= Spirometer
A \b[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,
$$
V(t)-V(0)=\int_0^t Q(s)\,ds.
$$
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.<Standardization of Spirometry 2019 Update by Brian L. Graham and others>[Equipment and Device 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 is
$$
V_m(T)-V_m(0)=g\,[V(T)-V(0)]+bT.
$$
An offset of $0.020\,\mathrm{L\,s^{-1}}$ 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%.<Standardization of Spirometry 2019 Update by Brian L. Graham and others>[Calibration and Quality Control: 3-L syringe, multiple flow rates and accuracy requirement]{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.
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