This all depends on to what application you refer. The only pneumatic system, that I am familiar with is legacy, HVAC control system for buildings. Hydraulic systems were never used in this application that I am familiar with. Hydraulic systems are typically used for transferring mechanical power, an application for which a pneumatic system would not be very practical. The reason that pneumatic systems were used for controls (this application has largely been replaced by direct digital controls (ddc)), is that the pneumatic control circuits can be employed with very small, lightweight, plastic tubing, driven by a very small compressor and relatively small, compact valves. Since the interior temperature of buildings remains fairly uniform, problems with pressure changes, due to environmental temperature change is not a problem. The ultimate problem with pneumatic control systems is that the need for more and more complexity in control systems would require continually larger and more complex valves and tubing. With the development of solid state, ddc systems, very small wires and compact, solid state, microcontroller panels could do the job of much larger, mechanically complex pneumatic control systems.
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1. Use pneumatic if the environment must be kept from leaking hydraulic fluid, like in the food industry.
2. Pneumatic controls respond much quicker to relatively small changes in pressure
3. Pneumatic has reserve capacity (air tank) while hydraulic usually needs a pump to produce
high pressure hydraulic fluid, though a hydraulic accumulator could be used.
4. Easier to transmit pneumatic pressure over long distances than hydraulic, since pneumatic
can be be piped in light duty tubing, while hydraulic needs high pressure tubing.
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I've used pneumatics for simplicity, cost, and cleanliness. If you can do the job with pneumatics great. If you are operating a front end loader you better have hydraulics, too much load and travel for pneumatics.