Atmospheric pressure of Venus? I have seen the calculations on the atmospheric pressure of Venus - 90 times that of earth.  With gravity very similar to earth, how does Venus do that.  The calculations I have seen assume constant volume at a high temperature as in a closed system.  Wouldn't Venus be an open system?  In an open ended furnace, as the temperature goes up, the pressure stays the same, the air density goes down.
 A: You're largely correct that when temperature goes up pressure stays the same, though with evaporation, that's not entirely true.  Surface water evaporation can increase the atmospheric pressure slightly though that's not an issue on Venus cause there isn't any surface water.  Rising temperature could significantly increase atmospheric pressure on Earth in a few hundred million or billion years or so in the future, unless a technologically advanced species figures out a way to move the planet away from the sun or shield it with giant space mirrors or something to that effect.
Venus atmospheric pressure has to do with how much atmosphere there is.   If the oceans were twice as deep (say, they had twice as much water), the pressure on the bottom of the ocean would be twice as much.  That's the basic principal.  Pressure is in simple terms the mass of the atmosphere above you.   14.4 lbs per square inch means a column of atmosphere 1 square inch weighs about 14.4 lbs.
Venus has about 99 times as much atmosphere by mass as Earth, but smaller surface area and lower gravity, it works out to 90 times the pressure.  Mars, by comparison, has about 150-200 times less atmosphere by mass than Earth.  (Mars Atmosphere varies quite a bit since it's atmosphere can partially freeze during winter).
The reason why Venus atmosphere is so much more massive than Earth's isn't entirely known, but part of it is that the life on Earth was able to capture a significant portion of earth's atmosphere and trap it in the ground and in living things.   Photosynthesis captures CO2 and releases O2 and the released O2 bonds with many elements on the Earth's surface, forming Granite from Basalt, and binding with the CH4 in the atmosphere, forming water.  
Trees can store converted CO2 underground, which over time becomes coal.   Ocean life forming shells is another means by which life removes atmosphere over time.  I've read different estimates of how thick Earth's atmosphere used to be.  There's a fair bit of uncertainty on that subject, but what is safe to say is a significant portion of Earth's atmosphere has been captured and is now part of the lithosphere.
Nobody knows if Venus might have once had life, but it's likely that it didn't have life long enough to drastically change it's atmosphere the way life on Earth has changed it's.
There are other possibilities, such as, Venus may have, by chance, been struck by larger comets more recently or, less large objects more recently if large enough meteors might blow some of the atmosphere off the planet.  (I don't know how much atmosphere a large meteor is likely to blow off a planet, so take that suggestion with a grain of salt).
Venus' has also gone through a recent resurfacing.   It's been theorized that because Venus doesn't have tectonic movement, Venus may have released a lot of trapped gas from below it's crust all at once when this massive resurfacing event occurred.  (though, personally, I rather like the Venus had a moon that broke into a ring system that spiraled into the planet's surface a few hundred million years ago idea better (credit to Local fluff for that one), but I've not read about that anywhere else, so that's probably a bit of a stretch.
My personal hunch is that it's some combination of all three.  (if I missed any, feel free to add).
A: It's related to the barometric formula. It's a (pseudo)exponential function of gas stacking.

CO2 and SO2 are 3-4 times heavier than N2 and O2 per molar weight. Adding this to the barometric formula allows for a far higher surface temperature.
The light elements are carried away from the stratosphere by solar wind, because Venus lacks a major magnetic field.
