Welding Duty Cycle & Gas Calculator
Duty cycle at any current, the amps your machine runs all day, cylinder life in arc hours, and the gas your hose dumps at every arc start.
The machine
The gas
What the current costs you
Same machine, different settings. The square law is steep in both directions, and the row you want is usually further down than you expect.
Two things worth knowing
Duty cycle is a square law, not a linear one. Heating in the machine goes with current squared, so the time you get scales with the inverse square of the current: D₂ = D₁ × (I₁/I₂)². Turn a 60% at 200 A machine up to 250 A and you get 38%, not the 48% a linear reading gives — wrong in the dangerous direction. Turn it down to 150 A and you get well past 100%, which is to say it runs continuously. That second direction matters more: dropping a quarter of the current very often removes the duty cycle limit entirely, and people buy a bigger machine instead. The number no machine prints on its label is Irated × √(D/100) — the current it will run at all day.
Turning the gas up to cure porosity usually makes it worse. Shielding gas has to leave the nozzle as smooth laminar flow. Past a certain rate it goes turbulent and starts pulling the surrounding air into the pool — precisely what the gas is there to prevent. So the instinctive fix is as likely to deepen the problem as solve it. The usual real cause is a draught, and a piece of cardboard beats another ten cubic feet an hour every time. Check the nozzle for spatter build-up too: a partly blocked nozzle starves the shield while the flowmeter still reads correctly.
- The ten-minute period is fixed and you cannot bank it. 60% means six minutes of arc in any ten-minute window — not thirty minutes of welding followed by twenty off. The rating exists because the machine reaches thermal equilibrium on that cycle, and a long continuous run overheats it even when the average looks right.
- The hose dumps its contents at every arc start. Between welds it sits full of gas at regulator pressure, and the solenoid releases most of it in about a second. A 25 ft ¼″ hose at 50 psi holds roughly seven seconds' worth at 20 cfh — noise on a long seam, and most of your consumption when tacking.
- Cylinder life tracks arc time, not shop time. An 80 cf bottle at 20 cfh is four hours of arc — which might be ten hours at the bench or two days, depending entirely on what you're making. That's why it feels unpredictable.
- Ratings assume 40 °C and clean airflow. That's a warmer day than most shops but a much cleaner machine than most machines. A blocked filter, a dusty interior or a unit shoved against a wall all cut the real figure, and the thermal cutout is the only thing that knows.
How to use
- Enter the duty cycle and current from the machine plate.
- Enter the current you actually weld at.
- Pick the process and cylinder to get gas consumption.
- Set your average weld length — short welds waste far more gas than long ones.
Frequently asked questions
How does duty cycle change with current?
As the inverse square, which is much steeper than people assume. Heating inside the machine goes with current squared, so the available time is the rated duty multiplied by the square of the ratio of rated current to actual current. A machine rated 60 per cent at 200 amps gives 38 per cent at 250, not the 48 per cent a linear reading suggests. That is an error in the dangerous direction, and it is why machines trip out sooner than their owners expect.
What current will my machine run at all day?
Rated current multiplied by the square root of the rated duty cycle divided by a hundred. A 60 per cent at 200 amp machine runs continuously at 155 amps; a 25 per cent at 200 amp machine runs continuously at 100. Almost no manufacturer prints this figure, and it is usually the one that decides whether you need a bigger machine, because the square law means a modest cut in current often removes the duty cycle limit entirely.
Can I weld for thirty minutes then rest for twenty?
No. The period is ten minutes and it does not accumulate. A 60 per cent rating means six minutes of arc time in any ten minute window, not six-tenths of any longer stretch you fancy. The rating exists because the machine reaches thermal equilibrium on that cycle, so a long continuous run overheats it even though the average across the hour looks correct. Both the American and European standards use the same ten minute period.
Will more shielding gas fix my porosity?
Usually not, and it often makes things worse. Shielding gas has to leave the nozzle as smooth laminar flow, and past a certain rate it becomes turbulent and starts aspirating the surrounding air into the weld pool — which is precisely what the gas is there to prevent. The usual real cause is a draught, so a screen or a piece of cardboard beats another ten cubic feet an hour. Check the nozzle for spatter build-up too, since a partly blocked nozzle starves the shield while the flowmeter still reads correctly.
Why does my gas cylinder empty so fast?
Probably the surge at arc start rather than the welding itself. Between welds the hose sits full of gas at regulator pressure, and when the solenoid opens it discharges far faster than the set flow rate. A 25 foot quarter-inch hose at 50 psi holds around seven seconds worth of gas at 20 cubic feet an hour and releases most of it in about one second. On a long seam that is noise. On tack welding it can easily be more than half your consumption.
How do I cut the arc-start surge?
A shorter hose, a narrower bore, or lower regulator pressure all reduce the volume being stored. Bore matters most because the gas held goes with the square of the diameter, so halving the bore quarters the charge. Surge-control regulators and flow-control valves fitted at the torch end exist for exactly this and pay for themselves quickly on short-weld work. Simply not leaving 50 feet of hose coiled on the floor helps more than most people expect.
How long will a cylinder actually last?
Divide the cylinder volume by the flow rate and you get arc hours, not shop hours. An 80 cubic foot bottle at 20 cubic feet an hour is four hours of arc time — but gas only flows while the trigger is in, so that might be ten hours at the bench or two full days depending entirely on what you are making. That gap between arc time and shop time is why cylinder life feels so unpredictable from job to job.
Are duty cycle ratings realistic?
They are a ceiling in ideal conditions rather than a promise. The standards specify a 40 degree Celsius ambient and unobstructed airflow, which is a warmer day than most shops but a far cleaner machine than most machines. A blocked air filter, a dusty interior or a unit pushed against a wall all cut the real figure, and nothing tells you except the thermal cutout. If a machine trips well inside its rating, look at airflow before you doubt the arithmetic.
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