What waterlogged actually means
A pressure tank stores pressure, not water. That distinction is the whole thing. Inside the tank there's a volume of compressed air, and the pump packs water in against that air the way you'd compress a spring. When you open a faucet, the air pushes the water back out and holds your house at pressure while the pump sits idle. The gallons the tank can deliver between the pump's cut-in and cut-out points are called drawdown.
Waterlogged means the air is gone and water has taken its place. Water doesn't compress. A tank without air is just a steel container full of incompressible liquid, and its drawdown falls to almost nothing. The pump reaches cut-out the instant it starts, shuts off, and then falls to cut-in the instant anyone uses water.
You don't need a gauge to suspect it. The symptoms are consistent. The pump starts and stops rapidly instead of running a real cycle, which is called short cycling. Pressure at the shower surges and sags in time with the pump. The switch clicks constantly and the gauge needle sweeps its full range in seconds. Some people notice it first as a thumping in the basement that wasn't there last year.
The four step test any homeowner can do
First, kill power to the pump at the breaker panel. Find the breaker labeled for the well or the pump and switch it off. Don't rely on a disconnect or on the pressure switch lever. You're about to have a hand on the tank and on the air valve, and you don't want the pump kicking on while you do it.
Second, drain every bit of pressure out of the system. Open the lowest faucet you have, a basement laundry sink, a utility tub, or an outside hose bib, and let it run until it stops on its own. Then watch the pressure gauge on the tank tee. It has to reach zero, and it has to stay at zero. This step is the one people skip, and skipping it makes the rest of the test useless. Any water pressure still in the tank adds itself to the air reading and gives you a number that looks fine when the tank is dead.
Third, tap the tank. Make a fist and rap your knuckles down the side of the tank from top to bottom, listening as you go. A charged tank rings hollow across its upper portion and goes dull and solid where the water sits, and with the system drained most of the tank should sound hollow. Where the sound changes from hollow to solid is where the water line is. If the tank sounds dead and solid from the very top, there's no air space left in it, and that one test tells most people what they need to know before they ever pick up a gauge.
Fourth, put a gauge on the air valve. On the top of the tank, under a plastic or metal cap, is a Schrader valve, the identical stem used on a car tire. Take the cap off and press an ordinary tire pressure gauge onto it the way you'd check a tire. Read the number, then press the pin in the center of the valve with a key and see what comes out. Air is what should come out. Water is the finding that ends the investigation.
Reading the number you get
The target is 2 psi below the pump's cut-in pressure. Cut-in is the low number on your pressure switch, the pressure at which the pump turns on. Most systems here run 40/60, so cut-in is 40 and the tank's pre-charge should be 38 psi. On a 30/50 system, 28 psi. On a 50/70 system, 48 psi. The switch is stamped or labeled with those numbers under its cover, and the gauge on the tank tee shows you where the pump actually starts and stops if you'd rather watch it run.
That 2 psi gap exists for a reason. It lets the tank push its very last water out before the pump comes back on, which squeezes full drawdown out of the tank. Set the pre-charge above cut-in and the tank empties and then sits empty while the switch waits, so it bangs and the pump surges. Set it well below cut-in and you're carrying dead water the tank never gives back.
So a reading of 38 on a 40/60 system means the tank is fine and your short cycling is coming from somewhere else, likely the pressure switch, its sensing tube, or a leak. A reading of 15 or 20 means the tank has been slowly losing air and the fix may be as simple as adding some back. A reading of zero means it's flat, and it's the cause of whatever's driving you crazy in the basement.
Water at the valve stem overrides every other reading. Water in the air side of a bladder tank means the bladder is torn. There's no repair for that. The tank gets replaced.
Bladder tank or old galvanized tank
Which kind of tank you have decides whether waterlogging is a maintenance item or a death sentence, so identify it first. A modern bladder or diaphragm tank is painted steel, typically blue and occasionally tan or gray, with a domed top, one water connection at the bottom and an air valve up top. Inside, a heavy rubber bladder keeps the air permanently separated from the water. Labels read 20, 32, 44, 62 or 86 gallons.
