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Pressure terminology can confusion the beginner. To have a clear understanding of the many terms regarding pressure one must maintain focus on a "root understanding." Pressure is a property of a fluid at a point within that fluid. Just a step, further, the idea includes a fluid and a special "point" larger then the vanishingly small point of Newton. The "point" in mind relative to being a fluid is special. Misunderstandings of pressure abound in engineering texts. A clean start is not possible. Below some terminology regarding pressure, pressure gages and such, is discredited. The intent is to clear a path through erroneous ideas, to lead to a correct understanding. This clean-up is unavoidable and overdue.
Vacuum: As long ago as Aristotle and through time until Torricelli and today the word, vacuum (used as noun) meant and still means "space devoid of matter." Consequently wherever there is vacuum there is no fluid whatsoever, no consequences of motion of fluid particles, and no measurable or definable pressure.
Vacuum Pressure: Sometimes in writings one encounters the word-pair "vacuum pressure" or "pressure-vacuum. Vacuum, the leading word, means a place devoid of matter, whereas pressure is a condition of matter as a fluid continuum. According to modern astrophysics, there are no locations that qualify as vacuum. Satellites in Earth orbit are not surrounded by "vacuum." A synonym of the noun vacuum is the noun "nothing." But, as explained below, vacuum, when used as an adjective has meaning.
Zero Pressure: That pressure might exist at a point in a fluid and have zero as its numerical value is a contradiction. Pressure is evidence of existence of fluid. As the measured value of pressure at a location within a fluid diminishes so also, and equally as fast, does the existence of the fluid. Pressure is a continuum property. The limit of low pressure is not a pressure magnitude of zero; the limit of low pressure is "continuum destroyed, pressure no longer a property."
Absolute Pressure: Pressure at a location in a fluid is unambiguous. To place the adjective "absolute" before pressure adds nothing to its meaning and plenty to the misunderstanding of pressure. Most physics texts do not use the idea "absolute pressure." The pressure of atmospheric beside us (on average) is 101.3 kPa. Some say the atmospheric air beside us an absolute pressure of 101.3 kPa. Of what value or what extra meaning is provided by the qualification "absolute?"
Total Pressure: Return in thought to the effect of fluid particle collisions at some point. What, physically might the "parts of pressure" that should be "totaled."
Since vacuum does not exist, since pressure never has the value "0," since pressure, properly understood, equals absolute pressure and since pressure does not have "parts," the above terms are of little use, if any.
Pressure Gage A physical device (constructed and installed in equipment) the readings of which are used to determine the pressure of some fluid contained within confined spaces is commonly called a "pressure gage." With the majority of gage installations the pressure within the confined space is greater than in the room from which the gage will be read. The number indicated by the gage is not a pressure, it is a "pressure differences" (there is a difference between a pressure and a pressure difference). Confusion is avoided by calling the "number" indicated by any gage, a gage reading, G.R.. The gage reading, a positive number, tells us either of two differences which are:
(pspace - patm) or
(patm - pspace)
So which is it? Whoever installed the gage was the first to know. The "reference" of all gage installations is, virtually always, the atmospheric condition, the pressure of the region within which the person will read the gage. In operation, the gage is to tell the pressure within the system (inside some scientific apparatus or element of process machinery). The installer, knowing the range of expected system pressures select a suitable gage and dial face appropriate to the task and installs it.
The are two possibilities:
marked gage dial face pressure If with the pressure inside (within the confined space) will be less than ambient atmospheric pressure, the installer must use a gage with "vacuum" written on its dial. Then the pressure inside is atmospheric minus the gage reading ~ (pinside = patm - G.R.). More commonly gauge dials without "vacuum" written on the face are installed. The case here is "inside pressure" is greater than atmospheric:
pinside = patm + G.R.
Gage Pressure or Vacuum Pressure:.... This is incomplete.