FAQ: Pressure = Vacuum
Posted: Mon Sep 29, 2008 12:06 pm
There is no such thing as a pure vacuum, only varying degrees of pressure.
We live on an air filled planet where the pressure at sea level is about 14.7 PSI or 760mm of mercury which is 760 torr. Even deep space has gas molecules in it, rare though they may be. We also live in a world where our bodies are comfortable, for the most part. Temperature, thermal conditions, can increase or reduce air pressure as it varies. Thus, all earth bound vacuums are specified against something called "standard temperature and pressure" or "STP". This is basically what is termed normal "sea level pressure" in dry air and at fixed temperature that is arbitrarily chosen as zero degrees centigrade.
For the sake of keeping this FAQ short, we will discuss only the units of pressure that we, and most books you might read, deal with. The main unit encountered is the "Torr". This is one millimeter of mercury column pressure rise. Atmospheric pressure is about 760 mm of mercury or 760 Torr under standard conditions of temperature and pressure. This has about 25 Quintillion gas molecules in each tiny cubic centimeter of air. Pressures less than STP are referred to as being at varying degrees of "vacuum".
There is no such reading as zero torr. It doesn't exist in this universe, to our knowledge. There are always molecules of gas; even in intergalactic space.
There are many pressures that various technologies would call "its" optimum vacuum level. Remember, all vacuums have a pressure associated with them. We are, therefore, trained to speak of "vacuum pressures". As such, the term vacuum literally seems a misnomer once this is comprehended. For most purposes a "vacuum" is a gas pressure significantly less than that of normal atmospheric pressure.
For a neon worker, a vacuum of 1-100mm or 1-100 torr is normal. For hard scientific vacuums, often termed "High vacuum", one needs to be at or below 10e-6 torr. This is where most vacuum tubes operate. Special, "ultra high" vacuums are found below pressures of 10e-8 torr. It is extremely difficult to achieve, hold and even measure vacuums below a pressure of 10e-12 torr. Even at this deep vacuum there are millions of gas molecules in every tiny cc of the chamber.
We often use a term in our fusion effort called the micron. This is an old term, but still used a lot by workers haunting the area called "technical vacuums". The micron is supposed to denote a millionth of an atmosphere but it falls short of that hope.
The reasoning, flawed though it may be, is that the atmospheric pressure is sorta' close to 1000 torr and one millionth of that is 10e-3 torr. This is the micron. It is merely a word for 10e-3 torr.
The traditional fusor we run and operate runs at anywhere from 5 to 20 microns or 5X10e-3 to 2X10e-2 torr of gas pressure. For us, in our work, the nice whole micron numbers with no exponentials, as in torr, are a lot easier to both visualize and verbalize. In short, the fusor works in the millionths of an atmosphere range. We have pumped out a lot of gas molecules, but still have over 10 trillion gas molecules in every cubic centimeter of our fusors.
No professional, scientific paper would, however, be accepted using microns, but scientists and workers speak in microns all the time. Microns are the region of "glow discharge" gas pressures.
You will notice there are no minus pressures for any units. All units go fractional. a deep vacuum of 10e-10 torr is actually 0.0000000001 torr or mm of mercury.
One is allowed to speak of submicrons i.e., 0.2 microns or 2X10e-4 torr. However, below this final decade, it is a no-no to speak in hundredths of microns. You must now speak in exponential torr units again as you have left the technical or glow discharge vacuum pressures far behind and are in the scientific vacuum pressure range. Down here when asked about your pressure you might just respond verbally with..."About ten to the minus seventh". One assumes the torr is the unit of choice.
Just remember that all vacuums are really just varying degrees of pressurized gas environments that are well below normal atmospheric pressure.
Richard Hull
We live on an air filled planet where the pressure at sea level is about 14.7 PSI or 760mm of mercury which is 760 torr. Even deep space has gas molecules in it, rare though they may be. We also live in a world where our bodies are comfortable, for the most part. Temperature, thermal conditions, can increase or reduce air pressure as it varies. Thus, all earth bound vacuums are specified against something called "standard temperature and pressure" or "STP". This is basically what is termed normal "sea level pressure" in dry air and at fixed temperature that is arbitrarily chosen as zero degrees centigrade.
For the sake of keeping this FAQ short, we will discuss only the units of pressure that we, and most books you might read, deal with. The main unit encountered is the "Torr". This is one millimeter of mercury column pressure rise. Atmospheric pressure is about 760 mm of mercury or 760 Torr under standard conditions of temperature and pressure. This has about 25 Quintillion gas molecules in each tiny cubic centimeter of air. Pressures less than STP are referred to as being at varying degrees of "vacuum".
There is no such reading as zero torr. It doesn't exist in this universe, to our knowledge. There are always molecules of gas; even in intergalactic space.
There are many pressures that various technologies would call "its" optimum vacuum level. Remember, all vacuums have a pressure associated with them. We are, therefore, trained to speak of "vacuum pressures". As such, the term vacuum literally seems a misnomer once this is comprehended. For most purposes a "vacuum" is a gas pressure significantly less than that of normal atmospheric pressure.
For a neon worker, a vacuum of 1-100mm or 1-100 torr is normal. For hard scientific vacuums, often termed "High vacuum", one needs to be at or below 10e-6 torr. This is where most vacuum tubes operate. Special, "ultra high" vacuums are found below pressures of 10e-8 torr. It is extremely difficult to achieve, hold and even measure vacuums below a pressure of 10e-12 torr. Even at this deep vacuum there are millions of gas molecules in every tiny cc of the chamber.
We often use a term in our fusion effort called the micron. This is an old term, but still used a lot by workers haunting the area called "technical vacuums". The micron is supposed to denote a millionth of an atmosphere but it falls short of that hope.
The reasoning, flawed though it may be, is that the atmospheric pressure is sorta' close to 1000 torr and one millionth of that is 10e-3 torr. This is the micron. It is merely a word for 10e-3 torr.
The traditional fusor we run and operate runs at anywhere from 5 to 20 microns or 5X10e-3 to 2X10e-2 torr of gas pressure. For us, in our work, the nice whole micron numbers with no exponentials, as in torr, are a lot easier to both visualize and verbalize. In short, the fusor works in the millionths of an atmosphere range. We have pumped out a lot of gas molecules, but still have over 10 trillion gas molecules in every cubic centimeter of our fusors.
No professional, scientific paper would, however, be accepted using microns, but scientists and workers speak in microns all the time. Microns are the region of "glow discharge" gas pressures.
You will notice there are no minus pressures for any units. All units go fractional. a deep vacuum of 10e-10 torr is actually 0.0000000001 torr or mm of mercury.
One is allowed to speak of submicrons i.e., 0.2 microns or 2X10e-4 torr. However, below this final decade, it is a no-no to speak in hundredths of microns. You must now speak in exponential torr units again as you have left the technical or glow discharge vacuum pressures far behind and are in the scientific vacuum pressure range. Down here when asked about your pressure you might just respond verbally with..."About ten to the minus seventh". One assumes the torr is the unit of choice.
Just remember that all vacuums are really just varying degrees of pressurized gas environments that are well below normal atmospheric pressure.
Richard Hull