Note: The content of this blog was updated October 19, 2021 to provide more information for the reader.
The applications engineers here at Teledyne Hastings discussed topics for our blog. We all agreed that one of the more frequent questions fielded, involves the units used to measure vacuum levels. We find that the technicians who use their vacuum systems daily often seem to develop a sixth sense about the “health” of their systems. They know something isn’t quite right when the base pressure (or rate of pressure change) is not what they expect. So when pressure measurements are inconsistent from batchtobatch, that is the time when the user stops to ask the meaning behind the data that their vacuum measurement instrumentation is providing.
Measuring Vacuum Pressures
A vacuum exists when there is negative pressure, or when there is system pressure that is less than atmospheric pressure. Manufacturing processes generate different levels of vacuum when operating at peak efficiency for a given vacuum application. Absolute vacuum is the absence of all matter. Atmospheric pressure, also known as barometric pressure, is the pressure due to earth’s atmosphere. Atmospheric pressure is 760 Torr or 14.696 psia at sea level and changes with altitude. Generally speaking, the vacuum pressure scale is bookended by absolute vacuum pressure in the “ultrahigh” vacuum range and atmospheric pressure at the “rough” vacuum range. It should be noted that absolute vacuum, or perfect vacuum, is never truly attained.
Most users know that vacuum is commonly measured using units of pressure. There are a few different systems of pressure measurement and this blog will discuss those most commonly used. In Armand Berman’s book, Total Pressure Measurements in Vacuum Technology, pressure unit systems are divided into two categories: “Coherent Systems” and “Other Systems”.
Units for Vacuum Measurement
“Coherent Systems” of units are based on the definition of pressure (P) as the force (F) exerted on a chamber wall per unit area (A). P = F/A. The International System of Units, or SI units, is commonly used for pressure measurement. http://physics.nist.gov/cuu/Units/units.html The SI unit for pressure is the Pascal (Pa), and it is interesting to note that NIST (National Institute of Standards and Technology) published papers are always required to use SI units. Again, the SI unit for pressure (force per unit area) is the Pascal. 1 Pa = 1 N /m2.
As a unit of pressure, the Pascal is not always convenient to use because vacuum systems often operate in pressure ranges where collected data results in large numbers. For example, near atmospheric pressure, we would measure approximately 100,000 Pa. So a more convenient unit, the bar, was developed. (1 bar = 100,000 Pa)
Moving lower in pressure, it is very helpful to then use the mbar (1 mbar = 0.001 bar), which has become the predominate unit of measure in Europe, as the basis for describing pressure levels. As a specific example, look at the base pressure specification of a turbo pump, which will typically be given in terms of mbar (e.g. Base Pressure < 1 x 10^{10} mbar).
“Other Systems” of units include the Torricelli system, which is based on an experiment (shown in the diagram below) conducted by the Italian scientist, Evangelista Torricelli. In this experiment, the pressure exerted on the mercury can be shown to be P = hdg, where h is the height of the mercury column, d is the density, and g is the acceleration due to gravity.
By measuring the mercury column height, the pressure can be determined. The Torr unit (named after Torricelli) has been defined as 1 millimeter of mercury (1 Torr = 1 mmHg). This unit, as well as the use of the mTorr unit (1 mTorr = 0.001 Torr), is commonly used in the United States. Historically, pressure was sometimes described in terms of “microns”, which simply meant a mercury column height of one micron (1x10^{6} m). Note that the micron and the mTorr are the same.
Lastly, it should be noted that occasional confusion arises between the use of different, but seemingly similar, units of pressure. As explained above, the mbar and the mTorr are not the same. One mbar has the same order of magnitude as one Torr (1 mbar = 0.75 Torr).
Unit Conversions
The table below gives some conversion values between various commonly used units of pressure and vauum. A useful website for conversions:
http://www.onlineconversion.com/pressure.htm

Pa

mbar

Torr

mTorr (micron)

