The Gas Test
Ideal Gas Law Formula Shmormula. I wanted to see for myself what different pressure changes I would see between N2, "air", and CO2 so I came up with my own test. I decided I could not use tires for comparison testing because of the numerous potential variables like stem leaks, bead seals, tread damage, flaws in the rubber, etc. I also wanted to test the gases in the most extreme temperature range as possible, as quickly as possible so fitting the gases in the freezer would also be a plus. Instead of tires I used high pressure aluminum bottles fitted with a valve and a pressure gauge. All were leak-tested at 600 PSI to ensure a leak-free test vessel. I did two tests; one at a low pressure representing normal car or light truck tire pressures and the second at a high pressure representing RV and tractor tire pressures. My findings are nearly identical to a pressure calculator created by Martin Shmaltz of Boston University Physics department. The calculator can be found by clicking here (note how there is no input for gas type). Here are my results:
Low Pressure Test
Figure 1 - I used an infra red surface thermometer to measure bottle temperatures. Each bottle was allowed to stabilize to temperature for at least three hours to ensure that the gas temperature was the same as the surface temperature. Our freezer got our bottles down to -11ºF in this test for a good starting temperature.
Figure 2 - This photo shows the three bottles fresh out of the freezer at -11 degrees F and all with a starting pressure of 40 psi. Almost immediately after removal from the freezer the bottles started to frost up on the surface. Each bottle is marked with its gas contents (CO2, N2, Air).
Figure 3 - Our shop temperature was 88ºF so this became our next test temperature. All bottles were allowed to stabilize at this temperature for at least three hours. As you can see the pressure on the gauges of all of the bottles is virtually identical at 52 PSI. So far all of the gases have increased in temperature by 99ºF and in pressure by 12 PSI.
Figure 4 - We found out that if you stick a black aluminum bottle on top of a steel car hauler in the afternoon sun for a few hours you can get some damn hot bottles. You are seeing a bottle temperature of 117.3ºF