The first automatic timer, the dashpot timer has been used in many different machines and has many variations. Pneumatic, hydraulic-action, and mercury displacement timers. Being used in a variety of things such as printing presses, motors, and even irrigation systems, the dashpot timer has seen many applications. Even in modern times with electrical and digital timers, these old mechanical timers are still in use due to their simplicity and ability to function in tough environments.
The dashpot timer is a fluid time-on-timer that can be used in definite time motor acceleration starters and controllers. A dashpot timer is a container, a piston, and a shaft. The dashpot timer functions when a magnetic field forces a piston to move within a cylinder when the coil is energized. The movement of the piston is limited by fluid passing through an orifice on the piston. The amount of fluid passing through the orifice is controlled by a throttle value, which determines the delay. If the fluid used to move the piston is air it is actually known as a pneumatic dashpot. If the fluid is oil, it is known as a hydraulic dashpot. Another kind of dashpot timer is the mercury displacement timer, this model uses mercury to contact electrodes.
The pneumatic timer consists of a timing disk, filter, diaphragm, solenoid coil, operating spring and a solenoid core. When the pneumatic timer is energized, the solenoid core moves up into the coil. When this occurs, the core applies pressure on the diaphragm. This moves the diaphragm into the top chamber, air trapped in the chamber is expelled through the needle valve timing disk. In pneumatic timers the amount of delay that occurs can be altered by adjusting the needle valve. Pneumatic timers are very reliable and have a very long operational life expectancy
Hydraulic dashpots or hydraulic-action timers are similar in appearance and operation to pneumatic timers. Hydraulic-action timers work by energizing the solenoid coil which pulls the hollow core into the center of the coil. Fluid in the hollow core is then forced to go through an orifice at the top, a one way check valve at the bottom of the hollow core prevents the fluid from escaping through the bottom. After the fluid is expelled, the core completes its upward movement and closes an air gap in the core, which in turn increases its electromagnetic field strength. When the coil becomes de-energized, it releases the core, and fluid is forced back into the hollow area of the core through the check valve, so the fluid is used again next time the coil is energized. Hydraulic action timers are usually designed for a specific time, which is set in the factory during their manufacture. These timers are also very reliable.