Nitrous oxide/compressed air ratio (noob ?)

[rpbarry58]

My first post here...please be nice.

I'm in the process of restoring/rebuilding a 1945-era nitrous oxide injection system designed in Germany during WW2. It uses an 85L insulated nitrous oxide tank which is not pressurized by the nitrous oxide evaporation itself, but uses compressed air for injection. The tank is rated at 8 atu, but a safety valve vents at 6 atu. The compressed air is stored in two 7L bottles at 150 atu and runs through a pressure reduction valve to bring it down to 4.5-5.5 atu before entering the tank. I have a stack of captured paperwork and notes regarding the system's design/use and one of the comments reads (basic translation): "Compressed air quantities for the system are in triplicate volume to the supplied quantity of (nitrous oxide) at a calculated operating pressure of 5.5 atu. It is estimated that 33% will be lost due to a sudden drop in temperature to minus 90*, 33% lost by absorption, and the remaining 33% usable for injection." My question is perhaps an uneducated one, but is as follows: would these calculations be relative to the amount of nitrous oxide in the tank, or relative to the interior volume of the tank itself? It would be far too expensive for me to actually fill this thing with 85L of nitrous, especially since it has an evaporation rate of roughly 25% every 12 hrs., so I am thinking of using more like 4 liters; so would I still require both of the 7L compressed air bottles for the injection or could I get away with just one? Or does the inner volume of the tank dictate the need for 14L@150 atu total compressed air reserves? This system was designed for continuous use, and not for very short bursts of injection which is how I plan on using it.

I have diagrams and pictures if needed for an accurate answer to my question.

[John Cuthber]

My first thought is that if the tank loses something like 20 litres of N₂O every 12 hours you had better keep it outside.

[rpbarry58]

There are 3 vent lines from the system leading outside...one 25mm-diameter ventilation line which is opened when the tank is filled, one 10mm vent line from a hand valve which is supposed to stay open between filling the tank and is closed when the tank is about to be pressurized with the compressed air, and a 10mm vent line from the safety overpressure valve. Both 10mm lines are plumbed into the main 25mm vent line, which leads outside. But yeah, the system looses 1/2 it's capacity in 24 hours...a trade-off for not having a huge tank pressurized to 900+ psi, and having a fixed injection pressure vs. an injection pressure affected by ambient temperature of the tank itself (in the interest of stoichiometry; this system provided extra oxygen for combustion).