Tissot spirometer

Tis·sot spi·rom·e·ter

(tē-sō'),
a very large water-sealed spirometer designed for accumulating expired gas over a long period of time; the counterbalancing of the bell (almost frictionless) is compensated for by the bell's change in buoyancy as it emerges from the water, keeping the contained gas precisely at ambient atmospheric pressure.

Tissot,

Jules, early 20th century French physiologist.
Tissot spirometer - a large water-sealed spirometer designed for accumulating expired gas over a long period of time.