ACCUMULATOR TANK: This is a pressure vessel meant to store energy in the form of compressed air with fluid. It holds air which will compress when the fluids are pumped in since water does not really compress in itself. The idea is that the pump pressurizes the accumulator, filling it with the nutes, until the desired psi is achieved by the pump and it shuts off. A solenoid is located down line at the mist nozzles that can open and close, initiating the misting cycles. Most people try to get by using a pump without an accumulator, and it yields poor results. I have wasted alot of time trying to figure out how to exclude the accumulator, but there is no known way to get the proper results without one. HPA is all about control. As discussed earlier, the nozzles wil output various micron mist size under different pressures. There is a ramping up and down of the pump pressure as it starts and stops, during which lower pressures are affecting the nozzle's spray. The mist cycles on HPA are generally fractions of a second to 2 seconds per cycle, the pump spends a portion that much time just getting to pressure, causing undesirable large drops spitting out of the nozzles which will cause wet roots, and undermine the benefits were trying for here in HPA. It's like a shooting competition where you get infractions for every shot outside the bullseye, with an accumulator you have a sniper rifle with a scope, without the accumulator you're shooting with a shotgun. Sure, you hit the bullseye with the shotgun for 10 points, but you also lost 10 points for all the droplets hitting outside the bullseye. Even if the proper 50 micron mists are created during part of the cycle, the larger droplets will land on top of them and wet them all together as if it never happened. A good accumulator will have a bladder that can be initially charged with air that will compress and exert pressure upon the liquid you pump in, and force it out under such pressure when the solenoids open. You'll want to pressurize the bladder with air at 2psi less than the minimum pressure you plan to run in your system. Tanks often come precharged to 40 psi or so, and more air will have to be added before connecting them to the system- using an accurate digital air gauge preferably. If using a pressure cut in switch set to 80psi, then the idea is to set the accumular air charge to 78psi. The interior will be coated with a plastic coating to resist the nutes from rusting it out. Most are only rated for 80-100 psi max- but I've stumbled upon a cheap place that has them rated for 150 psi which is better, even higher levels are great, as long as the pump can match them, but they tend to get more expensive. The larger volume the accumulator can hold, the longer you can go without the pump even coming on. If you size your accumulator correctly, you can go days or a week without the pump even needing to come on. Some people use a large accumulator tank along with a manual hydraulic pump. They only have to hand pump once a week for a few minutes and have the timers/solenoids on 12v battery... Off the grid and reliable no matter what the electric company is doing. A 5 gallon accumulator rated to 150 psi can be had for $60 usd or so- 34 gallon only $220 or so...