Oxygen quantification & forecasting
Contributors: Sky Vanderburg, Cornelius Sendagire, MMed, Michael Lipnick, MD
Date last updated: Aug 29, 2023
The COVID-19 pandemic highlighted the urgent need for tools to support a range of users seeking to understand oxygen supply and demand. This includes for example:
- Clinicians at the bedside trying to understand how long their supply will last or how much oxygen a new delivery device might consume
- Hospital operations lead or biomedical engineer/technician trying to estimate supply capacity or how often to order refills of LOX or cylinders
- Hospital administrator trying to estimate costs
- A Ministry of Health trying to design distribution of national or regional capacity to care for a surge in patients
- Advocacy or policy teams seeking to improve access to oxygen locally or globally
- Planners trying to determine which oxygen supply systems are optimal for a given setting
Oxygen consumption is commonly estimated based on need (e.g. number of beds, gas wall outlets, historical consumption), however, such estimates have major limitations and challenges. Flow measurement systems are uncommon in many facilities including LMICs, and also may not account for true need (e.g. waste, leak, rationing etc).
Below are descriptions of some widely used tools. A comparison summary table is coming soon.
Oxygen quantification tools
Open Critical Care Oxygen Calculator
Open Critical Care Oxygen Calculator
- Target audience: clinicians, facility decision-makers, advocates
- Purpose: support rapid forecasting and rapid calculations to help non-engineers understand supply and consumption; to track facility-level consumption over time
- Quantification Level: Facility, ward or patient
- Format: online web/mobile
- Inputs: facility-level data including comprehensive list of device types, oxygen supply models; also allows input of minimal data (# beds and severity of disease) to model demand forecasts
- Outputs: oxygen demand at health facility; oxygen consumption by device/patient; graph of oxygen consumption over time (daily) by device type; data can be exported to csv or pdf
- Potential limitations: unvalidated; requires Internet
- Link to latest version (new version coming January 2023)
Oxygen quantification formulas
The formulas described below are used in the OCC Oxygen Calculator.
Manufacturer specifications must always be referenced especially when using delivery devices with bias flow, turbines and compressors.
When estimating device consumption one must always consider device leak or leak at the patient interface which is commonly encountered.
Below are cylinder constants used in the OCC cylinder duration calculations that follow:
- ‘Size C’: 0.085
- ‘Size D’: 0.17
- ‘Size E’: 0.34
- ‘Size F’: 0.68
- ‘Size G’: 1.7
- ‘Size J’: 3.40
- ‘Size K’: 3.55
Remaining Supply =
Remaining Time =
Pressure in psi and Flow in LPM
Pressure (bar) × Cylinder water volume (L) = Total gas volume (L)
* Of note: cylinder size terminology and volumes are not universally standard. You must check with the local manufacturer for cylinder capacity and size/capacity.
*Service pressure for aluminum cylinders is approximately 2000-2200 psi (137-150 bar), while service pressure for steel cylinders may vary more widely. Always check with the manufacturer for specifications.
The OCC cylinder size calculator uses volumetric calculations based on height, circumference (or width), and wall thickness to estimate volume of the cylinder (see formulas below). This calculator assumes the shape of a cylinder which will vary from the actual shape of a gas cylinder – see ISO 7866 and ISO 9809 for details. Steel and aluminum cylinders have different wall thicknesses and knowing which type of cylinder you have is necessary for accurate estimations.
Calculated cylinder volume may vary from actual or manufacturer reported cylinder volume due to differences between the entered and actual wall thickness, shape of the bottom or top of the cylinder, or the fill pressure used to define capacity, among other potential factors. See ISO7866 and ISO9809
Liquid water volume capacity: Liquid volume is given by approximating the volume of a cylinder, by
(We do not multiply the wall thickness x2, as for this estimate we account for the thickness of the cylinder base only and not the dome)
Gaseous oxygen volume: Boyle’s Law is used to convert from a known volume and pressure to another known volume and pressure. For example, if we have an E cylinder with a volume of 4.7 L water and 2000 PSI (137 bar), and convert that to gaseous volume at ambient atmospheric pressure (P = pressure; V = Volume):
Further simplified, Pressure (bar) × Cylinder water volume (L) = Total gas volume (L) at sea level. If pressure units are in PSI, then this must be accounted for (1 atm pressure at sea level = 14.70 psi):
If you have the dimensions of the cylinder, the pressure in the cylinder and know ambient pressure, then the above equations can be arranged as follows:
(Volume in liters; Pressure in any unit; diameter, wall thickness and height in mm)
Nasal cannula, facemask, non-rebreather
As used in the OCC Demand Calculator, FiO2 is assumed to be 1.0 and flow rates are adjustable in liters per minute.
High flow nasal oxygen
As used in the OCC Demand Calculator, FiO2 is adjustable (defaulted to 1.0) and flow rate is adjustable in liters per minute.