Compressed air offers a number of advantages over electrical actuators for haunts, and has accordingly been used for many decades in dark attractions. The hiss of air accompanying a scare was a hallmark of the dark rides I grew up with. Utilizing compressed air is the most efficient method to produce sudden and startling kinetic effects. It is safe to use on damp ground outdoors, allows quick and powerful movements without the risk of blown fuses or overheating, and allows for the relatively simple construction of devices that would be expensive or impossible to duplicate using other methods.

The amount of force generated at the output of an air actuator (usually a piston and rod housed in a cylinder and connected to an object to be moved) by even a modest amount of input pressure can be amazing. If you've never considered an air system because it seemed too daunting, peruse this article and discover the basics. It may be just what you're looking for.

The basics of compressed air power systems are not hard to understand. A compressor feeds air into a holding tank, much as a well pump feeds water into a reservoir. Fortunately, air is compressible (unlike water) and a lot of it can fit into a relatively small, stout tank. The tank is essential to the proper functioning of the system, and the tank's pressure is always higher than the pressure required by any single device connected to it. Attached to a fitting on the tank is a limit switch that shuts off the compressor when the tank has been filled to its highest safe pressure, along with a spring-loaded valve, which releases excess pressure from the tank if the limit switch fails. On the main outlet is an adjustable regulator, which reduces the tank's pressure to that required by the connected system. On this regulator are two gauges, one reading tank pressure and the other showing output pressure. The regulator setup may vary, depending upon the size and cost of the equipment, but it is essential to be able to read both tank pressure and the output pressure of the regulator.

As air lines run to multiple applications, they branch by means of T-connectors and valves on the main line. The valves allow any single application to be shut off as needed, allowing the system to continue operating. If a single actuator (a device converting air pressure to kinetic movement) is connected to a branch, the valve may be electrically activated for automation purposes. A secondary regulator (and, in a large system, a smaller filter/trap) may be required at this point. Typically, the actuator's feed pressure must be limited to a lower pressure than the higher feed pressure, as this is usually higher than the actuator requires or can safely handle.

As described above, actuators include cylinder-piston assemblies, but also include air turbines, inflatable bladders, and spray nozzles. (In the context of a haunted attraction, you can move an object suddenly, inflate a figure which materializes out of an enclosure in the blink of an eye, cause an object or surface to vibrate, or startle your guests with a blast of air.)

When several closely proximal branches are required, multiple-tap connector blocks, known as manifolds, are employed. These may optionally be equipped with manual or electric valves that control the flow of air to each output line individually. Valved manifolds are useful for automating the movements of a device comprised of interacting actuators, such as an animatronic figure. When a manifold is used, a main regulator may be placed at its input to control the overall pressure feeding the device, along with other regulators at each actuator in the setup, where necessary.

The master air storage tank at the compressor holds a surplus of air as a buffer. Its pressure should not decrease significantly, even if a number of low pressure devices all fire at once. At each device that requires a strong, sustained surge when acting, a small in-line tank or reservoir (spherical or cylindrical) is installed, such that extra air does not need to travel a long distance through a restricted line to reach it. Unlike electricity, air is slowed significantly when it must travel through long, narrow conductors. Without reservoirs, actuators may work more slowly and unpredictably, especially if they are triggered often, or if several devices are triggered at once

What does all this mean to the phantasmechanic? Here are some tips on setting up your system.

NOTE: A new generation of affordable compressors features 'oil free' operation, and typifies the offerings available at suppliers like Home Depot. The principle advantage of this design is that it allows inexpensive construction, but with the sacrifice of relatively quiet operation and long-term reliability. Be sure to discover whether you can live with the noise produced by these units before you buy. We recommend that you look into compressors featuring the traditional oil-filled crankcase if you intend to use air power in a commercial haunt.

If you are just beginning to experiment with air power, I recommend that you consider a used compressor, as new ones can be rather expensive. Shop garage sales, and watch the want-ads. Call rental equipment stores, and ask about purchasing a used unit. Be sure to acquire an electrically powered compressor. Internal combustion motors are noisy and messy, must be operated outdoors, and will ruin the ambience of your haunt.

