Shielding Gas Calculator
Work out shielding gas flow, how long a cylinder lasts, and the cost per hour and per foot of weld - and why turning the flow up makes porosity worse.
How to use this calculator
- 1Use arc-on time, not hours at work. Gas only flows while the trigger is down.
- 2Set the flow with a flowmeter. A pressure gauge on a MIG regulator is not measuring flow and changes meaning when anything downstream changes.
- 3If the weld is porous, check for draughts, a spattered nozzle and leaks before touching the flow rate.
- 4Count the arc starts on tack-heavy work. The surge at each trigger pull can be most of the cylinder.
How the calculation works
Gas used (cu ft) = flow rate (CFH) x arc hours + surges
Cylinder duration (hr) = cylinder volume / flow rate
Cost per arc hour = flow rate x (cylinder cost / cylinder volume)
Nozzle rule: about 2 CFH per sixteenth of bore- CFH
- Cubic feet per hour, read on a flowmeter - not on a pressure regulator, which measures something else
- Arc hours
- Time the trigger is actually down. Gas only flows then, plus pre-flow, post-flow and the start surge
- Surge
- The gas dumped when the solenoid opens and the pressure built up in the hose releases
Flow rate is a volumetric measurement at standard conditions, so a cylinder rated at 125 cubic feet contains that much gas at atmospheric pressure regardless of the pressure it is stored at.
The nozzle rule of two CFH per sixteenth of bore falls out of needing enough volume to fill the nozzle exit area at a sensible velocity. It gives about 20 CFH for a 5/8 in nozzle, which is where the trade sets it.
Above roughly 30 CFH on a standard MIG nozzle the flow becomes turbulent rather than laminar, and turbulent flow entrains surrounding air rather than displacing it. That is the mechanism behind more gas producing more porosity.
Worked example
A day of MIG on a 125 cu ft cylinder
- 1.Short circuit MIG through a 5/8 in nozzle at 22 CFH - right in the usual 20 to 25 band, and close to the two-per-sixteenth rule which suggests 20.
- 2.Four hours of arc uses 4 x 22 = 88 cubic feet.
- 3.A 125 cu ft cylinder therefore gives 125 / 22 = 5.7 hours of arc - not much more than one busy day.
- 4.At $90 a fill that is $0.72 per cubic foot, so about $16 of gas per hour of arc time.
Result: 88 cu ft - a 125 cylinder lasts 5.7 arc hours
The welder who turned it up
- 1.The same job with the flow opened to 45 CFH to chase some porosity.
- 2.Gas use doubles to 180 cu ft - more than the cylinder holds - and the cylinder now lasts 2.8 arc hours.
- 3.And the porosity gets worse, not better. Past about 30 CFH through this nozzle the column goes turbulent and starts dragging room air in with it.
- 4.The porosity was almost certainly a draught, a spattered nozzle or a leak. Doubling the flow doubled the cost and added a second cause on top of the first.
Result: 180 cu ft, half the cylinder life, and more porosity
A day of tacking, where the surge dominates
- 1.Six hundred tacks over a day amounts to only half an hour of actual arc time - 11 cubic feet of flowing gas.
- 2.But every trigger pull dumps the pressure built up behind the solenoid, roughly 0.02 cu ft a time.
- 3.That is 12 cubic feet of surge - more than the gas that flowed during the welding.
- 4.On tack-heavy work a surge guard or a length of small-bore hose ahead of the gun pays for itself in weeks.
Result: 23 cu ft, over half of it in the start surges
What shielding gas is doing
Molten steel will take up nitrogen and oxygen from the air at a great rate, and both do damage. Oxygen forms oxides that end up as inclusions; nitrogen dissolves in the pool and comes out of solution as it solidifies, forming the gas pockets that show up as porosity. The whole job of the shielding gas is to keep air away from the arc and from the metal while it is hot enough to care.
It does that by displacement, not by pressure. A smooth column of gas leaves the nozzle, spreads across the work, and pushes the air out of the region. What makes it work is that the flow is laminar - orderly, layered, not mixing with what surrounds it.
That is why more is not better. Push the flow too high and the column becomes turbulent: it tumbles, it mixes with the room air at its boundary, and it carries that air down into the very place it was supposed to protect. The transition happens somewhere around 30 to 35 cubic feet per hour on a standard MIG nozzle, and above it every extra unit of flow makes the shield worse and the cylinder emptier.
Porosity and the wrong reflex
The reflex when a weld comes out porous is to turn the gas up, and it is almost always wrong. In order of how often they are actually to blame: a draught, a nozzle partly blocked with spatter, a leak in a hose or fitting, contamination on the plate, and too long a stickout holding the nozzle away from the pool.
