Quoting Removed, click Modern View to seeIt isn't linear or even close. The best dielectric (harder to strike an arc) is the vacuum, then it gets easier to strike the arc (poor dielectric) with a near-vacuum/some air present/about 0PSI, then as pressure increased to atmospheric (14.7PSI) it becomes a better dielectric (more difficult to arc), and this situation continues along the 'more difficult' trend as pressure further increases to say a cylinder pressure of 150PSI.
As can be seen from the graphs, general ratios are about 4X better dielectric at a 300,000 feet than at 150,000 feet and about 3X better at atmospheric than at 150,000 feet and 4X better dielectric at 150PSI than at atmospheric. It isn't good to become too focused on exact values of volts/inch, because the shape of the electrodes, the place up & down the whole voltage range (i.e., 8,000v or 70,000v) will all alter the specific arc over voltage. The text book atmospheric number it typically given as 76,000 volts per inch, but no HV circuit designer would use that number in determining spacing from ground...more typically 15,000v/in at high voltages and on a printed circuit board at lower voltages & close etch spacing, typically 8,000 volts per inch. Many variables. Whole books have been written on just dielectric properties of atmosphere. Part of the explanation of the behavior of these curves is in number of molecules present and the free travel path available in which to gain energy before striking another molecule. At vacuum extreme, no gas molecules present, no molecules to collide & result in ionization, thus infinite dielectric strength and in the middle pressures, some molecules, long free travel paths in which to gain energy, resulting in ion producing collisions & poor dielectric strength. Then at high pressures, the dense molecules have very short free travel paths in which to gain the energy required to disrupt another molecule & resulting ionization, thus high dielectric strength. Various contaminants can alter dielectric strength, some for the better such as sulfur hexafluoride, some for the worse such as water vapor. Hope you find these tid bits interesting.
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Today's Featured Article - An Old-Time Tractor Demonstration - by Kim Pratt. Sam was born in rural Kansas in 1926. His dad was a hard-working farmer and the children worked hard everyday to help ends meet. In the rural area he grew up in, the highlight of the week was Saturday when many people took a break from their work to go to town. It was on one such Saturday in the early 1940's when Sam was 16 years old that he ended up in Dennison, Kansas to watch a demonstration of a new tractor being put on by a local dealer. It was an Allis-Chalmers tractor dealership,
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