A reverse osmosis storage tank holds water in a bladder squeezed by a cushion of air on the other side, and that air charge is what pushes water back out at the faucet. When the charge drifts off the manufacturer's figure, you get a weak dribble, a tank that feels heavy but delivers little, or a membrane that runs far more often than it should.
Why the air charge is the whole story
The membrane fills the bladder slowly at line pressure. Nothing else in the system pushes water to the faucet, so the compressed air outside the bladder is the delivery mechanism. Too little air and the tank fills with water it cannot expel, which feels like a full tank and a starved faucet. Too much air and the bladder can barely fill at all, so you get a strong first glass and then nothing. Both faults look like a failing membrane and are not.
Checking it correctly, with the tank empty
The reading only means something on an empty tank. Close the tank valve, open the RO faucet and let it run until it stops completely, then read the Schrader valve at the base of the tank with a low range gauge. Compare that reading to the figure the tank manufacturer prints on its own label or specification sheet, since the correct pressure is a property of that tank, not a universal number. Add or bleed air in small steps, rechecking as you go.
Symptoms that point at the tank rather than the membrane
A slow trickle at the faucet with a heavy tank is a low air charge. A short burst then nothing, with a light tank, is an overcharged tank or a ruptured bladder. If air hisses out of the Schrader valve as a fine spray of water, the bladder has split and the tank is finished, since it cannot be repaired. A membrane fault behaves differently: the tank fills very slowly regardless of air charge, and taste and clarity usually degrade with it.
What else starves an RO faucet
Before condemning the tank, rule out the cheap causes. Low incoming line pressure, a partly closed feed valve, a clogged sediment prefilter, a kinked quarter inch tube or a fouled flow restrictor all reduce delivery. Check the prefilter first, because it is the most common and the least expensive, and our reverse osmosis maintenance advice keeps that on a schedule so it never becomes a mystery in the first place.
Questions people ask about ro storage tank pressure
What pressure should an RO tank be set to?
Use the figure printed on your own tank label or its specification sheet, measured with the tank fully drained. Tanks differ by size and maker.
Do I check the pressure full or empty?
Empty. Drain the tank through the RO faucet with the tank valve closed, then read the Schrader valve, or the water inside will distort the reading.
Water sprays out when I press the air valve. What now?
The internal bladder has failed and water has crossed into the air side. Replace the tank, since bladders are not serviceable in these vessels.
Can a bad tank charge damage the membrane?
Not directly, but it makes the system cycle far more often, which wears the automatic shutoff valve and wastes drain water on every short run.