_Fnord wrote:He could, but getting the right pressure without a regulator can be tricky sometimes. It's easy to overshoot if your only flow restriction is a ballvalve.
Look closely at the head of a standard Bernzomatic torch.
1. The valve is not a ball valve, it is probably a needle valve.
2. There is a very small orifice in the end of the pipe. Small as in smaller than the diameter of a hair. The orifice is so small that my crappy eyes can't even see the hole without a magnifying glass.
When you crack open the cylinder's valve it'll fill the meter at a pretty leasurly pace. Very easy to shut the cylinders valve when the proper pressure in the meter has been reached.
I just did a quick measurement by water displacement. Using a standard (un-modified) Bernzomatic torch head, with the air diffuser removed, cylinder temperature of 50F, cylinder valve fully opened, the standard orifice delivers about
7cc/second against atmospheric pressure. For a 100in<sup>3</sup> chamber (1,640cc) it will take about 9 seconds to pressurize the meter with the correct amount of propane.
If you chop the end of the pipe off, thus removing the orifice, the cylinder probably delivers more like 100's of cc/second.
The cylinder's valve can be easily closed down to the point where it will only deliver ~1cc/second. So it could take a minute or more to fully pressurize the meter pipe using the standard orifice.
The tricky part of this approach is that the pressure in the meter pipe will continue to rise even after the cylinder's valve is closed since there is a fairly large amount of propane, at the cylinder's pressure, between the valve and the orifice. It takes a little trial and error to figure out what pressure the gauge need to read when you close the cylinder's valve so that the pressure rises to the correct final value.
Using two ball valves and a pressure regulator is certainly a good way to build a meter. I'm just saying that you can save a $20 or $30 bucks if you omit the regulator, one valve, and a couple of the fittings, and just use a pressure gauge and the standard Bernzomatic torch head valve and orifice.
