Shielding gas flow rate and cylinder planning
How CFH flow, cylinder capacity and arc-on time combine, why bottles empty faster than the math predicts, and how to set flow for clean welds.
What flow rate really controls
The flowmeter on your regulator sets how many cubic feet of shielding gas leave the torch each hour, measured in CFH. That stream of argon, CO2 or a blend pushes air away from the molten pool so the weld does not pick up oxygen and nitrogen and turn porous. More flow is not automatically better: too little lets air creep in, while too much creates turbulence that actually sucks air back into the shield. The goal is the lowest steady flow that still keeps the puddle bright and clean.
Turning flow and time into gas used
Because flow is steady, the gas a weld consumes is simply the flow rate multiplied by the time the arc is running. This calculator divides your minutes by 60 to get hours, then multiplies by CFH. Twenty CFH for ninety minutes of arc-on time uses thirty cubic feet, and thirty-five CFH for a full hour uses thirty-five. Knowing the per-job consumption lets you price gas into a quote and predict when a bottle needs a swap before it runs dry mid-weld.
How long a cylinder lasts
Cylinder run time is the bottle's capacity in cubic feet divided by the flow rate in CFH, which gives the hours of continuous arc it can support. A common 80 cu ft bottle at 40 CFH lasts exactly two hours of pure arc time, while a large 250 cu ft cylinder at 35 CFH stretches to roughly 7.14 hours. Remember this is arc-on time only. Between welds the gas is off, so a shop bottle usually spans several working days even though its arc-on rating sounds short.
Why bottles empty faster than the numbers say
Real gas use almost always beats the ideal calculation. Pre-flow and post-flow keep gas running before and after the arc, a burst of gas surges every time you pull the trigger, and drafts in the shop force you to crank the flow higher to hold the shield. Leaks at hose fittings and the torch connection bleed gas silently around the clock. Tighten every joint, keep flow as low as clean welds allow, and use a gas lens or a draft screen outdoors to make each cylinder go further.