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How To: Acceptance Volume vs. Tank Volume

Tank Volume: The overall volume of the tank, including air and water.

Acceptance Volume: The amount of water inside the tank.

Maximum Acceptance Volume: The maximum amount of fluid inside the tank before stretching the bladder/diaphragm.

Acceptance Factor: The percentage of of tank volume storing fluid, expressed as a decimal.

Drawdown Volume: The amount of water that is drawn out from a starting tank pressure down to the minimum pressure.

A tank’s drawdown volume is the amount of water that is stored between a high and low pressure, which is usually determined by a pump switch.  In real-world plumbing systems, the tank should not start at the maximum pressure and the air cushion pressure should not fall to zero.

The volume of water in a tank, called the acceptance volume, is sometimes expressed as a decimal or percentage against overall tank volume, called an acceptance factor. However, the tank should be charged on the tank’s air side to the minimum pressure needed within the system and filled to the maximum pressure in water required to adjust the tank’s pressure correctly. (pressure in the tank should be set accordingly: 10% below pump cut-in pressure (hydropneumatics); equal to pressure reducing valve pressure (hydronic), equal to line pressure (thermal – domestic water heater protection). As a standard, Wessels Company offers tanks pre-charged to 40 PSI, which can easily be adjusted and reduced in the field to fit the pressure needed within the system.

All replaceable bladder tanks Wessels offers are considered a full volume acceptance tanks, since the bladder inside can fully expand within the tank, allowing all the available space inside the tank to be used.  Fixed-diaphragm tanks are not full acceptance tanks because the diaphragm is confined to only a portion of the tank.

You can tell if a tank is a full acceptance tank by looking at its chart either online, on the submittal, or in the literature for each style of tank.  If the chart says tank volume, that means the usable volume inside the tank is the total volume of the tank.  For example, in a Wessels Company NLA series tank, the total volume of the tank is also the number of gallons the tank can hold, making the tank a full acceptance volume tank. A fixed-diaphragm tank like the NTA, however, lists both the tank volume and the maximum acceptance volume.

If you have more questions about tank volume and Wessels products, feel free to reach out by chatting with us in the chat window at the bottom right of our website, by emailing us at, or by calling our offices at 317-888-9800.