SpudFiles

Do people use metering pipes for additional air or do they just pressurize directly from their air source?

If they use metering pipes, is it the same pipe used for the fuel or do they have a second pipe for air?

The only reason people use metering pipes is because the amount of propane needed is so minute that it wouldn't show up accurately on a gauge if put directly into the chamber. There is not that issue with the air, since you are putting large amounts in, and it doesn't have to be so accurate.
The simple answer: People just pump it straight in, though pumping through the metering pipe would terbulize (yes that is a word. *shifty eyes* ...well now it is) the propane better in the chamber.

I realize the issue with injecting fuel. I was just curious if people worried about similar precision with their air. I mean, it may have a larger margin of error, but it DOES matter.

nobody to my knowledge is that nitpicky, unless you were to say, use O₂ from a tank, then maybe?

There are a few incidences that come to mind of this being used, but both were metering pure O₂. Spudblaster's oxy/MAPP hybrid, and Dumbascii's Taternator II.

My metering system uses gauges to measure chamber pressure, one for propane, which is then closed off before the air is added. With accurate enough gauges, it would be far cheaper than the monstrous meter pipes that would be required for the Pipe Dream.

Considering the budget of this project, I don't imagine that precision gauges would be that bad. If it is only going to be used at a maximum of 3x or so, you could just use a grade 1A or better 0-3 psig gauge for the propane, and a similarly accurate 0-45 psig gauge for the air. If you could afford even higher accuracy gauges, it could be more accurate than a regular meter, and either way involves far less hassle.

Here's a picture of my metering setup:



The ports, from left to right are: chamber connection, air input, fuel input, air pressure gauge (0-300psig, grade 1A), propane pressure gauge (0-15psig, grade 1A).