In refrigeration we use PSI (pounds per square inch) as a measure of pressure, but we often show it on different scales. PSIA (pounds per square inch absolute) is a scale that shows the pressure relative to a pure vacuum. PSIG (pounds per square inch gauge) is a scale that shows the pressure relative to the ambient atmospheric pressure at sea level.
The difference between PSIA and PSIG is a little less than 15PSI due to the atmospheric pressure. That doesn’t seem like a lot of pressure, does it?
Here’s a video showing a cannon that uses that atmospheric pressure to destroy a watermelon.
