The pressure involved in medical oxygen decompression cylinders can be divided into two main parts: the pressure inside the cylinder and the output pressure after decompression.
Summary: Two key pressures
Cylinder pressure (high pressure): This is the compressed oxygen pressure stored in the oxygen cylinder itself, typically 15 MPa (megapascal) or 150 kg/cm2. This pressure is determined by the manufacturing standards and filling process of the gas cylinder.
Output pressure (low pressure): This is the safe pressure at which oxygen is delivered to the patient after being reduced by a pressure reducing valve. This pressure is very low and is usually not directly displayed as a value, but controlled and monitored through flow rate (unit: liters/minute). The safe pressure range in clinical practice is usually below 0.2-0.6 MPa, but patients do not need to pay attention to this value, only the flow rate.
Part 1: Pressure inside the gas cylinder (high pressure)
This is the actual pressure of the gas cylinder after filling, which directly reflects the amount of oxygen stored in the cylinder.
1. Standard working pressure (nominal pressure)
The most common standard is 15 MPa (approximately equal to 150 kgf/cm2). This is the standard operating pressure for medical oxygen cylinders as stipulated by Chinese and many international standards. You can find this value on the steel stamp of the gas cylinder.
There are also some older or special specification gas cylinders, which may have a working pressure of 12.5 MPa or 20 MPa, but 15 MPa is currently the absolute mainstream standard.
2. Relationship between pressure and cylinder capacity
The capacity of a gas cylinder is determined by its “water capacity” (such as 4 liters, 10 liters, 40 liters).
Oxygen storage capacity (volume of oxygen at atmospheric pressure)=cylinder water volume x pressure
For example, a 10 liter gas cylinder has an oxygen storage capacity of approximately 1500 liters (at atmospheric pressure) at a pressure of 15 MPa. That’s why it can last for a long time.
How to check the pressure inside the bottle?
The gauge with a larger range on the pressure reducing valve (high-pressure gauge) is used to display the remaining pressure inside the bottle.
When the pointer is at full range (pointing to 15 MPa): it indicates that the gas cylinder is full.
When the pointer drops: it indicates that the oxygen inside the bottle is constantly being consumed. The reading of the pressure gauge is directly proportional to the remaining oxygen level. When the pointer drops close to zero, it needs to be refilled..
Part 2: Output pressure after depressurization (low pressure)
This is the core function of a pressure reducing valve, which lowers high-pressure oxygen to a safe and usable level.
1. Function of pressure reducing valve
The pressure reducing valve automatically reduces the high pressure at the inlet (such as 15 MPa) to a stable and lower output pressure through a spring and diaphragm structure inside. This output pressure is usually a pre-set fixed value.
What is the output pressure?
For medical oxygen inhalation, the output pressure of the pressure reducing valve is usually set within the range of 0.2-0.6 MPa (approximately 2-6 kgf/cm2). This pressure is sufficient to overcome the resistance of the oxygen supply pipeline and drive the humidification bottle, while also being very safe.
Important reminder: As a patient or family member, you do not need to adjust or care about the specific value of this output pressure at all. What you need to care about and adjust is the next parameter – traffic.
3. From output pressure to flow rate – the role of flow meters
After the pressure reducing valve ensures stable output pressure, oxygen will flow through a flow meter (the transparent conical tube with a float).
You can change the size of the airflow channel by rotating the knob to precisely control the flow rate of oxygen, measured in liters per minute (L/min).
The “flow rate of 2 liters per minute” instructed by the doctor refers to adjusting the flow meter to align the center of the float ball with the position of the scale “2”.Summary and Pressure Process Overview
To help you understand more clearly, the pressure change process of oxygen from the cylinder to the patient is as follows:
High pressure inside the gas cylinder (15 MPa)
↓ [Reduced pressure through pressure reducing valve]
Stable output low pressure (approximately 0.2-0.6 MPa, which users do not need to adjust or care about)
↓ [Controlled by flow meter]
Safe and on-demand clinical flow rates (such as 1L/min, 2L/min, which users must precisely adjust)
↓ [Humidification through humidification bottle]
Patient Inhalation
Core points and safety reminders
Focus on flow rate, not output pressure: The only thing that patients and their families need to operate and pay attention to is the liter/minute (L/min) value displayed on the flow meter, and ensure that it complies with medical advice. This is a guarantee of the effectiveness and safety of medical oxygen therapy.
Be cautious of high pressure: The 15 MPa pressure inside the cylinder is extremely high, and it is necessary to operate it correctly to prevent the cylinder from tipping over or hitting the valve.
Prohibition of grease: It is absolutely forbidden for any grease to come into contact with the high-pressure oxygen system, otherwise it may cause severe combustion or even explosion.
Slowly open the valve: When opening the valve of the gas cylinder, it must be done slowly to avoid the high-speed airflow instantly impacting the pressure reducing valve and damaging the internal structure.