Air is compressible, and that changes everything
The single property that separates pneumatics from hydraulics.
Air is compressible and water is not, and that single difference is the entire reason a system driven by compressed air behaves like a spring, storing energy and giving slightly under load, while a system driven by pressurised liquid behaves like a rigid rod, transmitting force and motion almost instantly with barely any give at all.
Compressed air is the most common form of factory automation, and it behaves in ways that intuition borrowed from water gets wrong on almost every point, since the two fluids are pushed through nearly identical-looking pipes and valves while obeying very different rules underneath.
Same plumbing, different fluid
Pneumatics, the use of pressurised air to do mechanical work, and hydraulics, the equivalent use of a pressurised liquid, look almost interchangeable from the outside, both moving pistons and turning valves through pipes and cylinders that could pass for each other on a drawing. The behaviour underneath that similar plumbing diverges sharply. Water resists being squeezed into a smaller volume so strongly that, for ordinary engineering purposes, it can be treated as incompressible, while air readily shrinks into a fraction of its original volume under only modest pressure.
A hydraulic cylinder's piston moves the instant pressurised liquid is pushed toward it, since there is nowhere for that liquid to go except into moving the piston. A pneumatic cylinder's piston can sit still for a real, measurable moment while incoming air is still being compressed into the space behind it, and that air spends its effort squeezing a spring of gas before any of the work becomes motion at the piston face.
Landing on a trampoline and landing on concrete
Jumping onto a concrete floor sends the full force of the landing straight back up through the legs, since concrete gives essentially nothing under the impact and all of the jump's energy has to be absorbed somewhere else, mostly in the legs themselves. Jumping onto a trampoline instead sinks the surface down under the landing, storing the jump's energy in the trampoline's own stretch and returning most of it a moment later as a bounce. Both surfaces are simply being pushed on by a falling body, yet the relationship between force and motion is completely different.
A hydraulic system behaves like the concrete floor, rigid and nearly instantaneous in how it transmits force. A pneumatic system behaves like the trampoline, absorbing and storing energy in the compressed air itself before that energy shows up as useful motion. That springiness is sometimes a nuisance and sometimes, as the last article in this set explores, exactly the behaviour a designer wants.
Why the squeeze costs time and energy
Squeezing a given mass of air into a smaller volume takes work, because the air resists compression the same way any spring resists being compressed, and that work has to be supplied before the compressed air can do anything useful downstream. A pneumatic cylinder receiving a fresh burst of pressure spends the first fraction of its stroke finishing the job of compressing whatever air was already sitting in the line and the cylinder chamber. Only once that is done does the full force arrive at the piston, which an incompressible hydraulic fluid would have delivered at once.
That energy is stored in the compressed volume and will come back out later, so in one sense nothing has been lost. It is still energy the cylinder cannot yet spend on the job it was asked to do, and the resulting delay is built into the physics of the fluid, however well the valves and seals are made.
Seven volumes of air in one
Air at a typical factory supply of about six bar on the gauge sits at roughly seven times atmospheric pressure in absolute terms, so a given mass of it is squeezed into about a seventh of the volume it would fill in the open room. Water taken to the same pressure shrinks by only about three hundredths of one per cent, a change so small that for most purposes it might as well be zero. Those two figures, a seventh against almost nothing, are the whole quantitative difference behind everything this article has described. One medium stores and releases energy across a large range of volume, and the other transmits force through a body that for practical purposes keeps its size.
The same ratio explains why filling a pneumatic system asks so much of the compressor. Every cylinder and hose on the machine is holding seven room-volumes of air in the space of one, and all of that has to be pushed in by a compressor before any of it can push on a piston.
Where the springiness helps
The same springiness that ruins precise positioning is what makes pneumatic tools comfortable to work with and forgiving of a sudden shock. A pneumatic system absorbs a jammed part, an unexpected collision, or a sudden overload by compressing further, instead of transmitting the full force of the surprise instantly into the surrounding structure. A rigid hydraulic system, offering almost no give, stresses its own components far harder in the same moment.
A trampoline is a poor surface for a surgeon's steady hand and an excellent one for absorbing a fall, and a designer choosing between air and liquid is very often making that same choice, precision against forgiveness, with neither fluid simply better than the other.
What the rest of this set builds on
Every design decision covered in this set follows from compressibility, whatever the valves, cylinders or fittings look like, since a compressible working fluid changes what can be promised about timing, position and force delivery however carefully the rest of the system is built. The next article follows the most immediate consequence, why a pneumatic cylinder that can slam confidently between two hard end stops struggles to stop reliably anywhere in between them. The final article returns to the trampoline side of this trade, and to the jobs a factory hands to compressed air precisely because rigid, instantaneous force transmission was never what those jobs needed.