The storage tank is the most misdiagnosed component in a reverse osmosis system. It contains a bladder with water on one side and a charge of air on the other, and that air is what pushes water out when the faucet opens. When it is wrong, the system behaves exactly like one with a failing membrane, and households replace the expensive part instead of checking the cheap one.
How it works
The membrane produces slowly, filling the water side of the bladder and compressing the air behind it. Opening the faucet releases that compressed air's pressure, which pushes the stored water out at a usable rate. So the tank is not just storage: it is the delivery mechanism. The air charge is set when the tank is empty, and it has to be lower than the pressure the system produces at, or the membrane cannot fill against it.
Checking and setting the charge
Close the tank's ball valve and drain the tank completely through the faucet, which takes longer than expected and must be thorough, because a tank with water still in it reads high and misleads. Then check the pressure at the schrader valve, usually under a cap at the bottom, with a low pressure gauge rather than a car tyre gauge, which is too coarse at these values. Adjust to the manufacturer's figure with a hand pump, in small increments, rechecking as you go.
The faults it explains
A trickle from the faucet with the tank apparently full is the classic low charge symptom: water is in there and nothing is pushing it out. A tank that seems to empty after a glass or two is the same thing. A system that runs to drain almost continuously can be a charge set too high, since the membrane never reaches the pressure needed to trip the shut off valve. And water coming out of the schrader valve means the bladder has ruptured, which means a new tank.
Where it sits in the diagnostic order
Before the membrane, always. Slow production or poor delivery should send you to three cheap checks first: the prefilters, the incoming line pressure and the tank charge. Only when all three are right does a dissolved solids comparison across the membrane become the meaningful test. Checking the tank takes ten minutes and a gauge, and it resolves a large share of the systems people are about to spend money on. Our RO installation guide covers commissioning the tank correctly.
Questions people ask about ro pressure tank
Why does my RO faucet only trickle?
Usually a low tank air charge. Water is stored but nothing is pushing it out, which mimics a failing membrane exactly.
How do I check the tank pressure?
Drain the tank completely first, then measure at the schrader valve with a low pressure gauge rather than a car tyre gauge.
What if water comes out of the air valve?
The internal bladder has ruptured and the tank has to be replaced.
What should I check before blaming the membrane?
Prefilters, incoming line pressure and the tank's air charge. All three cause slow production and all three are cheap to check.