Large ones are often used for babies who need special equipment. Oxygen passes through warming coils at a controlled rate to provide gaseous oxygen for administration. Large stationary units are stored in the home and can be used to refill smaller, lightweight versions of the larger unit. How long liquid and cylinder systems last before refilling depends on the amount of oxygen a person uses. Conserving devices extend the length of time.
Oxygen systems deliver oxygen continuously during inspiration and exhalation.
Conserving devices can be programmed to deliver oxygen during inspiration only, therefore reducing the amount wasted during exhalation. You can ask the Assistive Devices Branch about the different types of conserving devices available.