[rpbarry58]

 My first post here...please be kind.
  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 oxide, 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. 

[Borek]

My guess is that these 33% numbers are based on the tank volume, not on the amount of nitrous oxide.

Let's say you start with compressed air at 0 °C - that's 273 K. For a given sample of an ideal gas in a constant volume tank pressure is directly proportional to the temperature expressed in absolute scale. Going down to -90 °C is equivalent to lowering temperature (and in effect pressure) by one third, which nicely fits the 33% from the description.

[rpbarry58]

Many thanks for your reply...so the only way to decrease the required compressed air reserves by 50% would be to reduce the size of the tank by 50%? 
Looks as if I'll have to use the two 7L cylinders, in that case. 

[Borek]

Many thanks for your reply...so the only way to decrease the required compressed air reserves by 50% would be to reduce the size of the tank by 50%? 
Looks as if I'll have to use the two 7L cylinders, in that case.

That would be my understanding.