Effects of temperature and composition on the viscosity of respiratory gases

The steady-state sensitivity of resistance pneumotachographs is proportional to viscosity. Dynamic characteristics of pneumotachographs, pressure transducers, and mass spectrometers are also viscosity dependent. We derive linear equations to approximate the viscosities of O₂, N₂, CO₂, H₂O, He, N₂O, and Ar for temperatures between 20 and 40°C by using published viscosity data and a nonlinear extrapolation equation. We verify the accuracy of the extrapolation equation by comparison with published data. Our linear equations for pure gas viscosities yield standard errors <0.35 μP. We also compare a nonlinear equation for calculating the viscosities of mixtures of gases with published measured viscosities of dry air, humid air, and He-O₂ and N₂-CO₂ mixtures. The maximum difference between published and calculated values is 1.3% for 10% CO₂ in N₂. All other differences are <0.38%. For saturated humid air at 35°C, a linear concentration-weighted combination of viscosities differs from our nonlinear equation by 4.9, 2.1, and 1.7% at barometric pressures of 32, 83, and 100 kPa, respectively. By use of our method, the viscosity of normal respiratory gases can be calculated to within 1% of measured values.