[Rate of evaporation of tear fluid in the human compared with a physical model]

Evaporation of tears occurs by diffusion, free (thermal), and forced convection. Under resting conditions only diffusion and free convection play a role. Therefore, a device should be applied with only these two parameters working. Creating a stationary known pressure gradient of water vapor over the eye, the evaporation rate was found to be 24.8 +/- 6.8 mg cm-2 h-1 in 21 test persons (no spectacle or contact lens wearers, no known general or eye disease, average age 25.6 years). The same test arrangement over a water surface of 1.7 cm2 gave a value of 43 mg cm-2 h-1 and simplified calculation, applying Fick's law of diffusion, 54 mg cm-2 h-1. These results suggest that there is a water vapor pressure of less than half the saturation water vapor pressure of less than half the saturation water vapor pressure corresponding to 32 degree C over the surface of the eye. Calculation yields a value of 230 mg per eye per 16 hours, corresponding to 32 degrees C (50% air humidity) over the surface of the eye and 22 degrees C (50% air humidity) room temperature. Assuming a tear secretion of 0.6 g per eye per 16 hours (Schirmer), about 40% of this amount evaporates under normal conditions.