A water pipe percolator is a small additional sub chamber within the shaft of a water pipe

Bong

A bong is a filtration device/apparatus generally used for smoking cannabis, tobacco,or other herbal substances.In construction and function a bong is similar to a hookah, except smaller and more portable...

 that
provides in-line smoke-water interaction via heat exchange

Heat exchanger

A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

 and dissolution

Solvation

Solvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...

. Percolators come in different forms, such as dome, pedestal, tree and the more recent double helix. Depending upon the form of the percolator, it may or may not be diffused. However, the primary purpose of the percolator is to act as an extra chamber to filter smoke through water.

Function and purpose of a diffuser

Diffusers can mostly be found on male downstems. A diffuser works by utilizing a pressure differential between its bottom and top in/outlets. Reduced pressure at the outlet end is usually provided by the users lungs. The fluid

Fluid

In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

 at the inlet (i.e.: a smoke, vapor, and air mixture) is directed to the bottom of a column of water, where the pressure differential causes the inlet fluid to pass though the water in small pockets (liquid bubble

Liquid bubble

A bubble is a globule of one substance in another, usually gas in a liquid.Due to the Marangoni effect, bubbles may remain intact when they reach the surface of the immersive substance.-Common examples:...

s), and then rise to the outlet. In short, diffusers make smoke cooler, and rise evenly through the pipe, due to the water bubbles hitting the surface simultaneously. The following is a diagram illustrating the diffusion process of a tree percolator:
Figure: Schematic of the flow through a percolator (inlet at bottom, outlet at top).

1. Initial state
2. Water displacement
3. Flow

The diffuser serves the primary function of cooling the smoke and combustion vapors that pass through it. The surface of the bubbles come into direct contact with the water, where the temperature difference causes an exchange of heat

Heat exchanger

A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

 between the hot smoke/vapor, and the cool water. The tree percolator design incorporates a number of small openings instead of large singular ones in order to diffuse the smoke into a larger number of smaller bubbles. This serves the purpose of increasing the surface area of the smoke that is in contact with the water and increasing the efficiency of heat transfer

Heat transfer

Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...

. In short, the primary function of the diffuser allows for cooler, cleaner and even rising smoke.

A secondary function of a percolator is to trap certain components contained within the smoke. Water soluble substances enter into solution as the smoke/vapor contacts the water, while less soluble substances pass through with relative ease. However, studies conducted by MAPS

Multidisciplinary Association for Psychedelic Studies

The Multidisciplinary Association for Psychedelic Studies is a membership-based 501 non-profit research and educational organization working to develop psychedelics and marijuana into legal prescription drugs...

 and CA-NORML

National Organization for the Reform of Marijuana Laws (United States)

The National Organization for the Reform of Marijuana Laws is an American non-profit organization based in Washington, DC whose aim is to move public opinion sufficiently to achieve the legalization of non-medical marijuana in the United States so that the responsible use of cannabis by adults is...

 have demonstrated that filtration by solvation is ineffective when consuming smoked cannabis. The studies infer that the solubility of Cannabinoids, the active components in the smoke, are similar to or greater than that of tar, and so no (or negative) "filtration" takes place. There is no data available for tobacco or other substances.

A tertiary function of the diffuser is to hydrate the air-smoke-vapor mixture. Humidity of the air passing through the water is increased. The increased humidity reduces the irritation to the upper respiratory tract experienced by users according to anecdotal claims.

Types

There are a number of percolator designs on the market today. The two most prevalent are the tree and dome designs. Pedestal, double helix, serpentine, inline, dewar, ring, coil, and tree twist percolators also exist, but are uncommon. All of these are ordinarily known in scientific production as 'traps'. Each chambered percolator, or trap, filters smoke. Multiple percolators will cause increased filtration and minimize elements such as nicotine or any other resin. Multiple percolators in one piece can also cause cleaning issues and vacuum pressure resulting in trapped particulate matter, or water travelling into the bushing for glass on glass water pipes. A new trend for more artistically shaped percolators has resulted in many new styles. This has inspired the 'Zoa' movement.

Tree

Tree percolators utilize between 1 and 8 (or more, if the manufacturer is capable) individual flow paths for the smoke to enter the water. A sub-category 'supra-flow', with increased numbers of arms. In general, smaller work is more difficult to manufacture. These designs are more recent than the dome design, and represent the original percolator creating complete (two stage) diffusion. Recent variations include bent arm and twist trees. The size of each slit and the number of slits combined should equal the inside diameter of the up stem for equivalent air flow. A sub-category of the tree called a 'twist' resembling a jellyfish is thought to be the first 'Zoa'-type percolator system.

Dome

Dome percolators instead use a single encircling cylindrical piece to direct smoke to the bottom of the water column. Dome percolators can be diffused in many different ways, some have triangular slits along the bottom. While others have either a few large holes or many small holes around the bottom of the dome. The size and number of holes, or slits is determined by the inside diameter of the up stem for correct air flow. Sometimes colored art is applied to the dome resulting in a 'zig zag' pattern common in traditional soft glass paperweights and vases. pH(x) glass holds the patent for the dome percolator design (USPTO 6,935,345 ) and has prosecuted other paraphernalia manufacturers in US Federal Court.

Coil

Coils limit or increase air flow based on the number of wraps and outside diameter of the tubing. They were the first associated with the 'slow-draw' style trap for water pipes.

Dewar

Dewar percolators correct an early problem associated with domes, whereby a vacuum creates back pressure pushing water up the bushing. The Dewar contains a jacketed dome that stands up off the bottom of the percolator chamber eliminating the issue. *Jacketed Dewar sections containing a coil immersed in freezable glycerin are believed to represent the first models.

Ring

The Ring is a variation on the tree containing a single bent up stem attached to a u-bend. The u-bend wraps around the up stem and contains either holes or slits. This would be a "slow-draw" percolator and is usually found in shorter pieces. Altering the air flow for intentional drag is claimed to create a smoother hit, thus allowing harsher tobacco products to be consumed. While no studies have shown this to be proven, recent federal laws on tobacco products exhibit a trend in this direction.

Pinwheel

Artistically combining a Dewar with low-set arms in a swirl pattern the Pinwheel is a re-engineered version of the Ring. Only two are known to exist, although tested and shown to be 'slow-draw' water pipe percolators; the 'slow-draw' system is not yet proven to provide benefits. *Footnote.

Multiple Action Dewar

M.a.d.s are jacketed Dewars with round or compressed spheres inside. Holes at the top of the sphere inside the Dewar allow smoke to billow intensely filling the jacket before channeling out slits in the bottom. They act as air diffusers and water percolators in one piece. A sub-category of M.a.d.s include the Atomic Mushroom and Snake Egg.

Inline

Inline percolators resemble the tree with one larger arm sealed inside a jacketed Dewar. The Inline contains slits and is usually found in the first chamber of the water pipe. The Inline can be made with slits of varying diameter, allowing increased diffusion and creating visually entertaining bubble patterns. Coined 'Harmonic', the pattern resembles waves of bubbles similar to a sound wave pattern. The glass artist 'Hops' is credited, along with 'Toro', for origination of terminology and design.