# Vacuum - Measurement

Measurement

The quality of a vacuum is indicated by the amount of matter remaining in the system, so that a high quality vacuum is one with very little matter left in it. Vacuum is primarily measured by its absolute pressure, but a complete characterization requires further parameters, such as temperature and chemical composition. One of the most important parameters is the mean free path (MFP) of residual gases, which indicates the average distance that molecules will travel between collisions with each other. As the gas density decreases, the MFP increases, and when the MFP is longer than the chamber, pump, spacecraft, or other objects present, the continuum assumptions of fluid mechanics do not apply. This vacuum state is called high vacuum, and the study of fluid flows in this regime is called particle gas dynamics. The MFP of air at atmospheric pressure is very short, 70 nm, but at 100 mPa (~1×10−3 Torr) the MFP of room temperature air is roughly 100 mm, which is on the order of everyday objects such as vacuum tubes. The Crookes radiometer turns when the MFP is larger than the size of the vanes.

Vacuum quality is subdivided into ranges according to the technology required to achieve it or measure it. These ranges do not have universally agreed definitions, but a typical distribution is as follows:

pressure (Torr) pressure (Pa)
Atmospheric pressure 760 101.3 kPa
Low vacuum 760 to 25 100 kPa to 3 kPa
Medium vacuum 25 to 1×10−3 3 kPa to 100 mPa
High vacuum 1×10−3 to 1×10−9 100 mPa to 100 nPa
Ultra high vacuum 1×10−9 to 1×10−12 100 nPa to 100 pPa
Extremely high vacuum <1×10−12 <100 pPa
Outer space 1×10−6 to <3×10−17 100 µPa to <3fPa
Perfect vacuum 0 0 Pa
• Atmospheric pressure is variable but standardized at 101.325 kPa (760 Torr)
• Low vacuum, also called rough vacuum or coarse vacuum, is vacuum that can be achieved or measured with rudimentary equipment such as a vacuum cleaner and a liquid column manometer.
• Medium vacuum is vacuum that can be achieved with a single pump, but the pressure is too low to measure with a liquid or mechanical manometer. It can be measured with a McLeod gauge, thermal gauge or a capacitive gauge.
• High vacuum is vacuum where the MFP of residual gases is longer than the size of the chamber or of the object under test. High vacuum usually requires multi-stage pumping and ion gauge measurement. Some texts differentiate between high vacuum and very high vacuum.
• Ultra high vacuum requires baking the chamber to remove trace gases, and other special procedures. British and German standards define ultra high vacuum as pressures below 10−6 Pa (10−8 Torr).
• Deep space is generally much more empty than any artificial vacuum. It may or may not meet the definition of high vacuum above, depending on what region of space and astronomical bodies are being considered. For example, the MFP of interplanetary space is smaller than the size of the solar system, but larger than small planets and moons. As a result, solar winds exhibit continuum flow on the scale of the solar system, but must be considered as a bombardment of particles with respect to the Earth and Moon.
• Perfect vacuum is an ideal state of no particles at all. It cannot be achieved in a laboratory, although there may be small volumes which, for a brief moment, happen to have no particles of matter in them. Even if all particles of matter were removed, there would still be photons and gravitons, as well as dark energy, virtual particles, and other aspects of the quantum vacuum.
• Hard vacuum and Soft vacuum are terms that are defined with a dividing line defined differently by different sources, such as 5 psi, one Torr, or 0.1 Torr the common denominator being that a hard vacuum is a higher vacuum than a soft one.

### Other articles related to "measurement, measurements":

Measurement - Definitions and Theories - Quantum Mechanics
... In quantum mechanics, a measurement is an action that determines the location of an object, its momentum, its polarity (if it is a photon), etc ... Before a measurement is made, the wavefunction of what is to be measured gives the range of probabilities for the outcomes of measurement, but when a measurement is accomplished that results ... The unambiguous meaning of the measurement problem is an unresolved fundamental problem in quantum mechanics ...
Inch (disambiguation) - Units
... Inch as a measurement of rain, measured by a rain gauge Inch of mercury, unit of pressure Inch of water, unit of pressure Scottish inch, Scottish unit of measurement Column inch ...
Electrical Capacitance Tomography
... distribution in the interior of an object from external capacitance measurements ... Potential applications include the measurement of flow of fluids in pipes and measurement of the concentration of one fluid in another, or the distribution of a solid in a fluid ... Although capacitance sensing methods were in widespread use the idea of using capacitance measurement to form images is attributed to Maurice Beck and co-workers ...
Correction For Attenuation
... Spearman (1904), to "rid a correlation coefficient from the weakening effect of measurement error" (Jensen, 1998), a phenomenon also known as regression dilution ... In measurement and statistics, it is also called disattenuation ... The correlation between two sets of parameters or measurements is estimated in a manner that accounts for measurement error contained within the estimates of those parameters ...
Measurement Tower
... A measurement tower is a free standing tower or a removed mast, which carries measuring instruments with meteorological instruments such as thermometers and wind velocity ... Beside measurement towers, which are equipped with meteorological measuring instruments, there are also measuring towers for measuring the radiation pattern of antennas ...

### Famous quotes containing the word measurement:

That’s the great danger of sectarian opinions, they always accept the formulas of past events as useful for the measurement of future events and they never are, if you have high standards of accuracy.
John Dos Passos (1896–1970)