Measuring air pressure in a bladder type water pressure tank (C) Daniel FriedmanHow to Set or Adjust the Air Pressure in Bladder Type Water Storage & Pressure Tanks

  • WATER TANK BLADDER PRESSURE ADJUSTMENT - CONTENTS: How to check and adjust air pressure in an internal bladder well water tank. Where to measure air pressure or water pressure on a water tank. Typical water tank actual draw-down volume vs. tank rated size. Questions & Answers on internal-bladder water pressure tanks. Well pump & water tank diagnosis & repair procedures
  • POST a QUESTION or READ FAQs about setting the air precharge pressure in internal-bladder type water pressure tanks

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How to set the air pressure in a water tank using an internal bladder:

this article describes how to determine the proper air pressure setting in a bladder-type well tank, when, and how to actually change the air pressure in the water tank.

We explain how to determine the proper air charge for water pressure tank and how to adjust the actual tank air pressure to that number. We detail how to fine tune the water pressure tank air charge to match the pump pressure control switch settings.

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How to Select the Proper Air Pressure Charge for a Bladder-Type Well Tank

Bladder type Well X Trol Water Tank (C) Daniel FriedmanReaders of this document should also see WATER PUMP REPAIR GUIDE an specific case which offers an example of diagnosis of loss of water pressure, loss of water, and analyzes the actual repair cost. The page top photo shows the author testing the air pressure in a Well-X-Trol bladder type well tank.

[Click to enlarge any image]

According to Well-X-Trol, bladder type well tank air precharge: typical factory set air pressure on a bladder-type residential well water tank is 18 psi. Some models may come with different pressure settings however. WELL-X-TROL™ water pressure tanks are shipped from the factory with a standard precharge of

  • 18 psig for models WX-101 and WX-102
  • 30 psig for models WX-103 - WX-203
  • 38 psig for models WX-205 through WX-350.

Before discussing doing anything to a bladder-type water pressure tank, read this safety warning from WELL-X-TROL, a leading well tank manufacturer:


When the Well-X-Trol has been in service and a change to a higher pre-charge pressure is necessary due to a required change in the pressure switch setting, failure to follow instructions below can cause a rupture or explosion, possibly causing serious or fatal personal injury, and/or property damage.

  • Do not adjust or add pressure if there has been a loss of air.
  • Do not adjust the pre-charge pressure if there is visible exterior corrosion.
  • Do not adjust the pre-charge pressure if there has been a reduction of the pump cycle time or the pre-charge pressure compared to its initial setting. This is because reduction in pump cycle time can result from loss of tank air pressure which in turn can mean there may be internal corrosion and any repressurization or additional pressure could result in rupture or explosion.

How to Read Water Tank Pressure or any Pressure Gauge Accurately

If you are fine-tuning water tank bladder or air pressure be sure that you are using an accurate pressure gauge and that you know how to obtain an accurate pressure reading.


Well Tank Air Pressure at Time of Tank Installation

Photo of a water tank air valve for adding air to a water pressure tank

Set the well tank air pressure to 2 psi below the well pump control switch cut-in pressure.
We detail this procedure just below.

Remember to use your separate air pressure (tire) gauge to check the actual water tank pressure when your well pump turns on and off - that's because the dial gauge installed on your water tank may be inaccurate.



Well Tank Air Pressure Adjustment during Water System Maintenance or Repair

If you need to tune your bladder-type water tank to work exactly properly with your pump pressure control switch, or if you know that someone has already modified the pressure in your water tank and you want to restore it to factory specifications, a reasonable procedure is the following recommended by the manufacturer:

  • Remove all water from the water tank. This means turn off the water pump, open a nearby plumbing fixture, run water until the water stops entirely - your water tank should be empty or close to empty.
Water tank air valve (C) Daniel Friedman Water tank air pressure (C) Daniel Friedman
  • Check the air pressure in the water tank by using an accurate air pressure gauge (such as a dial type or digital tire pressure gauge) to measure the air pressure at the schrader valve on the water tank. The tank pressure should be within 10% of the original factory water tank pressure setting. Our photo (above left) shows the water tank air inlet valve on the top of our Well-X-Trol well tank. We removed the cap and label for the photo.

    Our photo at above right shows our digital air pressure gauge measuring the air pressure in the water tank. The reading of 39 psi tells us that the dial pressure gauge that we illustrated
    at How to Read Water Tank Air Pressure Accurately (and located on this same water system) agrees with our pressure gauge within 1/2 psi.

    Because water tank pressure controls normally use a pressure difference of 20 psi between the cut-in pressure and cut-off pressure at the water tank, a gauge error of one or two psi is not a concern. Thanks to Jeff Garmel for suggesting text clarification in this discussion of water pressure tank diagnosis.
  • Set the air pressure in the empty water tank to 2 psi below the well pump pressure switch cut-in pressure. Bleed air pressure out of the water tank, or add air pressure into the water tank, until the tank pressure. For example, if your well pump pressure control switch is set to "cut in" (start pumping water from the well) at 30 psi, then set the pressure tank to (30 - 2) = 28 psi.
  • Replace the air cap on the tank air pressure valve and be sure that any warning labels that you took off during cap removal are back in place.

