The portable E cylinder is the most common source of compressed oxygen gas for individuals requiring mobility or for use in emergency medical settings. These tanks allow people to maintain necessary oxygen therapy while away from a stationary oxygen concentrator. Understanding the capacity of the E cylinder and estimating its remaining duration is important for safe and effective use.
Standard Capacity of an E Cylinder
The E cylinder is a standardized size in the medical gas industry, defined by its physical dimensions and oxygen capacity. A full E cylinder is pressurized to approximately 2,000 to 2,200 pounds per square inch (psi) with medical-grade oxygen. This high pressure allows a significant volume of gas to be stored in a relatively small tank.
When released and converted to standard atmospheric pressure, the E cylinder contains between 622 and 680 liters of oxygen (about 22 cubic feet). This total capacity is a fixed value, regardless of the flow rate setting.
The volume of gas remaining is directly proportional to the pressure reading on the gauge, making pressure the primary factor used in duration calculations. The tank’s aluminum alloy construction is designed to safely contain this high pressure.
Calculating How Long the Oxygen Will Last
The duration an E cylinder lasts depends entirely on the prescribed flow rate in liters per minute (LPM). Since the total volume is fixed, a higher flow rate causes the oxygen to deplete more quickly. To estimate the remaining time, a standard formula incorporates the current pressure reading and a specific conversion factor for the E cylinder.
The formula for calculating the duration in minutes is: (Remaining PSI \(\times\) Conversion Factor) / Flow Rate (LPM) = Duration in Minutes. The E cylinder conversion factor is 0.28, derived from dividing the tank’s maximum volume (625 liters) by its maximum pressure (2,200 psi). This factor is a constant multiplier specific to the E cylinder size.
For example, if a tank reads 1,500 psi and the prescribed flow rate is 2 LPM, the calculation is \((1,500 \times 0.28) / 2\), which equals 210 minutes (three and a half hours). If the flow rate is doubled to 4 LPM, the duration is halved to 105 minutes. It is important to account for a safe residual pressure, usually 200 psi, which is the pressure at which the tank is considered empty.
Reading the Pressure Gauge and Flow Regulator
The two components used to manage the oxygen supply are the pressure gauge and the flow regulator. The pressure gauge is a dial that registers the remaining pressure inside the cylinder in pounds per square inch (psi). This gauge determines how much oxygen is left and should be checked whenever the cylinder is turned on.
The flow regulator, often a separate dial or knob, controls the rate at which oxygen is released, measured in liters per minute (LPM). This regulator converts the high internal pressure into a safe, controllable flow. The user must adjust this knob to the exact flow rate prescribed by their physician.
The gauge and regulator measure different things: one measures remaining content, and the other controls the output rate. The flow regulator maintains the set LPM until the tank is depleted or manually shut off, while the gauge shows a continuous drop in pressure as oxygen is used.
Essential Safety Guidelines for E Cylinders
Oxygen supports and accelerates combustion, so strict safety guidelines must be followed when using and storing E cylinders. Oxygen equipment should be kept at least five feet away from all heat sources, including stoves, fireplaces, and radiators. Open flames, smoking, or sparks are prohibited in any area where oxygen is being used, as a small spark can lead to a serious fire.
Cylinders must be secured at all times using a stand, cart, or appropriate holder to prevent them from falling over. If the valve post breaks off, the rapid pressure release could cause the tank to become a dangerous projectile. Additionally, petroleum-based products like oil, grease, or certain creams must never be used near the valve, regulator, or connection points, as oxygen reacts violently with these substances.