Setting the air charge in a reverse osmosis storage tank takes ten minutes and a gauge, and doing it slightly wrong produces a confident reading that is not the pressure you think it is. The two common errors are measuring a tank that is not actually empty and using a gauge too coarse to read these values, and both send people on to replace parts that were fine.
Empty the tank properly first
Close the tank's ball valve, then open the RO faucet and leave it open until flow stops completely, which takes longer than expected. Then close the ball valve if it was open, disconnect the tank if the manual directs, and let any remaining water out. Water left inside pushes against the bladder and the gauge reads high, which is the single most common reason a tank is set wrong. If the reading falls as you continue draining, the tank was not empty when you started.
Use the right gauge
The pressures involved are low, and a car tyre gauge is too coarse to resolve them usefully: its scale starts where this measurement finishes. Use a low pressure gauge, ideally one intended for expansion or storage tanks, and be aware that every measurement releases a small amount of air, so check as few times as necessary and top up afterwards. The schrader valve is usually under a cap at the base of the tank.
Adjust in small steps
Set to the manufacturer's figure for that tank, adding air with a hand pump in small increments and rechecking, because it is easy to overshoot and awkward to let precisely the right amount out again. Too low and the tank stores water it cannot push out, which presents as a trickle from a full tank. Too high and the membrane cannot fill against it, which presents as a system that produces very little and may run to drain continuously.
After setting it
Reconnect, open the feed and the tank valve slowly, and let the system fill fully before judging anything, which takes a while. Then draw from the faucet and see whether the flow is strong and sustained. Check for water at the schrader valve while you are there: if any appears, the internal bladder has ruptured and the tank needs replacing rather than recharging. Our RO installation guide covers commissioning the whole system.
Questions people ask about ro tank pressure
How do I set the tank pressure?
Drain the tank completely, measure at the schrader valve with a low pressure gauge, and adjust to the manufacturer's figure in small steps.
Why does my reading seem too high?
The tank probably is not empty. Water inside pushes against the bladder and inflates the reading.
Can I use a tyre gauge?
Not usefully. These pressures are low and a tyre gauge's scale begins around where this measurement ends.
What if water comes out of the air valve?
The bladder has ruptured. The tank needs replacing rather than recharging.