Atm

1 Pa =

1

0.01

0.0075

7.50

~ 10^{5}

1 mbar =

100

1

.75

750.06

~ 10^{3}

1 Torr =

133.3

1.333

1

1000.0

~ 10^{3}

1 mTorr (micron) =

0.1333

0.00133

0.001

1

~ 10^{6}

1 Atm =

101,325

1013.25

760

760,000

1

Absolute Pressure and Gauge Pressure
In conclusion, keep in mind that vacuum measurements can be referenced to ambient pressure, gauge vacuum pressure measurement or absolute vacuum pressure measurement (perfect vacuum). Absolute pressure will often be designated by the letter “a” after the unit of measure; “psia” for example. Relative, or gauge pressure, will often be designated by the letter “g” after the unit of measure; “psig” for example.
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Original Content posted Sept 22, 2014
FAQ Corner  Units for Vacuum Measurement
an overview of units used to measure pressure
Earlier this year, the applications engineers here at Teledyne Hastings discussed topics for our blog. We all agreed that one of the more frequent questions that we discuss with folks involve the units used to measure vacuum levels. We find the technicians who use their vacuum systems daily often seem to develop a sixth sense about the “health” of their systems. They know something isn’t quite right when the base pressure (or rate of pressure change) is not what they expect. So when pressure measurements are not consistent from batch to batch, that is the time when the user stops to ask the meaning behind the data that their vacuum measurement instrumentation is providing.
Now, most users know that vacuum is commonly measured using units of pressure. There are a few different sets of pressure units and this blog will discuss the more commonly used ones. In Armand Berman’s book, Total Pressure Measurements in Vacuum Technology, pressure unit systems are divided into two categories: “Coherent Systems” and “Other Systems”.
Coherent Systems of Units are based on the definition of pressure (P) as the force (F) exerted on a chamber wall per unit area (A). P = F/A. The International System of Units, or SI units, is commonly used for pressure measurement. http://physics.nist.gov/cuu/Units/units.html The SI unit for pressure is the Pascal (Pa). It interesting to note that at NIST (National Institute of Standards and Technology), published papers are always required to use the SI set of units. Again, the SI unit for pressure (force per unit area) is the Pascal. 1 Pa = 1 N /m^{2}.
Now, the Pascal as a unit of pressure is not always the most convenient because vacuum systems are often operating in a range of pressures where we would need to collect data using large numbers. For example, near atmospheric pressure, we would measure approximately 100,000 Pa. So a more convenient unit, the bar, has been derived. (1 bar = 100,000 Pa)
Moving lower in pressure, it is very helpful to then use the mbar (1 mbar = 0.001 bar). So many vacuum users, especially in Europe, use the mbar as the basis for describing pressure levels. As a specific example, look at the base pressure specification of a turbo pump, it will be given in terms of mbar (e.g. Base Pressure < 1 x 10^{10} mbar).
Another system of pressure units is based on the Torricelli experiment (shown in the diagram). In this experiment, the pressure exerted on the mercury can be shown to be P = hdg, where h is the height of the mercury column, d is the density, and g is the acceleration due to gravity.
By measuring the mercury column height, the user can determine the pressure. The Torr unit (named for the Italian scientist Torricelli) has been defined to be 1 millimeter of mercury (1 Torr = 1 mmHg). This unit is very common, especially in the United States. It is also common to use the mTorr (1 mTorr = 0.001 Torr). Many years ago, pressure was sometimes described in terms of “microns”, which simply meant a mercury column height of one micron (1x10^{6} m). Note that the micron and the mTorr are the same.
One last word about the units used to measure vacuum: on occasion, there is confusion between pressure units. As we have seen above, the mbar and the mTorr are not the same. One mbar has the same order of magnitude as one Torr (1 mbar ≈ 0.75 Torr). The table below gives some approximate conversion values. A useful website for conversions:
http://www.onlineconversion.com/pressure.htm

Pa

mbar

Torr

mTorr (micron)

Atm

1 Pa =

1

0.01

0.0075

7.50

~ 10^{5}

1 mbar =

100

1

0.75

750.06

~ 10^{3}

1 Torr =

133.3

1.333

1

1000.0

~ 10^{3}

1 mTorr (micron) =

0.1333

0.00133

0.001

1

~ 10^{6}

1 Atm =

101,325

1013.25

760

760,000

1

Douglas Baker is the Director of Sales & Business Development of Teledyne Hastings. Antonio Araiza prepared the Torricelli experiment drawing. Antonio is the head of Technical Documentation at Teledyne Hastings (and is among the best soccer referees in the Commonwealth of Virginia).