The temperature/pressure behavior of N20 when it is above the critical temperature (Tc - 97.5°F / 36.4°C) is of major importance in any discussion about heating or over- pressurizing nitrous bottles and system components.

In the context of preventing over-pressurization of bottles and nitrous systems, it is of primary importance.

Below Tc, in the “normal” range of operating pressures, the pressure is dependent only on the temperature. Bottle pressure at a given temperature is the same if the bottle is 50% or 100% full.

Above the critical temperature (Tc), N2O pressure becomes dependant on not only the temperature but also the

__mass contained__and the

__volume of__the container.

Above Tc (more than 97.5°F), more or less mass N2O in the same volume bottle will have more or less pressure at the same temperature. (This TxM/V = P behavior of fluids in the super-critical state is common to all fluids, not just nitrous.)

A small increase in N2O mass, such as from over-filling, will dramatically elevate bottle pressure at temperatures above Tc (97.5°F).

The following information is extrapolated from a calculator for this purpose in the NIST (http://www.nist.gov/index.html ), which is a .gov domain. Presumably, the information is reasonably accurate in a .gov website. The pressures calculated there are equivalent to the pressures calculated with the physics math in the permanent memory of a HP48GX scientific calculator.

Confusingly, the HP calculator and the several sources of information in several physics books I have and those available on the web are not in exact agreement of the calculated pressures. However, they all are similar and show the same trend of rapidly increasing pressures with increasing temperatures above Tc. The most important common fact they all agree on is the extreme pressure sensitivity to small amounts of additional mass in the bottle at a given temperature above the critical point.

At the following above critical temperatures, with the following amounts of N2O in a standard 10 lb. bottle (5.9L volume) the pressures can be:

9 lbs. N2O @ 110°F = 1570 PSI

10 lbs. N2O @ 110°F = 2150 PSI

11 lbs. N2O @ 110°F = 3350 PSI

9 lbs. N20 @ 120°F = 1850 PSI

10 lbs. N2O @ 120°F = 2550 PSI

11 lbs. N2O @ 120°F = 3850 PSI

9 lbs. N2O @ 130°F = 2100 PSI

10 lbs. N2O @ 130°F = 2940 PSI

11 lbs. N2O @ 130°F = 4350 PSI