The main objective of this paper was to develop a mathematical model for predicting the optimum vacuum pressure of a precision vacuum seeder using onion seed properties, vacuum characteristics and the hole geometry of seed plate. The onion seed properties were linear dimensions (length (L), width (W) and thickness (T)), one thousand seed mass (m), seed cross sectional area (As), projected area (Ap), geometrical mean diameter (Dg), sphericity (f), seed density (ρs), repose angle (q), the dynamic coefficient of friction (m), terminal velocity (Vt) and the drag coefficient, (Cd). The vacuum characteristics were the vacuum velocity in general flow (Vv), vacuum density at normal temperature (rv), vacuum pressure at normal temperature (Pv) and the Reynolds¢s number (NRe). The hole geometry of seed plate were the effective hole diameter (Do), the radius in sink flow (R) and the conical entrance angle (a).
Experiments were carried out under laboratory and soil bin conditions. Laboratory experiments were conducted to determine the onion seed properties. Engineering calculations were performed for estimating the vacuum characteristics and also for calculating the hole geometry of seed plate. On the other hand, soil bin tests were carried out to verify the accuracy of developed model at various levels of blower (vacuum pump) speeds using three seed plates with different hole diameters. The blower speeds (BS) used were 4000, 4500, 5000 and 5500-rpm and the different hole diameters of seed plates were 0.8 mm, 1 mm and 1.2 mm.
Results indicated that the developed model could satisfactorily describe the parameters affecting on determining the vacuum pressure in the hole with correlation coefficients ranged from 0.97 to 0.99. It also showed that the final model could be used for predicting the optimum vacuum pressure of a precision vacuum seeder of onion seeds with an efficiency of 0.99