Thread from the WWn Wiki - originally developed by Andrew
3.1 Standard (Elastic) CPH
4 Design elements
4.1 Standard (Elastic) CPH
4.1.1 Cylindrical PC
4.1.2 Spherical PC
4.1.3 Diaphragm PC
A Constant Pressure Homemade (CPH), can refer to any homemade water blaster which provides almost constant pressure, to it's stream, over the duration of the shot. This results in a minimal drop-off in range. Traditionally the term refers to elastic bladder based blasters, based on the Super Soaker CPS technology, but can include other, more recent, methods of maintaining water pressure. This design is a favourite amongst homemade builders, as it combines very high performance and a long shot time with maximum range during the entire shot. It is recommended for enthusiasts who have some experience modifying or repairing water blasters and who have already made an Air Pressure Homemade. If properly constructed, a CPH can outperform any manufactured water blaster, and most APH blasters aswell.
Although most elastic pressure homemades are a variant of CPH (Constant Pressure Homemade), not all CPH's use elastic pressure chambers (CAPH for example). Also not all elastic homemades are strictly CPH's (spring based homemades for example) but are still found on this page.
For CAPH (Constant Air Pressure Homemade) see Air Pressure Homemade
The first recorded elastic pressure CPH was Ben's SuperCPS, in 2004. Like the standard APH, the SuperCPS featured interchangeable nozzles and a homemade pump. It also featured a homemade backpack reservoir based on a solar shower bag, providing up to 20 litres of reservoir water. The SuperCPS used latex rubber tubing to emulate the cylindrical CPS bladder design, used in the CPS 2000. Since then many builders have created variations of this blaster, some opting to use spherical bladders constructed from, high quality, rubber balloons.
 Standard (Elastic) CPH
CPH Firing Steps (Top:Spherical PC, Bottom:Cylindrical PC)
The basic Elastic Pressure Homemade operates on the principle of the elastic expansion of rubber. During the first stage, the reservoir is filled with water at room temperature and pressure. During the second stage, water is drawn into the pump through the first check valve. During the third stage, the pump is retracted, forcing water through the second check valve and into the rubber chamber. As water is pumped into the rubber chamber, it expands, and the rubber applies pressure inwards onto the water. Due to the nature of rubber expansion, the pressure remains constant, as long as ther is water in the chamber. In theory this means that a single pump worth of water in the pressure chamber will reach the same distance, when fired, as a full pressure chamber, although the shot will be substantially shorter. This is not always the case, however, as there tends to be a slight drop in pressure when the bladder is almost empty. Also if the nozzle is too large, then there will not be enough water to maintain the stream over the full distance. During the fourth stage, the outlet is opened and the rubber chamber forces the water out of the nozzle.
Note: Although a spring is, in theoy, an elastic mechanism, it does NOT provide constant pressure, as the force exerted is proportional to the distance by which the spring has compressed, according to Hooke's Law:
Where F=Force, k=Spring Constant, x=Spring Displacement
Elastic bladder designs are generally more powerful than stock blasters because they lack a pressure relief valve and are constructed from sturdier materials. A good bladder chamber design is often more powerful than most APH's, and may have more range aswell. It is much more feasible to create an elastic bladder homemade with a remarkably longer shot time than stock blasters, than it is with an APH, as the range and power remain constant over the entire shot. Like the APH, the simple construction and lack of complex moving parts makes a basic elastic design much more durable and reliable than stock blasters.
By definition, a CPH blaster has constant pressure over the entire duration of the shot, allowing the blaster to reach maximum range even with a nearly empty pressure chamber. Also, due to the lack of air anywhere in the pressurised system of an elastic bladder CPH, it can be fired at any angle without any chance of a 'mist shot'
An elastic bladder CPH can prove difficult to adjust and vary in power as you cannot simply control the amount of air being pressurised to increase power. Infact it is very hard to increase the the performance of a bladder CPH, without some form of power mod, as the pressure exerted onto the water is controlled directly by the thickness of the bladder. An elastic based CPH can also have reliability issues. The rubber used for the PC has a finite lifespan, which is shortened by repeated expansion-contraction cycles. Over time the bladder will exert less pressure on the water, reducing performance, and may eventually rupture when you need it most.
 Standard (Elastic) CPH
The basic CPH is similar in design to the basic APH as it is also composed of three main sections. These are; the pump, the pressure chamber, and the reservoir. A check valve is placed between the reservoir and the pump and also between the pump and the pressure chamber. A trigger valve is placed between the pressure chamber and the environment. Like the APH the orientation and positioning of any of these sections is flexible, but unlike the APH the oulet in the pressure chamber does not have to be located at the "bottom" of the chamber. Instead it is advised that the PC, the outlet valve and the nozzle are inline with each other to make the flow more laminar.
 Cylindrical PC
The most popular elastic pressure chamber type, by far, is the cylindrical PC, originally used in the CPS line of Super Soaker's such as the famous CPS 2000. They are usually constructed out of latex rubber tubing (LRT) which is easily available in the US, and is easy to work with. When water is pumped into the LRT, it expands outwards and lengthways. In total it expands to around three times it's original length. It is this lengthway's expansion that makes this type of elastic PC more efficient, as the pressure exerted on the water is in the opposite direction to the majority of its expansion. This means that most of the pressure exerted, on the water, is in the direction of the nozzle, providing a more linear design with better flow and less turbulent streams. Some have had success with Cylindrical PC's constructed from bicycle inner tubes, but these are much less effective than LRT.
 Spherical PC
The second most popular pressure chamber is the spherical PC, and is also design based on the CPS line, but from weapons such as the CPS 1200. In some areas, LRT can be hard to find, so some opt to use spherical PC's in their CPH. In CPH's spherical pressure chambers are generally constructed from helium grade balloons layered on top of each other. This method is similar to the K-Mod power enhancement for spherical CPS bladders, except that the ballons themselves are the PC and over 75 ballons are usually required. Balloons tend to be a lot harder to work with than LRT and are more easilty broken. As the spherical PC expands in all directions when inflated, it is also less efficient than the cylindrical PC, as the majority of the force exerted is not in the direction of flow.
 Diaphragm PC
Diapghragm PC's were originally used by Buzz Bee Toys in their Water Warriors Blasters up until 2010, in an attempt to avoid falling foul of the Super Soaker CPS patent. The PC is essentially a flat sheet of rubber, which is clamped to a flat surface with a water inlet in the centre. When water is pumped in beneath the diaphragm, it expands outwards pressurising the water within. Although this design reduces the amount of 'dead space' inside the blaster, the rubber tends to expand unevenly, causing it to deteriorate more quickly. Due to the design of such a PC, the very end of the shot experiences more noticeable drop-off in pressure, than the cylindrical and spherical alternatives.
Currently no CPH blaster has been made using a diaphragm PC, as it is more difficult to construct, and performs less well than the cylindrical and spherical PC's.