So what IS in a vacuum? Nothing? Something? Everything?! Well first we need to define what a vacuum is. In day to day life we consider any gas that has less pressure than its surroundings to be in a state of ‘vacuum.’ That doesn’t mean that it has NOTHING in it, it just really means it has less in it. So what’s a good practical example of this? A vacuum cleaner: it produces an area of lower pressure than the surrounding atmosphere giving it the ‘sucking’ capabilities that we use to clear an area of dirt or dust. Now I assume that most people know what a vacuum is already from the broader sense of things. But when we really get down to it, we can’t actually make a perfect vacuum; we can only get to a higher and higher quality of a partial vacuum.
Measurement of a vacuum is important because we can use it to quantify exactly what the quality of a vacuum is. A true vacuum is one that is absolutely devoid of any particles of any kind, it really is just a void. Even when there is a perfect vacuum it is not truly empty because of various energy waves that are able to transmit through with no medium such as many that are found in outer space. Note that even Space isn’t a true vacuum! If we quantify this perfect vacuum as having a pressure of 0, then we can start to build from here. In our day to day life generally we see pressure in two different formats, pounds per square inch (PSI – Imperial Unit), and kilopascals (kPa – SI), and this is very helpful in pressures ranges we generally deal with for day to day life. But when we measure vacuum generally it is measured in either Torr (1 Torr = 1/760 atmosphere) or Pascals (Pa). I won’t get into exactly what the units of all of these are, or how they are found as most people who are reading this should already be familiar. If you aren’t familiar however I highly suggest sliding on over to Wikipedia and brushing up on pressure and what exactly it is.
There is some debate out there as to which of the two vacuum measurement units should be used, considering that SI units are already used in about 95 percent of the world, and using its units down to low pressures is just moving a decimal point like any other operations with it, I lean towards the side that is rooting for Pa and nPa! Check out this post by Jake Leachman for a bit more info about the SI system, especially as it pertains to Engineering Education.
Instruments to measure a vacuum come in many shapes and sizes. Not all of them work for all vacuum ranges however, the higher the vacuum you have (read: closer to 0 pressure), the harder it is to get a higher vacuum AND to measure it. From the “High-Vacuum” technology book (by Marsbed H. Hablanian) there are three mains types of vacuum gauges:
- Force measuring (105 – 10-2 Pa)
- Heat transfer (10 – 10-2 Pa)
- Electrical charge transfer (ionization) (100 – 10-9 Pa)
Each of these have pros and cons to their usage. In my experimental work I will be aiming for a vacuum of approximately 10-5 Pa (10-7 torr) and we use a FRG-700 Inverted Magnetron Pirani Gauge manufactured by Varian. It is a combination of a Pirani Gauge (heat transfer method measurement) and a Cold Cathode Gauge (ionization), and changes between the operating modes depending on the vacuum range.