The draught is worth dwelling on. A breeze of five miles an hour - barely noticeable, an open roller door on a mild day, a pedestal fan pointed at the welder for comfort - will strip the shield off a weld at any flow rate a machine can produce. It cannot be out-flowed. It has to be blocked, with a screen or a curtain or by closing the door, and it is the reason gas-shielded processes are indoor processes and self-shielded flux-cored exists for outdoor work.
Leaks are the quiet one. A perished hose or a loose fitting can bleed a substantial fraction of the flow, and the flowmeter reads the same because it is upstream of the leak. Soapy water on every joint once a season finds them in a few minutes.
- Draught — Five mph strips the shield. Screen it - no flow rate beats a breeze.
- Spattered nozzle — Disrupts the column into turbulence. Clean it and use anti-spatter.
- Leaks — Upstream of the flowmeter, so the reading looks fine. Soap test the fittings.
- Contamination — Mill scale, oil, paint, moisture and galvanising all gas off into the pool.
- Long stickout — Holds the nozzle too far away for the shield to reach the pool.
What the gas actually costs
Gas is usually treated as an afterthought next to wire and electrodes, and on a busy machine it should not be. At 22 cubic feet an hour and typical cylinder pricing, shielding gas runs somewhere in the region of ten to twenty dollars per hour of arc time - which on a job with several hours of welding is comparable to the wire consumed.
Two things make it worse than it needs to be. The first is small cylinders: filling costs are dominated by handling rather than by contents, so a 40 cubic foot cylinder costs a large fraction of a 330 while holding an eighth as much. Unless the work genuinely has to be carried, the big cylinder is dramatically cheaper per cubic foot and has the additional virtue of not running out in the middle of a pass.
The second is rental. On low-volume use, monthly rental on a cylinder that lasts six months can exceed the cost of the gas inside it. Owning the cylinder, or moving to an exchange arrangement where you swap rather than rent, usually beats it - and it is worth doing the arithmetic rather than assuming, because the answer depends entirely on how fast the shop actually gets through gas.
What this assumes, and where it stops
Assumptions
- Flow rates are typical published ranges by process and nozzle size.
- Cylinder volumes are the nominal contents in cubic feet at standard conditions.
- The start surge is taken at roughly 0.02 cubic feet per trigger pull, which varies with hose length and bore.
- Arc time is time the trigger is down; pre-flow and post-flow are treated as part of the flowing time.
Limitations
- It does not distinguish between gas mixtures. Argon, CO2, 75/25 and tri-mixes cost different amounts and behave differently, but they flow at the same rates.
- Nozzle geometry matters as much as bore. A gas lens produces a far more stable column at the same flow and extends the usable stickout considerably.
- The turbulence threshold is approximate and depends on nozzle design, stickout and the gas itself - CO2 and argon have different densities and behave differently.
- Back purging for stainless and titanium root passes uses considerably more gas than the torch does, and is not included.
- It says nothing about which gas to use, which is a metallurgical and transfer-mode question rather than a flow one.
Common questions
How long does a bottle of argon last?
Divide the cylinder size by the flow rate. A 125 cubic foot cylinder at 22 CFH gives about 5.7 hours of arc time - which is roughly one busy day of manual welding, since arc-on time is only 20% to 30% of the working day. A 330 gives about 15 hours at the same flow.
What flow rate should I use for MIG?
Twenty to twenty-five CFH for short circuit through a normal 1/2 to 5/8 inch nozzle. Spray transfer wants 30 to 40 because the arc and nozzle are bigger. The working rule is about two CFH per sixteenth of nozzle bore, which puts a 5/8 nozzle at 20.
Will more gas fix my porosity?
Almost certainly not, and past about 30 CFH it will make it worse - the gas column goes turbulent and starts dragging room air into the arc. Check for a draught first, then a spattered nozzle, then leaks in the hose and fittings, then contamination on the plate. Those are the real causes in roughly that order.
Why does welding outdoors go badly?
Because a five mile an hour breeze - which you would barely notice - strips the shielding gas off the pool faster than any flow rate can replace it. It cannot be solved by turning the gas up. Either screen the work properly or use a self-shielded process such as flux-cored, which carries its own protection in the wire and does not care about wind.
Is a regulator the same as a flowmeter?
No. A regulator sets pressure, and the flow that results depends on every restriction downstream - change the hose, the gun or the nozzle and the same pressure gives a different flow. A flowmeter measures the flow itself. Many MIG setups ship with a pressure gauge marked in flow units, which works only for the specific hose it was calibrated against.
Sources
- Shielding gas selection and flow guidance — Miller Electric
- Welding fume - occupational exposure — US Occupational Safety and Health Administration
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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