, on TIG) after the arc stops. Set pre-flow to 0.5-1.0 seconds. Set TIG post-flow to 5-10 seconds (longer at higher amperages). MIG post-flow of 1-3 seconds is typical.
Using a damaged nozzle or cup. A cracked MIG nozzle or chipped TIG cup disrupts gas flow patterns even at correct flow rates. Inspect regularly and replace damaged parts.
Never calibrating the flowmeter. Ball-in-tube flowmeters lose accuracy over time. The tube gets dirty, the ball gets scratched, and the spring weakens. If your flow rate seems right but you’re getting porosity, try cleaning the flowmeter tube or testing with a known-good meter.
Gas Savings Tips
- Use a gas lens for TIG. Saves 30-40% gas with equal or better coverage.
- Shorten the gas hose. A shorter run from regulator to machine means less hose volume to pressurize, reducing pre-purge waste.
- Close the cylinder valve during breaks. Even 10 minutes of solenoid bleed adds up over a day.
- Fix leaks immediately. A tiny leak that you can hear but ignore wastes more gas in a week than you’d use in a day of welding.
- Match nozzle/cup size to the joint. Don’t use a 3/4" nozzle on a small fillet weld. A 1/2" nozzle provides adequate coverage at lower flow.