gas consumption for a typical tip used to weld, say, 1/8 inch thick material (tip numbers vary widely among brands and series) should be about 5-8 cubic feet per hour each gas - thereby 75/5 = 15 hours continuous use .
See if you can find a good tip chart for your specific tips, with flow stats in cubic feet per hour(cfh).
Acetylene cylinders are invoiced by WEIGHT - each 15 cubic feet equals approx. 1 pound. Your 75 cubic foot cylinder should weigh about 5 pounds less when empty than when full. This is one reason that you will see that they are stamped with a "tare weight". I don't believe that the Tare Weight includes the acetone that is added to stabilize the acetylene under cylinder pressure. Acetylene in acetone is kind of like the fizz (CO2) dissolved in pop/beer. That's why withdrawal rates, and avoiding laying acet. cylinders on their sides is important.
Also - acetylene tends not to behave like oxygen in a straight line (pressure vs. quantity). e.g. 2200 psi full - 1100 psi 1/2 full. Acetylene, as it is dissolved in a liquid, pressure tends to stay around 200-250 for a long time, diminish to 50 or so, and then there may still remain a significant quantity remaining. If you are withdrawing too fast, or little or none remaining - your flame will burn with a purple color and lose heat (acetone burning). If you use a low flow rate, or weld for brief periods, you can probably get much more from your cylinder. When watched CAREFULLY, when a cylinder gets "low", I will set the Acetylene outlet (torch) pressure at 10-12 psi and use it 'til it drops just below that.
Hope this helps.
PS - always work with equal pressures when welding, and never starve a torch by throttleing it down - go to the next smaller tip instead - to keep the velocity of the burning gases outside of the tip and thus avoiding backfires and flashbacks. ALSO - Consider investing in reverse flow check valves and( /or in combination) flashback arrestors.
Thanks for the great information.
I am begining to think that my old torch has some metering issues as I recall it erratically switching from a slight hiss to a quiet flame a few times. I also have a Victor torch (100?) in nicer condition but it is not as easy to hold, and it needs bigger tips for what I am working on.
I do use the flashback arresters.
The responses are appreciated.
This is probably only of historical interest but there used to be torches that used the energy of the oxygen flowing through a venturi to suck in the acetylene. I assume they date to the use of acetylene generators which made acetylene by reacting calcium carbide with water. In general I believe those generators produced only low pressures (until they exploded which I hear was not unusual). Those torches expected oxygen pressures several times the acetylene pressure.
See injector type torches table here-
http://www.fortunecity.com/village/lind/247/weld_book/Ch11.htm
as well as lots of info about oxy/fuel in general.
Bill