The gauge is not a fuel gauge
An 80 cubic foot argon cylinder rated at 2265 psi reading 1400 has about 49 cubic feet in it, because for a gas that stays a gas all the way down, contents track pressure closely enough to divide one by the other. Stop using it at 150 psi and 5.3 cubic feet stay in the bottle, so 44 are actually yours. That is the whole of the gauge arithmetic and it is worth knowing it is an approximation at both ends — a nearly full bottle holds slightly less than the linear figure says, and a cold bottle reads low without having lost anything.
None of that applies to CO2. A CO2 cylinder is mostly liquid, and the gauge reads the vapour pressure sitting above the liquid, which stays roughly where it is until the liquid has gone. A CO2 bottle can read full and be nearly empty. The only way to know is to put it on a scale and subtract the tare weight stamped on the shoulder, which is why the form has a second mode rather than a footnote.
Where the eleven percent goes
At the defaults in the form — 35 CFH, two minutes of arc a part, four arc starts, two and a half seconds of pre-flow and post-flow on each — a part burns 1.1667 cubic feet on the arc and 0.0972 on the starts. Eight parts a day plus two line purges at 0.2 cubic feet is 10.51 cubic feet a day, of which 1.18 never shielded an arc. That is 11.2 percent of every bottle, or about nine cubic feet of an eighty.
Add a tack-heavy job and it climbs fast, because the overhead scales with trigger pulls and not with weld length. A bracket with twelve tacks and forty seconds of actual arc is mostly pre-flow. This is not an argument for turning the post-flow down — that gas is doing a job — it is an argument for putting it in the estimate, where almost nobody does.
The rental and the gas
A twelve dollar monthly rental on an eighty that costs sixty-five to fill works out at 0.81 dollars a cubic foot of gas against 0.05 of rental at the usage in the form, which is 221 cubic feet a month. The rental is noise. Drop the usage to a weekend welder burning fifteen cubic feet a month and the rental is 0.80 a cubic foot, which has doubled the price of the gas without a single extra weld being made. The form prints the crossover — 14.8 cubic feet a month at these prices — because where you sit relative to it decides whether owning a cylinder is worth the capital or not, and the answer is genuinely different for a production shop and for somebody who welds a gate twice a year.
What this does not know
It does not know your flowmeter is telling the truth. A ball floating at 35 in a tube calibrated for argon reads differently on a mix, a leaking fitting downstream of the meter costs you gas that the meter has already counted, and a draught through an open roller door means the gas that arrives at the nozzle is not the gas that shields the puddle. It also does not know how much gas goes into a hose left pressurised overnight against a slow leak, which in some shops is more than everything above put together. Weigh or read the bottle at the start of a week and again at the end, compare it with what the page says you should have used, and the gap is your own answer to all of that at once.
Questions people ask
How long does an 80 cubic foot argon bottle last?
Divide the usable gas by your consumption. At 35 CFH a bottle holding 44 usable cubic feet is 75 minutes of continuous arc, but almost nobody welds continuously. At two minutes of arc a part with four tack starts each, the same bottle is 34 parts, or a bit over four working days at eight parts a day. The flow rate and the number of trigger pulls matter more than the bottle size does.
Why does the gauge on my CO2 bottle stay at the same pressure?
Because the CO2 in it is liquid, and the gauge is reading the vapour pressure above the liquid rather than a quantity. That pressure stays roughly constant while any liquid remains, then drops quickly once it is gone. A CO2 cylinder has to be weighed: gross weight now, minus the tare weight stamped on the cylinder shoulder, gives the pounds of gas, and pounds times the cubic feet per pound figure for the gas gives the volume.
How much gas do pre-flow and post-flow use?
Flow rate times the seconds, times the number of arc starts. At 35 CFH, two and a half seconds is 0.0243 cubic feet a start, so four starts on a part is 0.097 cubic feet before any welding. On a part with two minutes of arc that is eight percent on top; on a part with twenty seconds of arc and a dozen tacks it is more gas than the welding uses. It is not waste, but it does belong in the number.
Is it cheaper to own a cylinder or rent one?
It depends entirely on how much gas you move through it, and the form works out the crossover for your own prices. Rental is a fixed monthly charge spread over whatever you use, so the cost it adds per cubic foot falls as usage rises. At sixty-five dollars a fill for an eighty and twelve dollars a month rental, the rental costs more than the gas below about fifteen cubic feet a month. Above that the rental fades into the noise fast.
Does the temperature change the gauge reading?
Yes, and by more than people expect. Pressure in a fixed volume rises and falls with absolute temperature, so a bottle brought in from a cold yard reads low and the same bottle sitting in afternoon sun reads high, with the same amount of gas inside it either way. Read the bottle warm and with gas flowing, and do not chase a hundred psi of apparent change between morning and afternoon.