An old galvanized tank is bare zinc coated steel, dull silver, often horizontal or standing tall on legs, and it has no bladder at all. Air sits directly on top of the water inside. Plenty of them are still in service in houses that were plumbed in the 1950s and 1960s, and some have been going for sixty years.
The galvanized tank waterlogs as a matter of physics, not failure. With air touching water directly, the air slowly dissolves into the water and leaves the house through your faucets. That's why those tanks came with air volume controls and snifter valves to put air back in, and why they've always needed periodic recharging. A galvanized tank that's waterlogged is recharged, not condemned. If the steel is sound and it isn't weeping rust at the seams, put the air back and keep it.
A bladder tank is the opposite. It doesn't lose air to the water, because the rubber won't let it. If a bladder tank has gone waterlogged, either the air valve is leaking, which soapy water at the stem will show you, or the bladder has failed, which isn't fixable in the field. Bladder tank plus water at the Schrader valve equals a new tank, and anyone who pumps air into a torn bladder is selling you a couple of weeks.
Putting air back in, and knowing when it won't hold
Recharging is straightforward on a tank that deserves it. Power off, all pressure drained to zero at a faucet, then attach a bicycle floor pump or a small compressor to the Schrader valve exactly as you would to a tire. A floor pump is genuinely fine for a small tank at a low target pressure, and a pancake compressor is faster on an 86 gallon tank. Add air in bursts and check as you go, because it's easier to sneak up on 38 psi than to bleed back down to it.
Add air only with the system drained. If there's water pressure in the tank while you pump, you're fighting it and the number you land on will be wrong. Once you hit the target, cap the valve, close the faucet, turn the breaker back on and let the system come up to cut-out. Then run water and count. If the pump now runs a minute or more per cycle instead of a few seconds, you fixed it.
Here's how you know it won't hold. If air comes out spitting water or the pressure won't build at all no matter how long you pump, the bladder is torn and the air you're adding is going straight into the water side and out the house. If the tank holds 38 psi today and reads 10 psi in a week, it's leaking, either at the valve core or through the bladder, and a valve core is worth trying once before you condemn the tank. If the tank sweats, weeps at a seam, or shows soft rust at the bottom, don't put pressure in it at all. A rusted tank under 60 psi is a hazard, and that's a call to make rather than an experiment to run.
How long a tank lasts, and how big yours should be
A bladder tank runs 5 to 15 years. The wide spread is not luck. What shortens a tank's life is pressure cycling, so the same short cycling waterlogging causes will chew up the tank that replaces it if the underlying reason isn't fixed. Sediment and iron cutting at the bladder shorten it. So does a tank that was undersized from day one, because it cycles more. A tank set up right, on clean-running water, on a properly sized pump, will make the far end of that range.
Sizing is where most homeowners get shortchanged, and it's the cheapest place to buy reliability. The rule the trade works to is that the pump should run at least one full minute per cycle. Work backward from that and a two or three person house on a standard pump wants a 32 to 44 gallon nominal tank at minimum. A family of four or five with two and a half baths, laundry and a dishwasher belongs on a 62 gallon tank. Six or more people, a big house, or a high output pump belongs on an 86. Going one size up costs a hundred and change and buys fewer starts for the life of the pump.
That's the real argument for taking a tank seriously. The tank is a few hundred dollars and it lives in your basement where anyone can reach it. The pump is thousands and it lives at the bottom of a 400 foot hole. Every start the tank absorbs is a start the pump doesn't have to make, and starts are what wear a submersible motor out.
If you've run the four step test and you're not sure what you're looking at, or the tank checked out fine and the pump is still cycling, call 973-853-1550 and describe what you found. Tell us the cut-in and cut-out numbers, the reading at the valve stem, and whether you got air or water. That conversation is short and it's free.