Sometimes...

... you can think of nothing to post about and then a dozen brilliant topics turn up at once. I might make two or more posts today, to make up for recent slacking.

First of all, we were watching Scrapheap Challenge, a fantastic show in which teams battle it out to build the best contraption that must then defeat their opponents in a contest or two. The only resources at their disposal are what they can find in a scrapyard. I often feel I would really enjoy such an experience, despite having absolutely no practical skills whatsoever, and only a basic knowledge of things like engines, axles and so on.

In the episode I watched last night, they were attempting to build battleships, and one team decided to use Archimedes Screws to provide propulsion for their craft, which were powered by hydraulics.

This made me realise that I had completely forgotten the difference between hydraulic and pneumatic, and in case you have ever wondered what the difference may be, here it is.

Hydraulics in general refers to the applied science of managing the mechanical properties of liquids, based upon the theoretical foundation of fluid mechanics.

A hydraulic system is one that makes use of liquid fluid power to carry out work. This means that a hydraulic fluid (usually some form of petroleum oil with additives; edible oil or water is sometimes used in factories where food is produced) is pumped through tubes and hoses in order to provide force or torque multiplication. It is popular due to the ability to apply the force over a long distance, with no need of mechanical gears or levers.

It is a complicated technology with very simple principles: If you push on one end of something, the force will be applied at the other; by altering the pressure of the medium being pushed, it is possible to apply the force at a different rate. I shamelessly ripped the example below from Wikipedia, which, as I am sure you're aware, is where I have been getting most of the facts for my posts from.

Cylinder C1 is one inch in radius, and cylinder C2 is ten inches in radius. If the force exerted on C1 is 10 lbf, the force exerted by C2 is 1000 lbf because C2 is a hundred times larger in area (S = πr²) as C1. The downside to this is that you have to move C1 a hundred inches to move C2 one inch.
(lbf or 'pound-force' is a stupid unit, by the way, and is equal to exactly 4.4482216152605 Newtons)

The units and measurements of the components is unimportant from a theoretical perspective; the ratio is what matters here. At the other end of C2, you will need an actuator such as the brakes on your car. This is not one of those fancy engineering subjects that has very few real world applications: Hydraulic machinery is used in all kinds of situations, most notably on the linkages of heavy equipment such as excavators, diggers and earth movers.

You can see the darker hydraulic cylinders all over the arm that work as linear actuators here. Yes, I know, it's a big one.

Pneumatics, on the other hand, revolves around the use of pressurized gases in order to achieve a similar effect of mechanical motion to hydraulics, but with one or two differences. Pneumatic systems use any easily compressible gas such as air or pure gases like nitrogen or carbon dioxide.

Compared to hydraulic machines, pneumatic ones are simpler to design and control, and more reliable with a longer operating life, because the gases used are compressible whereas the liquids used in hydraulics are not, meaning excess force caused by a shock to the system can be absorbed by the gases, but not by the hydraulic liquids. Of course, this also means that energy is lost in the transfer of force, due to the compressed gas absorbing it. Hydraulics are capable of exerting far more force due to the incompressibility of the liquids at work.

So basically, there you have it: Hydraulics uses liquids and is slow and powerful, whereas pneumatics use gases and are quicker but weaker.

Pneumatics should not be confused with the pneumatic class of humans from Gnosticism. The pneumatic saw himself as escaping the doom of the material world via the secret knowledge, and outsiders could only know these secrets by joining a gnostic group. To be a gnostic is to believe in three planes of existence: the pure unknown (demiurge), the material world of coitus and comfort, and the pure spiritual realm of ascension or escape. Pneumatics were the highest class of humans, above psychics and hylics, the latter of which were the lowest and concerned only with the physical and material world... but that's another blog post.