<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">I'm a mechanical engineer, it's also a part of my job to be sure that something is safe<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
I'm glad to see that you're safety-minded, but what you fear just isn't possible, for many reasons:
• As several have already stated, the propane concentration in the chamber will be well above the upper explosive limit in a very short amount of time. If someone leaves both valves open, there will simply be too much gas to ignite.
• <b>Even if</b> someone managed to click the ignition at exactly the right moment when the mixture was ignitable, there would not be enough pressure to cause backflow. Pressure during ideal stoichiometric combustion in a perfectly insulated closed chamber at 1atm cannot exceed 102 psig, and typically does not exceed 50 psi in normal use. Since 50 psi is significantly lower than the typical propane bottle pressure of 80-90 psi, backflow simply cannot occur.
• <b>Even if</b> the pressure were high enough (with a sealed, insulated chamber), the regulator vents excess pressure overboard. High pressure gas would pass out through the regulator vent, instead of back into the bottle.
• <b>Even if</b> gases in the meter pipe were to be forced backwards past the regulator, into a nearly empty bottle (as in: all the liquid has been consumed and bottle pressure has dropped), the flow would be slow enough that the flame would never reach the orifice. This is due to the valve on the propane bottle, which allows relatively low flow.
• <b>Even if</b> the flame were able to reach the bottle valve, it would most likely be extinguished, as the narrow orifice through cold metal would act as a flame arrestor.
• <b>Even if</b> the flame could get through the orifice, there would be no ignition, as the bottle does not contain any significant oxidizers. Without an oxidizer, combustion does not occur.
If you're <i>still</i> paranoid, use a detachable meter.
I've actually fired my gun this way intentionally. With the bottle-side valve open, I set the pressure. I then turned on my stungun ignition and opened the chamber-side valve. As soon as the mix was ignitable, it ignited. As there was no more oxygen left to burn in the chamber, there was no more flame. Propane continued to flow into the chamber without combusting, even with the stungun still firing several nice fat sparks per second.
In summary: Know what you're talking about before you claim to know what you're talking about, or someone who <i>does</i> know what they're talking about <i>will</i> call you on it.