Fresh air control in a mushroom tunnel is by adjustment of the ventilation damper in the air handling unit, and this has a major impact on the climate of the tunnel. Damper angle affects the proportion of fresh to recirculated air, i.e. the mixing ratio. The primary objective of this study was to determine the mixing ratio as a function of the damper angle, thus facilitating predictive climate control strategies. Measurement of mixing ratio was achieved by psychrometric measurement of the fresh, recirculated and mixed airflows. A full range of damper angles, from horizontal to vertical, i.e. 0–908, at 98 intervals were used. Compared to mixing ratio models from the literature, results showed that the effect of damper angle on mixing ratio can be better modelled by an exponential relation, but a much better model is a sigmoid. The benefit obtained for an exponential model was a fit improvement of 37%, but a sigmoid model gave a 583% fit improvement. In practice, model parameterization could be by low-cost automatic calibration during periods when the tunnel is unoccupied, subject to the constraint that a minimum psychrometric distance between the fresh and recirculated airflows is maintained during data collection.