 Spirometer

A spirometer is an instrument for measuring gas volume changes associated with breathing. Its output supports spirometry. A displacement design follows the movement of a gas-containing chamber; a flow-measuring design records volumetric flow rate and integrates it. In a pneumotachograph, a measured pressure difference across a flow-resistive element supplies the signal used to estimate volumetric flow rate.

For an ideal piston with cross-sectional area $A$ and displacement $x$, the transferred volume is $V=Ax$. For a flow-measuring instrument,These are different measurement chains for the same volume change. The 2019 primary technical statement explicitly discusses volume and flow instruments and their quality assurance.^[ref]

An original error model makes integration drift visible. If a flow sensor reports $Q_m(t)=gQ(t)+b$, with constant gain $g$ and zero offset $b$, its integrated result isAn offset of $0.020\mathrm{L}{\mathrm{s}}^{-1}$ contributes 0.020 L during one second but 0.20 L during ten seconds. It can therefore change a long-expiration total more than an early timed volume, even though the flow offset is constant. A gain error instead scales both in proportion when no offset is present.

Spirometer calibration verification uses a known gas transfer to test the assembled measurement chain. The 2019 statement specifies at least daily verification with a 3-L syringe at several flows and a measured-volume accuracy requirement of ±3%.^[ref] For a stipulated ideal 3.000 L transfer, a report of 3.090 L differs by 3%; identical total volumes at different transfer speeds can also expose a flow-dependent response.

Measurement calibration, leak checks and correct timing address different errors. Agreement with a syringe transfer establishes a specific instrument response; it does not by itself show that a person's breathing maneuver was complete or physiologically comparable to another attempt.