- Spirometer Definition: A spirometer is defined as a biomedical device that measures lung capacity and lung volume.
- Working Principle of Water Seal Spirometer: The water seal spirometer uses a water-filled cylinder and a bell jar to measure changes in air volume during breathing.
- Breathing Pipe Mechanism: Breathing into the spirometer moves the bell jar up and down, changing the weight’s position.
- Kymograph Production: A pen attached to the weight records breathing patterns as a graph called a Kymograph.
- Electrical Signal Conversion: The vertical movement of the weight can be converted to an electrical signal for display using a linear potentiometer.
A spirometer measures inspired or expired volume or flow as a function of time. Common spirometry results include forced vital capacity (FVC) and the volume expired in the first second (FEV₁). Simple spirometry does not measure residual volume or total lung capacity, because air remaining after complete expiration is not collected. Modern instruments are usually flow-sensing systems; the water-seal model below explains an older volume-displacement design.
Water Seal Spirometer
A water-seal spirometer has a water-filled tank and an inverted bell that can move vertically while remaining sealed by the water. The bell’s usable volume and dimensions depend on the instrument rather than a fixed 6-8 litre specification. A counterweight and pulley reduce the force needed to move the bell. A breathing tube connects the person to the gas space under the bell.
Gas entering the bell increases the enclosed volume and raises it; gas leaving reduces the volume and lowers it. Bell displacement is calibrated in units of volume. The direction of movement during inhalation or exhalation depends on whether the person breathes into the bell, from it or through a configured closed circuit.
Older instruments coupled the moving mechanism to a pen that traced volume against time on paper wrapped around a rotating kymograph drum. The trace is a spirogram; the recording drum is the kymograph.
Bell movement can also drive a transducer for electronic recording. A linear potentiometer is one possible displacement sensor, although other sensors may be used. A spirometer must preserve the direction, timing and scale of the breathing signal. A volume-displacement model measures volume directly and calculates flow from the rate of volume change; a flow-sensing model measures flow and integrates it to obtain volume. Clinical equipment also requires calibration verification, quality checks and infection-control procedures.
Other types of spirometers include the Wedge Spirometer and Ultrasonic Spirometer. Wedge-bellows instruments are volume-displacement devices. Ultrasonic and pneumotachograph instruments sense flow and calculate volume. Test interpretation depends on acceptable, repeatable manoeuvres and appropriate reference values, so a tracing alone should not be used for diagnosis.
