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Originally Posted by sanddrag
negative pressure? Is there such thing?  I thought zero pressure like the true vacuum of space is the least pressure you can have. |
Yes there is such a thing, there is +/- and 0 pressure. If you deal with a few or even call manufactures that deal with items such as these. They will tell you they will work at +/-/0 in X amount of elevation, Y amount of humidity, Z amount of temperature.
Positive pressure is a pressure within a system that is greater than the environment that surrounds that system. Consequently if there is any leak from the positively pressured system it will ingress into the surrounding environment.
However, use is made of positive pressure to ensure there is no ingress of the environment into a supposed closed system. A typical example of the use of positive pressure is the location of a habitat in an area where there may exist flammable gasses such as found on an oil platform.
Hospitals may have positive pressure rooms for patients with compromised immune systems. Air will flow out of the room instead of in, so that any airborne germs that may infect the patient are kept away.
Negative pressure is a term used to describe a pressure less than that of a surrounding fluid (such as the air). The term comes from Gauge pressure pressure gauges, which measured a pressure against air pressure. These were extensively used in steam engines because the pressure was relative to air pressure, which was the final pressure of the exhaust steam. The standard pressure gauge design (Bourdon tube) measures gauge pressure by default, making this an easy design choice. Negative pressure meant that the pipe or vessel being measured was below atmospheric pressure.
Except in special circumstances, the lowest possible pressure is a vacuum, which corresponds to -1 atm pressure. Actually what is meant is that there is a pressure difference between a region of high pressure and another region of low pressure. A room with a lower pressure than its surroundings will cause air to flow into the room from the outside whenever a door or window is opened. It is often used in a hospital setting to quarantine people afflicted with highly contagious, deadly, airborne diseases. Some player pianos used vacuum to drive the mechanism and measured things in negative inches of water pressure.