The following lines were added (+) and removed (-):
In consideration of the pressure balanced mixing valve in a home shower fixture, there is a cartridge in the single lever design. Once the lever is removed the cartridge is located inside where the hot and cold supply lines converge. Adjusting the shower water temperature is accomplished by changing the Rotational Limit Stop (RSL) position which limits the shower handle range of movement. This is a mechanical piece located between the handle and the inner cartridge. The Rotational Stop Limit (RLS) sets how far the shower handle can be moved toward the hot or cold side. Hard water and mineral deposits as well as rust from aging galvanized pipes commonly causes the cartridge to fail prematurely. This is because the inner device within the cartridge is mechanical and constantly moving back and forth while you are showering. The hot and cold water enters from both sides via the supply lines and is mixed in the cartridge cap then flows out the base to the outlet in back of the shower valve body. Rubber flapper type backflow prevention valves are necessary for the automatic pressure balance valve inside the lower housing to operate. Variations in design have resulted from pressure valve evolution. In the early design a balancing disk was in the center (where the cartridge is on the modern design) and the disc would slide back and forth on a piston. It moves very quickly grabbing a little hot from one line and a little cold from the other, constantly mixing it and doing so quickly enough that you feel a constant temperature.Disc piston as a design was replaced by a spool type pressure balance valve. In the spool design the spool itself slides towards the supply with the lower pressure. This cuts off the volume of water entering the valve on the high pressure side and allows more water from the low pressure side. Output water temp cannot vary more than 3 degrees. There tends to be much less mechanical motion in this design.