TL;DR
Not every breathing device is solving the same problem. Standard volumetric incentive spirometers fit basic post-op lung expansion and guided deep breathing.
Inspiratory exerciser formats can offer a different visual-feedback style. Respiratory muscle trainers are built for a different training goal than basic post-op spirometer use.
Spirometry systems fit testing and measurement, not simple home recovery breathing support.
Not every breathing device is built for the same use. Some products....
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