Oxygen FAQ

Up to date, expert answers to frequently asked questions (FAQ) about oxygen supply systems, respiratory care and pulse oximetry written by OCC & collaborators.


Ventilator connections vary but NIST connectors are common. Most ventilators are supplied with a high pressure hose with a NIST fitting on one or both sides and the other end of the hose with a fitting suitable to match the oxygen source (e.g. Shrader quick release)

Low pressure outlets are most commonly attached to, and driven by, a flowmeter attached to a pressure regulator as shown, these types of outlets are simple push fit and similar to the outlets on oxygen concentrators (e.g. use smooth bore oxygen tubing)

  • Without access to an oxygen analyser it can be difficult to determine the gas contents in a cylinder. International standards for color of cylinders are not straightforward.
  • Most cylinders are filled using pressure swing adsorption (PSA) oxygen plants and thus have a maximum FiO2 of 95%  and often considerably less.
  • Older oxygen cylinders may be black with a white top. Outside of these standards many variations also exist.  All the cylinders in the images below contained oxygen.
  • Oxygen analysers should be used to determine gases before use. Cylinders require routine testing and cleaning by certified technicians. Outlet fittings should be examined for damage or corrosion.

Most oxygen regulators will have a gauge attached, a full J size oxygen cylinder full will have a weight of 78kg (172lb) the oxygen contained in the cylinder weighs approximately 9kg (20)lb, Therefore gauging the volume of oxygen in a cylinder is difficult.

Long story short: If you have an oxygen plant that can fill and produce 32 cubic meters per hour, can expect 100-120 cylinders (42L) to be filled in a 24-hour period. Manufacturer gives you rate of capacity if operating every minute, every hour. Assume 80% in real life. Plants are also rated at 1000ft above sea level or lower – much of Sub Saharan Africa is above this so have to derate the plants significantly.

Also, need to consider what size of cylinder you are using – too small, too large cylinders not good – middle is where you get best economy and life span.

Additional details for understanding these conversions:

Cylinder sizes are commonly referred to by volume of liquid (water) such as 42 Liters. Then you must covert the liquid liter size to gas volume.  For example, a cylinder that is 42 Liters (liquid) actually contains 6.287 (or round to 6.3) CM of oxygen in a gas state, when the cylinder is filled under pressure to 152 Bar (or 2200 psi).

PSA Plants are built to a specification that identifies a “rated” capacity of production. For example, the plant is rated to produce 16 Cubic Meters per hour.  Therefore, you may calculate that 24 hour in a day X 16CM Per hr. = 384CM per day.  Once the total CM per day is understood you can divide by the common size of cylinder to know how many cylinders of a given size can be produced per hr or per day.  However, it is not likely that the plant will operate every minute of every hour, therefore it is reasonable to assume 80%-90% of the rated capacity will be the actual working capacity. When a PSA plant is ordered, the manufacturer will need to know the location where the plant is to be installed to design the plant with consideration to altitude above sea level, and environmental common temperature range.

FAQ by Assist International

Ventilators without a turbine or compressor generally require both high pressure oxygen (green) and high pressure air (yellow) input to function appropriately, and cannot function at all without at least one of these

Ventilators with a turbine or compressor have the ability to entrain room air directly without a compressed air source; and have variable gas inputs depending on the manufacturer, including the ability to have some combination of:

  1. low pressure oxygen (e.g. from a portable concentrator), via common smooth bore oxygen tubing (this may require a reservoir to augment FiO2 and an adapter to connect to the device)
  2. high pressure (55psi/4bar) oxygen (from central pipes or a cylinder)
  3. high pressure air (usually not, as the turbine or compressor provides this)

High pressure oxygen sources are capable of delivering oxygen at ~50psi/4bar to a device. These include oxygen cylinders (via regulator), oxygen plants via a compressor, liquid oxygen (via vacuum insulated evaporators) and very few portable oxygen concentrators (via additional compressor). High pressure oxygen is required for most ventilators, high flow nasal cannula and non-invasive positive pressure ventilators when taking care of critically ill patients.

Low pressure oxygen sources deliver oxygen at far less than <50psi/4bar. These include oxygen from a low flow flowmeter or a portable oxygen concentrator. Generally, these cannot deliver high enough oxygen concentrations to take care of severely hypoxemic patients.

***Some ventilators (e.g. LTV2200, Zoll 731) can operate with either high pressure or low pressure oxygen input. Other ventilators (e.g. PB560) may only be capable of utilizing a low pressure oxygen input. Of note – when utilizing low pressure oxygen input, some ventilators may not be able to provide high enough concentrations of oxygen to care for critically ill patients. Check manufacturers’ reports for maximum oxygen concentration delivery.

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