You are transporting your patient, who has emphysema, from your unit to
radiology for a CT scan. Without oxygen at 6 L/min, your patient's O2
saturation on room air drops to 75%. It will take you 10 minutes to get
there and back. The portable O2 tank you grabbed has 500 psi left in
it. Do you have enough oxygen left for the trip? See the example
below

At 500 psi with a flow rate of 6 liters per min, there are
28
minutes of oxygen left. Yes, there is enough for the trip.

Formula

My text book uses the following formula and data:

capacity (in L) / service pressure (in psi) = remaining contents (in L) / gauge
pressure (in psi)

The service capacity for an e-cylinder carrying oxygen
is 1900 psi.

The volume of oxygen in an e-cylinder is 660 liters.

Now entering the values listed above:

660 L / 1900 psi = remaining contents (in L) / gauge
pressure (in psi)

I find it easier to look at this formula this way:

0.35 x psi on gauge L/min to be delivered

The 0.35 comes from dividing 660 by 1900.

In the example above:

0.35 x 500 psi 6 L/min

= 28 min

Reference:

Nagelhout, J. J., & Zaglaniczy, K., L. (2001). Nurse anesthesia (2nd ed.). Philadelphia: W.B. Saunders Co.

*The service pressure of 1900 psi and capacity of 660 are
included in table 14-1 which references data from Standard specification for
minimum performance and safety requirements for components and systems of
anesthesia gas machines [F1161-88], Philadelphia, 1988, American Society for
Testing and Materials; and NFPA 99: health care facilities [Table C-12.5],
Quincy, MA, 1990, National Fire Protection Association, p 184. "Note that
slightly different values may be found in different sources." The
answer in example is rounded down to the nearest whole number. Calculator
also rounds answer down to nearest whole number.