When you examine a used compressor, give it a thorough going-over. How does it look? Is it caked with gunk and garbage? Is the intake filter (attached to the pump) black with residue? Look for oil leaks at the pump assembly. Does it look neglected or rusty? If the tank has noticible areas of rust on it, forget buying that compressor. (A tank explosion is something you never want to experience, outside of a movie!) Is there a user's manual, detailing the maintenance and lubrication schedule? Ask to see it. If the owner has discarded the intructions, this may be a sign of haphazard maintenance.

Is the electrical wiring in good shape? Is insulation missing on any of the connecting wires, or are any leads dangling? Are there exposed connections or terminals? You will need to correct any of these problems if you buy the unit.

Run the compressor, and listen for rattles, squeaks and other 'expensive noises.' Is it too loud for your taste? Can you afford to install it so that it will not be heard throughout your haunt?

Stop the motor momentarily and listen. Is there an excessive amount of hissing? Are there leaks at the fittings on the tank itself? Look for a label on the tank showing the rated pressure, and make a note of it. (If this label is missing, decline the purchase.) Re-start the motor, watch the tank gauge, and see how high the pressure rises before the motor shuts off. (If it starts getting uncomfortably close to the maximum rated pressure, shut the unit down!) Does the relief valve act before the motor shuts off? It should not. If the motor does not shut off, don't let the owner tell you that the relief valve will be sufficient - you will need to replace the limit switch.

How much compressor power is enough for your haunt? Unless you are running several complex animatronics, or plan to have a large full-motion dinosaur in your front yard, a modest 1/2 to 1 horsepower compressor with a modest reservoir tank should be good enough.

If you find that a compressor is out of your price range, consider renting a large compressed-air cylinder and regulator from a local compressed-gasses company. One should do you for Halloween - if you have only a handful of effects that are fired infrequently, it can last for days. If you see the tank pressure falling off too quickly, you can always cut down the frequency of effect activation, and/or rent another tank.

Modest size cylenders draw very little air volume per firing. Let's say your compressor's reservoir tank holds air at an average pressure of 80 P.S.I. The regulator at its output could be set to 40 P.S.I., and a small number 'downstream' gadgets will happily sip from the reserve. It is better (in an aesthetic sense) to fire effects separately, and the price of this is multiple solenoid (electrical) valves.

Consider adding an in-line reservoir tank near each device, just before the firing valves, and you will not starve your effects for air. You don't want to end up with a weakly presented startle effect.

Test all of your home-brew devices with low pressures at first, and gradually determine a safe working range. Operate each effect such that it does not act violently enough to damage its own parts. If one particular gadget is tearing itself apart, add a small regulator to its input, just before the valve. Feed the device only as much air pressure as is required to produce the effect. Door closers and bicycle pumps are not industrial parts, and will wear out quickly if cycled constantly or with punishing force. (One correspondent admits to having produced a hole in his ceiling when an unfortunate bicycle tire pump blew apart under no-load conditions, while being tested for actuator use!)

If you see water vapor squirting out of your actuators, even with a filter present at the output of your compressor, consider adding small filters near your effects, and/or consider locating the compressor in a low-humidity (air-conditioned) environment.

Problems with leakage are usually limited to noise and slight air loss, causing compressors to have to cycle on more often. I've never been in a facility (other than Disney's) where compressed air devices were in use, in which there wasn't at least a slight hissing present. Nevertheless, you should try to limit this to a minimum, as it is distracting, and can warn your guests that something is about to happen.

The only real dangers in any air system come from: (1) a poorly maintained compressor with a faulty relief valve, (2) devices operated at excessive pressures which may cause them to self-destruct, sending parts flying, and (3) bad connections at fittings, which might allow a hose to come loose and knock someone senseless. A loose and flying air hose - especially one with a metal fitting attached to it - can really present a hazard even at moderate pressures, and the only way to stop it is to close the valve at the source. Don't try to grab it! You should have a master shutoff valve at your compressor's tank, just in case.