Hydraulic and pneumatic systems are used for many engineering applications. Hydraulic systems use the movement of a fluid to control the movement of something else, such as a pump, piston or other object. Probably the most familiar use of hydraulics is in the bucket of a front loader. The cylinders raising and lowering the bucket are hydraulic cylinders. Hydraulic systems use oil and run at a very high pressure when compared to their pneumatic counterparts. Pneumatic systems use air and run at a much lower pressure.
Examine the cylinder for a maximum pressure rating. If it is a pneumatic system, the pressure will only get to a few hundred pounds per square inch (psi). In a hydraulic system, the pressure could be as high as 5000 psi. A very high pressure rating suggests a cylinder designed for hydraulic use. The hydraulic cylinders are also larger than their pneumatic counterparts, so compare the cylinders to a cylinder you know is for a pneumatic or hydraulic use.
Compress or extend the cylinder by hand if it is part of a small system. Since oil does not compress easily, you will not be able to compress a hydraulic cylinder, although you may be able to compress a pneumatic cylinder. Be very careful if you choose to do this with a purge system; hydraulic and pneumatic systems can both produce massive amounts of force.
Listen for a compressor or other hissing of air. Hydraulic systems are always closed. This means the oil stays within the system and recirculates, while pneumatic systems are often open, with a compressor on one end that pressurizes air to put into the system and vents air out when the pressure gets too high. Some systems may have a mechanical compressor, while other systems may have a gas canister or gas cylinder.
Trace the system from the cylinder back to its source by following the pipes. Can you see a gas canister or air compressor? If you can find neither, the system is likely to be a hydraulic system. If you find one, the system is pneumatic. This is the only way to know for sure if a system is pneumatic or hydraulic.
Tips & Warnings
- Both pneumatic systems and hydraulic systems operate under very high pressure. Use caution when looking around them.
- Photo Credit hydraulic lock mechanism image by green308 from Fotolia.com