Effect of altitude on negative pressure (Vacuum)
At mean sea level, which is considered as datum, i.e. o mts., the barometric pressure is 760 mm of Hg(G) absolute.
Now as we go above the sea level the barometric pressure goes on decreasing. As per an estimate at every 1000 feet or 330mts, the barometric goes down by 1 inch.
That means as we go above sea level the pressure extracted by the atmosphere goes on decreasing.
PPI Vacuum Pumps give 710 mm of Hg (G) vacuum, at closed suction condition, at Ahmedabad site which is 100 mts above mean sea level and barometric pressure is equal to 751 mm og Hg(G).
Now suppose the same pump is taken to a new site which is 600 mts above mean sea level, what will be the effect of the vacuum produced ?
As stated earlier at every 330 mts the barometric goes down by 1 inch (25.4 mm), so at 600 mts the barometric pressure will be 714 mm of Hg(G).
That means a drop of 46 mm of Hg (G).
Now at Ahmedabad the barometric pressure is 751 mm of Hg(G), i.e 9 mm drop of barometric. We deduct this drop from 46 that we received earlier, so we get 37 mm of Hg(G).
Now this 37 has to be deducted from 710 (i.e the vacuum developed by the pump at works site-Ahmedabad), so we get 673 mm of Hg(G).
Thus the pump that was developing 710 mm of Hg(G) at Ahmedabad will develop 673 mm of Hg(G) at an another site which is 600 mts above the mean sea level.
Chart attached with this write-up will help in finding the same easily.