The amount of air that can be breathed in is measured by a Spirometer. Spirometers can also be used to determine the gas flow rates if the volume signal is separated, either electrically or manually, from the volume-time traces. The water-sealed bell spirometer is the most precise model. This counterbalanced, underwater-sealed bell is upside down. The displacement of the counterweight and the associated pen is directly linked to the change in volume, assuming the bell is a perfect cylinder. It is possible to calculate the rate of change of volume and, consequently, the expiratory and inspiratory flow rates if the recording kymograph is configured to move quickly.
Although this kind of Spirometer is by nature extremely straightforward, there are a few safety considerations that should be taken when using it. The bell will typically have a diameter that corresponds to the calibration on the recording device or the provided paper. Nonetheless, a check should be performed by infusing known gas volumes. Although the volumes within the lung are at body temperature and atmospheric pressure that is saturated with water vapour, the readout provides volumes at ambient pressure and temperature (BTPS). This should be adjusted appropriately by using the gas laws or the appropriate tables because it is often 10% higher than the measured volume.
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