The McGill Physiology Virtual Lab

Respiration Laboratory

Pneumotachometry> Recording airflow
  Many important aspects of lung function can be determined by measuring airflow and the corresponding changes in lung volume. Airflow can be measured directly with a pneumotachometer and a transducer.
A pneumotachometer converts the flow of gases through it into a proportional signal of pressure difference on either side of a central mesh whose design ensures a signal linearity over a range of flow rates with a minimum dead space.


The setup will consist of a flow head (pneumotachometer) and a transducer which will integrate volume from flow.
The flow head  

In this type of pneumotachometer, flow is derived from the pressure difference over a small, fixed resistance, offered by a fine metal mesh inside the plastic cone. The trumpet-like shape of the pneumotachometer is designed to achieve laminar flow over a wide range of flows (up to 12 L/sec). (When high flows give rise to a turbulent flow pattern, the pressure drop across the resistance would change more than proportionally with flow). Two small plastic tubes attached on either side of the mesh transmit the pressure difference across the mesh to the differential pressure transducer.
The transducer converts the pressure signal into a changing voltage that is recorded by the PowerLab and displayed with the Chart software. The volume, V, is then calculated as the integral of flow:

This integration represents a summation over time; the volume traces that you will see in the Chart View during the experiment are obtained by adding successive sampled values of the flow signal and scaling the sum appropriately.  The integral is initialized to zero every time a recording is started. 

The difference in the temperature of the ambient air and the air exhaled during expiration must be considered in accurate measurements of tidal volume. Since the volume of gases depends on temperature, recorded tidal volume has to be corrected accordingly. Correction must be used to convert flow and volume measured at ambient conditions to the conditions within the lungs.  The ambient conditions are called ATPS (Ambient Temperature and Pressure, Saturated with water vapour).  The conditions within the lungs are called BTPS (Body Temperature and Pressure, Saturated). 
Conditions affecting pneumotachometry  
The volumetric accuracy of pneumotachometry is primarily dependent on the flow condition. Day-to-day variation in ambient temperature and thus pneumotachometer temperature will affect pneumotachometer sensitivity as a result of change in gas viscosity. Moisture accumulation in the flow head, and non-ideal distribution of air flow across the wire mesh, may result in a drift in the zero setting. This component of drift is minimized by the use of disposable droplet filters.
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