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Shielding gas preflow is the short burst of argon that starts before the TIG arc. Its job is to purge air from the torch cup and create a clean gas envelope before the tungsten begins heating the metal. Set it too short and the first part of the weld can turn gray, porous, or contaminated. Set it excessively long and you waste gas without improving the weld.
What Preflow Does
When you press the TIG torch trigger, the gas valve opens before the arc starts. Preflow gives argon time to move through the hose, torch, collet body, and cup. This displaces oxygen and nitrogen around the tungsten and joint.
Preflow protects the tungsten during arc initiation and helps prevent a dirty, oxidized start. It matters most when using a gas lens, a large cup, a long torch hose, or a remote gas solenoid located some distance from the torch. It is also useful when welding stainless steel, aluminum, titanium, and other metals that show contamination quickly.
Preflow is not the same as postflow. Preflow happens before the arc; postflow continues after the arc ends to cool the tungsten and protect the hot weld. Changing one does not change the other.
Recommended Starting Settings
For most TIG work, start with 0.3 to 0.8 seconds of preflow. A setting around 0.5 seconds is a practical general-purpose starting point for a typical torch, standard cup, and short hose.
| Situation | Useful starting preflow | Reason |
|---|---|---|
| Short torch, standard cup, mild steel | 0.3–0.5 seconds | Small gas volume and forgiving material |
| Stainless steel or aluminum | 0.5–0.8 seconds | Cleaner starts reduce visible contamination |
| Large cup or gas lens | 0.6–1.0 seconds | More volume must be filled before the arc starts |
| Long torch lead or slow gas solenoid | 0.8–1.5 seconds | More time may be needed for stable flow at the cup |
| Titanium or highly contamination-sensitive work | 1.0–2.0 seconds | Extra purge time can protect the start |
These are starting points, not fixed rules. A machine with a fast electronic gas valve may need less preflow than an older welder with a mechanical valve. Your actual gas flow at the cup matters more than the number displayed on the front panel.
How to Set Preflow Step by Step
First, connect a clean argon cylinder and set the regulator or flowmeter. Pure argon is standard for DC TIG on steel and stainless, and for AC TIG on aluminum. A typical flow setting is 15 to 20 cubic feet per hour (CFH) with a standard cup. A gas lens may work well around 15 to 25 CFH, depending on cup size, drafts, and torch position.
Check for leaks before adjusting the timing. Brush leak-detection solution over the regulator connections, hose fittings, and torch connection, then watch for bubbles. A leak can make a correct preflow setting appear ineffective. Keep the argon gas leak detection solution away from the joint and clean any residue before welding.
Open the shielding gas valve and listen for the flow. If your machine has a digital menu, select preflow and enter 0.5 seconds. On machines with a knob, set it near the middle of the available low-time range. Put the torch in its normal welding position, press the trigger, and wait for the arc to start. The gas should flow smoothly before ignition, without a loud blast or pulsing sound.
If the arc starts while the tungsten still appears surrounded by turbulent gas, increase preflow in small steps of 0.1 to 0.2 seconds. If the weld starts cleanly at 0.5 seconds, there is usually no benefit in raising it to several seconds.
Signs the Setting Is Wrong
Too little preflow commonly produces a dark or dirty tungsten tip, a gray or sooty bead at the start, pinholes, and an unstable first fraction of a second. Stainless steel may show brown, blue, or black discoloration immediately after ignition. On aluminum, the arc can start with an unusually contaminated or erratic appearance.
Do not blame preflow first if the entire weld is discolored. Check gas flow, leaks, drafts, cup size, torch angle, and postflow. A shop fan or open garage door can blow argon away even when preflow is correctly set. Contaminated filler rod, oil, paint, and a dirty base metal can create similar symptoms.
Too much preflow wastes argon and slows the start. A long hiss before every tack is especially costly on a small disposable or rental cylinder. Excessive flow can also create turbulence, particularly when the flow rate is already too high for the cup. More gas is not automatically better.
Torch, Cup, and Gas Factors
A gas lens often improves coverage and allows the tungsten to extend farther for better visibility, but it does not eliminate the need for preflow. Large cups hold more air and can benefit from additional time. A #12 cup may need more preflow than a #6 cup, especially with a long hose.
Keep the ceramic cup and gas lens clean. Cracked cups, clogged screens, damaged O-rings, and loose collets can disturb shielding. If you need replacement parts, a basic TIG torch gas lens cup kit is often cheaper than troubleshooting repeated contamination. Verify the kit fits your torch series before ordering.
Use a flowmeter that reads in CFH or liters per minute and place it at the cylinder regulator. A torch-end flowmeter can provide a more accurate check, but it is not essential for ordinary work. A simple regulator is fine for a hobby welder if it is in good condition and the readings remain steady.
A Practical Tuning Test
Prepare clean scrap of the same material and thickness as the job. Fit the tungsten, cup, and gas lens you plan to use. Make several short arc starts without adding filler, using preflow settings of 0.3, 0.5, and 0.8 seconds. Inspect the tungsten and the first few millimeters of each arc mark.
Choose the shortest setting that gives a quiet, stable start and a bright, clean tungsten. Then repeat the test with the torch held at the actual working angle. If a setting works only with the torch pointed straight down, it may not provide enough protection in a real joint.
For most home and light fabrication TIG welders, starting at 0.5 seconds, 15–20 CFH, and adjusting only after inspecting the first inch of the weld is the sensible approach. Spend money on leak-free connections and a suitable cup before buying a more elaborate controller. Good gas delivery and clean preparation usually matter more than an extra second of preflow.