For us rural folk...
If you have a well you have one of these or at least some form of it. What the heck does it do?? The pressure tank performs a few tasks. First, it stores water, second it provides water pressure when the pump is not running, and third, it maintains a constant pressure in times of high demand. A well pump will probably run for the better part of forever if they are not cycling on and off. This cycling puts extra wear on pump components and can lead to premature failure. Anything that can be done to reduce the number of cycles is advantageous and this is where the pressure tank comes in. When you flip a faucet on the water pressure in the system starts to fall. At the "cut-on" pressure your pump turns on and pumps this pressure back up to the "cut-out" pressure where the pump turns off. Having the pump turn on and off constantly is not desirable so the pressure tank steps in to reduce this.
The pressure tank is partially filled with water and partially filled with air. This air is compressible (water is not) and is used as a buffer for the pump. As water pressure goes down the air exerts a force on the water in the pressure tank and maintains the system pressure until it is used up, then the pump turns on to restore pressure. The tank pressure should be set to 2 psi below the "cut-on" pressure of your pressure switch which is generally 30 or 40 psi in residential homes. If the pressure tank psi is too low it can cause reduced water flow at fixtures which may be exacerbated at upper levels of the home. If it is too high it can cause the pump to short cycle (turn off and on more often) which can shorten the life of the pump.
The pressure tank psi can easily be checked by removing the cap on the Schrader valve and putting a pressure gauge on it. You can see the cap in the photo above at the front/top of the unit, it looks just like a bike/car tire valve stem. Just be careful you don't let a bunch of air out when testing it. If it reads 28 or 38 psi you should be good to go. If it differs substantially you may have an issue with the pressure tank and a plumber familiar with well systems should be called in to evaluate the system. Ignoring this may lead to a hefty pump replacement bill down the road.
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