oximeter

noun2

1. •
   A device that measures the quantity of oxygen in something, particularly air in the atmosphere.

   • EUDIO′METER. [...] An instrument by which the quantity of oxygen and nitrogen in atmospheric air is ascertained: an oxymeter. — 1836
2. •
   A device that measures the oxygen saturation of arterial blood.

   • At 8 to 10 G, ear oximeter readings may fall to 75 percent saturation. The lungs, afterward, may show some atelectasis. — 1967