I can't help with the forging part of your question, but I can help with the questions regarding propane.
Propane is generally considered to weigh 4.11 lbs per liquid gallon. That weight has some fluctuation though – as temperature and humidity both effect the density of propane. Most refineries list the weight (of propane) at 4.11 lbs per gallon. A few refineries (or storage locations) list different weights (between 4.11 lbs per gallon and 4.2 lbs per gallon).
The only accurate means of measuring propane is weight. I know of no gauges that can provide anything other than a rough estimate of the amount of propane left in a tank.
Containers that are designed to be moved are called cylinders. Generally speaking, any propane container less than 420 lbs (capacity) are going to be cylinders. Containers designed for permanent placement are called tanks. Generally speaking, tanks will have a capacity that is larger than 420 lbs.
With rare exception, cylinders do not list a water capacity on the tank; they will list a lb capacity. Tanks will list the water capacity.
Neither tanks or cylinders should be filled beyond 80% of their capacity. This is for safety precautions, as the remaining 20% allows for expansion (of the propane) as the temperature increases.
Both tanks and cylinders will have a “bleeder valve” near the fill valve. This bleeder valve is often used as a means of insuring the cylinder/tank is not over-filled (when propane starts to spit out of the bleeder valve, the tank/cylinder is 80% full). Be aware of the fact that bleeder valves occasionally stick (it is rare, but it does happen). Any tank/cylinder that is over-filled must be drained down to the safe level. This is especially important when the temperatures fluctuate from cold to warm.
A 100 lb cylinder can hold approximately 24.33 gallons of propane (that would fill it completely – which would be over-filled). An 80% fill would be approximately 20 gallons (slightly less, actually).
All cylinders must have a tare weight stamped into the cylinder. That is the weight of the cylinder itself.
Suppose the tare weight of the 100 lb cylinder is 42.7 lbs. To fill that cylinder (safely) you would set the scale to a weight of 122.7 lbs (tare + 80% of 100 lbs) and fill the tank to the point that the scale just balances at 122.7 lbs. I would also recommend that the bleeder valve is opened slightly at this time – when it starts spitting the cylinder is filled to 80% of capacity.
In those cases when a tank (not completely empty) is to be refilled, weigh the cylinder (with some propane still in it) and then subtract that weight from the safely filled weight (tare weight + 80% of the capacity in lbs). Otherwise you would be paying for the propane still in the tank prior to filling.
Just to be clear on that last part: suppose your tare weight is 42.7 lbs, but the cylinder weighs 52 lbs. You have 7.3lbs of propane still in the cylinder.
Most places will not let the owner (of the cylinder) fill their own cylinder. Be aware of the fact that some places will try to cheat people out of the propane that was in the cylinder. They do that by subtracting only the tare weight from the total weight of a safely filled cylinder (omitting the extra weight of the propane left in the cylinder before filling).
There are charts available that can give you a general idea of how much propane it will take to do the task at hand. Normally, those specifics would be found via the manufacturer of the item that is burning the propane.
It's probably easier to weigh the cylinders before and after forging your items. The difference – in lbs – can then be used to determine how many gallons of propane you have used.
Check with your local propane companies; it might be cheaper for you to buy the propane through them (other than buying your own cylinder).
While propane is fairly safe, there are dangers involved and some points that should be remembered.
Propane is lighter than water, but heavier than air. It will always seek out the lowest spot in calm conditions.
Keep the cylinders/tanks outside – helping to prevent the accumulation of propane vapors in the event of a leak.
In the event of a leak, make sure there is plenty of ventilation and stay upwind (and on higher ground) until the propane is completely dispersed. With air movement, propane disperses quite rapidly: there is no harm to the environment
Propane itself is odorless, but mercaptan is added as a safety caution (it smells bad, so leaks can be easily detected).
Propane is pressurized. It is possible – since the liquid form of propane is much colder than the outside air – to “burn” your skin with the liquid form.
Use a brass wrench – or some other non-sparking material – to attach/detach lines to/from propane containers.
Check for leaks – using a soapy water concoction (you can buy it, or make it) every time you change cylinders. There can be leaks that aren't heard or smelled, so be sure to check for them.
