A Short Introduction to Pressure Terminology

The units of pressure were a bit confusing to me before I started this project, so to clarify things for the reader in advance, here is a quick tutorial about pressure and its units.

In order for the fusor to produce fusion reactions, the inside of the chamber must be at a very low pressure, or put another way, at a very high vacuum. Out in space, there is an almost perfect vacuum. On the earth, the atmosphere is filled with gas, which exerts pressure on everything. You are used to this pressure and do not notice it in your everyday life, but if you drive up a mountain or fly on an airplane, your ears will pop, as the atmosphere is thinning and the pressure is dropping.

The standard pressure at sea level is defined as 1 atm (atmosphere), which is roughly 15 pounds per square inch. The pressure of the earth's atmosphere at sea level will apply a force of 15 lbs on every square inch exposed to the atmosphere. Easy, right? Well, here is where things start to get confusing.

There are many different systems of measuring pressure. The SI unit of pressure is the pascal (Pa), but I have not encountered this notation very often in my online fusor research. More common is the Torr, which is roughly the fluid pressure exerted by a millimeter of mercury. One atm is equal to 760 Torr. However, vacuum pressure is much, much less than atmospheric pressure, so normally only a tiny fraction of a Torr is measured. For example, light bulbs are normally evacuated to 0.1 to 0.01 Torr. The vacuum pressure on the moon is roughly 10^-11 Torr.

From my online research so far, it seems that a vacuum of about 20-60 mTorr (m stands for 10^-3) is needed. Because one Torr is roughly equivalent to a millimeter of mercury, another symbol for a Torr is mmHg. Thus, one mTorr will be equal to one micrometer (10^-6 meters) of Hg, and so the term "micron" is interchangeable with "mTorr." Whew! Long story short, my fusor will need to obtain pressures of about a tenth of the vacuum pressure in a light bulb in order to operate.

There are many other measurements of pressure, and if you are really interested, wikipedia has good articles on pressure (http://en.wikipedia.org/wiki/Pressure) and vacuum (http://en.wikipedia.org/wiki/Vacuum) that I obtained most of the information above from. Hope this helps!