How to Fine Tune the Pump Pressure Control Switch to the Water Tank

Well-X-Trol offers excellent advice on adjusting the water pressure control switch which we paraphrase, edit, and expand below:

Because well pump pressure control switches are not precise, "lab grade" instruments, the actual pressures at which a pump control switch turn the pump on or off will sometimes be above or below the stated pressure setting on the package of the control switch or above or below the pressure range shown on the switch itself (if your switch has such markings, most do not).

A difference between the marked pressure settings on a control switch and its actual performance can cause a
delay between the time that a water pressure tank becomes completely empty and the time that the pump turns on. Usually this delay is brief, though some readers have reported that their pump simply did not turn on at all under these conditions.

We want to adjust the water pump pressure switch to be"tuned to the Well-X-Trol (or other brand) bladder-type water tank air pressure that was set at the factory (or is presently the actual pressure in the tank). Here is the procedure:

  1. Close off all plumbing fixtures that are using water in the building.
  2. Turn on the water pump and let the system run until the water pump cuts off.
  3. Open one or more plumbing fixtures to drain the water pressure tank; during this draining, watch and listen to the water tank and pump.
  4. If there is a momentary pause in the water flow from the time the well water pressure tank is emptied and the time that the water pump starts, adjust pump cut-in setting clockwise slightly to increase the pump cut-in (turn on) setting.

    If your water pump is in your well rather than on or next to your water tank, you can't hear the pump run. But you can listen for the "click" of the pump switch cutting in or out. For more photos and description of exactly which nut or screw to turn to adjust the well pump switch cut-in,
  5. Close the plumbing fixtures and let the water pump continue to fill the water tank until the water pump cuts off.
  6. Check time it took to fill the water tank.
  7. Open the same plumbing fixtures again, follow the steps above, this time to see if pause in water delivery has been is eliminated. If not, continue adjusting pressure switch.

Frequently-Asked Questions (FAQs) on internal-bladder water pressure tanks & draw-down water volume, pressure switch adjustment, and proper air pre-charge pressure

Question: what is the relationship of water tank rated size to actual water draw-down volume?

How much water should be in the water pressure tank intenral bladder? The water tanks are rated as 85 gallons. When the system cut-out water pressure is reached there is only about 5gal in tanks? - Stephen

Reply: Table of Water Tank Draw-Down vs Rated Size

Modern internal bladder type water pressure tanks give an "equivalent draw down" cycle to the actual "gallons" number on the equipment, but the actual physical volume of water may be significantly less.

5 Gallons does not sound reasonable. Typically on an "85 gallon" sized water tank, the actual water draw-down volume will be about 25 gallons. Below we include a table of rough estimates of actual water draw down volumes for different internal bladder water pressure tanks.

Table of Typical Water Tank Draw-Down vs Rated Size
Water Tank Rated-Size (approximate gallons) Actual Volume of Water Draw Down (approximate gallons)

Notes to the water tank draw-down volume table above:

  1. The "actual draw down" is the physical volume of water that is drawn from the water pressure tank before we expect the pressure drop in the water supply system to reach the well pump cut-in pressure.
  2. The exact water tank size and expected draw-down water volume will vary by manufacturer and tank model, so our data above is only approximate.
  3. The significance of the "draw-down" volume of water is that it determines the frequency with which the water pump has to cycle on and off which in turn affects equipment life.

Detailed Chart of Water Pressure Tank Sizing, Tank Volume, Water Drawdown Gallons

Well-Rite Tank Model Total Tank Volume Equivalent
Tank Size
Drawdown Water Volume
20/40 psi

30/50 psi

40/60 psi
  Gals Liters Gallons Gals Liters Gals Liters Gals Liters
WR45 14 60 30 Gal 5.2 19.68 4.3 16.28 3.8 14.38
WR60 20 80 43 Gal 7.4 28 6.2 23.47 5.4 20.41
WR80 26 100 82 Gal 9.6 36.34 8.1 30.66 7 26.5
WR100 32 120 82 Gal 11.8 44.66 9.9 37.47 8.6 32.55
WR120 33 130 82 Gal 12.3 46.56 10.3 38.99 9 34.06
WR140 44 170 120 Gal 16.3 61.7 13.6 51.48 11.9 45.04
WR200 62 240 220 Gal 22.9 86.68 19.2 72.67 16.7 63.21
WR240 81 310 220 Gal 30 113.55 25.1 95 21.9 82.89
WR260 85 325 220 Gal 31.5 119.23 26.4 99.92 23 87.05
WR360 119 450 450 Gal 43.6 165.03 36.8 139.29 32.1 121.5

Notes to the Wel-Rite Water Tank Sizing Chart

Data based on a Well-Rite® water pressure tank [1]

When choosing an internal-bladder type water pressure tank, take note of the operating pressure range of your well pump equipment. You will see in the table above that the higher you set the water pressure cut-in and cut-out, the smaller will be the draw-down volume provided by a particular water pressure tank.


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