Thermal oxidation rates of silicon are known to be a function of time, temperature and gas flows. Recent work has focused on pressure as a variable in the process. While attention has been placed on pressure equalization between the process gases and the exhaust, this paper demonstrates that this level of control is insufficient, since the absolute pressure also plays a contributory part. This is amply demonstrated by the relationship of atmospheric pressure to the rate of oxidation. Silicon oxidation processes at high altitude locations grow less oxide than those at sea level. The variability of the prevailing atmospheric conditions then has a direct impact on the resultant oxidation thickness--which is why so often the oxidation time is adjusted on a daily rate. A system is presented that control absolute pressure in the furnace irrespective of changes in the local ambient pressure.