Definition
An oxygen delivery system that supplies oxygen to the pilot only when they inhale (on demand), and automatically mixes the oxygen with cabin air in a ratio that varies with altitude. At lower altitudes the regulator dilutes the oxygen with more cabin air; as altitude increases, the regulator delivers a higher percentage of oxygen until, near the system's altitude limit, it delivers nearly 100 percent oxygen. A selector typically allows the pilot to override the dilution and deliver 100 percent oxygen on demand.
Plain English
A mask system that gives you oxygen only when you breathe in, and automatically blends it with cabin air — less oxygen at lower altitudes, more as you climb higher.
Context Anchor
Seen in high-altitude oxygen equipment discussions and when checking the oxygen system or mask settings before flight.
Derivation
‘Diluter’ refers to the regulator diluting (thinning) the pure oxygen with cabin air. ‘Demand’ means oxygen flows only on the pilot's demand — that is, when they inhale. The two words together describe exactly how the system behaves: it dilutes, and it delivers on demand.
Why Pilots Care
Conserves limited oxygen supply while ensuring adequate oxygen to prevent impaired judgment or loss of consciousness at altitude.
Intuition Check
Do not read “demand” as meaning the pilot manually asks for oxygen. Here, it means the system responds automatically when the pilot inhales.
Example Sentence 1
The aircraft was equipped with a diluter-demand oxygen system, so oxygen flowed only when the pilot inhaled through the mask.
Example Sentence 2
The diluter-demand regulator blended cabin air with oxygen during cruise so the small bottle lasted the entire flight.