Enter pipe, method and O₂ target to see how many volume changes and how long the purge takes, how much argon each joint uses and what it costs, with and without dams.
Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.
Exponential dilution model, with dams and for the whole pipe, at the same flow. Log scale.
Purge and welding argon for each method.
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When welding stainless, the inside of the root is exposed to air and oxidizes, forming dark "sugaring" that reduces corrosion resistance. The purge replaces the air inside with argon before and during welding until O₂ drops to the required level.
With the perfect-mixing dilution model, going from 20.9% to 50 ppm with argon carrying 5 ppm O₂ takes about 8.4 changes. Each change cuts O₂ to about 37% of the previous value; practical guides often quote 5 to 10 changes.
In a 4 in pipe (4.5 in OD × 0.12 in wall) with dams 16 in apart, there is about 0.14 ft³ to purge, including 0.01 ft³ of hose. At 40 cfh the purge takes about 1.8 min and uses about 1.2 ft³; adding 3.75 ft³ while welding (15 cfh for 15 min), each joint uses about 4.9 ft³, or about $1.24.
Almost always on long runs. In the example, purging the whole 20 ft pipe would take about 25 min and 17 ft³ of purge gas per joint; with dams, under 2 min and about 1.2 ft³. Water-soluble or inflatable dams are removed after welding or dissolve during hydrotest.
It depends on the spec. AWS D18.1 provides a reference color scale tied to purge O₂ content; sanitary tubing often calls for 50 ppm or less, and titanium much less. Always measure with an O₂ analyzer at the outlet before striking the arc.
Argon is an asphyxiant and heavier than air: it collects in trenches and low spots with no smell or color. In permit spaces follow OSHA 29 CFR 1910.146, monitor ambient O₂ (below 19.5% is oxygen-deficient) and ventilate; keep the purge vent open so the root is not pushed out.
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