A pressure tank does one deceptively important job: it stores water under pressure so that small demands can be met without starting the pump. Get it right and the pump starts a handful of times a day. Get it wrong and the pump starts every time someone washes their hands, and its life is measured in months.
How a diaphragm tank works
Inside the tank, a flexible butyl rubber diaphragm separates two spaces: water on one side, a sealed cushion of compressed air on the other.
- The pump runs and pushes water in. The diaphragm stretches, compressing the air.
- The pump stops at its cut-out pressure. The compressed air now holds the stored water under pressure.
- A tap opens. The air pushes water out — no pump needed.
- Pressure falls to the cut-in point. The pump starts and refills.
Rural Max pressure tanks use a polypropylene liner with a butyl rubber diaphragm, fixed to the tank wall with a steel lock ring, a brass valve sealed with an O-ring bonnet, and a stainless steel water inlet pipe. The wear regions of the liner and diaphragm are reinforced, and the shell carries an epoxy undercoat beneath double polyurethane paint for UV protection — which matters, because these tanks often live outdoors.
The number that matters: drawdown
Nominal tank volume is not usable water. An 80 litre tank does not give you 80 litres between pump starts. It gives you the volume of water that moves in and out between cut-in and cut-out pressure — the drawdown, sometimes called acceptance volume.
Drawdown depends on the tank size, the pre-charge pressure, and the pump’s cut-in and cut-out settings:
Drawdown = Tank volume × P_precharge × (1/P_cut-in − 1/P_cut-out)
All pressures absolute — that is, gauge pressure + 1 bar for atmosphere.
Worked example
Rural Max pressure tanks are set to a 2 bar pre-charge. With a pump cutting in at 2.2 bar and out at 4.0 bar:
Absolute: pre-charge 3.0, cut-in 3.2, cut-out 5.0.
Factor = 3.0 × (1/3.2 − 1/5.0) = 3.0 × 0.1125 = 0.3375
So each tank delivers about 34% of its nominal volume:
| Tank | Nominal volume | Usable drawdown |
|---|---|---|
| RM-R2 | 2 L | ~0.7 L |
| RM-R8 | 8 L | ~2.7 L |
| RM-R18 | 18 L | ~6.1 L |
| RM-R60 | 60 L | ~20.3 L |
| RM-R80 | 80 L | ~27.0 L |
Figures at 2 bar pre-charge with 2.2 bar cut-in / 4.0 bar cut-out. A wider pressure band gives more drawdown; a narrower band gives less.
This is the single most misunderstood thing about pressure tanks, and it explains why small tanks disappoint.
Sizing for pump life
The purpose of drawdown is to keep the pump running long enough per start. Electric motors are stressed by starting, not by running — inrush current heats the windings, and a motor that starts every 20 seconds will not last.
Aim for a minimum run time of about one minute per start for domestic pumps; 30 seconds is a hard floor for small units.
Drawdown needed = pump flow rate × desired run time
A RM-JP100 delivering 55 L/min at its operating point, wanting one minute of run time, needs 55 L of drawdown — which at 34.5% means a 160 L tank. That is more than most domestic installations carry.
This is the honest tension in tank sizing: the theoretically correct tank is often larger than people expect or want to fit. In practice:
| Situation | Practical choice |
|---|---|
| Small pump, occasional use, space limited | RM-R18 or RM-R60 |
| Household on tank water, normal use | RM-R60 or RM-R80 |
| Larger pump, several outlets, long pump life wanted | RM-R80, or multiple tanks in parallel |
| Water hammer suppression only, not cycle control | RM-R2 or RM-R8 |
| Pump life matters more than tank size | Consider a controller instead — see below |
Tanks can be combined. Two RM-R80s plumbed in parallel give roughly double the drawdown. This is often easier than sourcing one very large tank.
The small tanks have a different job. An RM-R2 or RM-R8 is not there to reduce cycling meaningfully — it is there to absorb water hammer, cushion thermal expansion, and smooth pressure fluctuation. Both are legitimate uses.
Setting the pre-charge
The single most important maintenance task on a pressure tank, and the one most often skipped.
Rural Max pressure tanks are pre-charged to 2 bar (about 29 psi). That is the figure to set them to, and the figure to check them against.
The pump must then be set to cut in just above the pre-charge — around 2.2 bar — with cut-out higher again, typically 4.0 bar. The tank cannot deliver any water below its pre-charge pressure, so a cut-in set at or below 2 bar leaves you with no usable drawdown at all.
If you are fitting a tank alongside an RMPC controller, note that the RMPC-10 and RMPC-30 start the pump at a fixed 1.5 bar — below the tank’s 2 bar pre-charge. In that combination the tank acts as a water hammer arrestor and pressure cushion rather than as a store of usable water, because the controller restarts the pump before the tank begins to discharge. That is a perfectly valid use of a small tank — just do not expect it to reduce pump starts.
How to check and set it
- Switch off the pump at the isolator.
- Drain the water side completely. Open the nearest tap or the tank’s drain until water stops and the gauge reads zero. The tank must be empty of water or the reading will be meaningless.
- Find the air valve — a Schrader valve under a cap on the top or end of the tank, the same fitting as a car tyre.
- Measure with a tyre pressure gauge. A dry, accurate gauge, not a service station airline with a built-in guess.
- Adjust to 2 bar. Add air with a bicycle or foot pump; release by depressing the valve pin.
- Re-cap, refill, restart and confirm the system cycles correctly.
What happens when it is wrong
| Pre-charge | Result |
|---|---|
| Below 2 bar | Tank fills mostly with water, drawdown collapses, pump short-cycles |
| Above 2 bar | Tank empties of water before cut-in, pressure drops suddenly at the tap, pump cycles hard |
| Zero (diaphragm failed) | Tank is useless; pump cycles on every draw |
Check the pre-charge every six months. Tanks lose air slowly through the valve and the diaphragm — this is normal, not a fault.
Testing for a failed diaphragm
Press the air valve pin. Air should come out. If water comes out, the diaphragm has ruptured and the tank needs replacing. A waterlogged tank also feels uniformly heavy and gives no drawdown at all.
Where the tank goes
- On the discharge side of the pump, after any non-return valve.
- As close to the pump as practical.
- Supported on a firm, level base. A full RM-R80 is heavy, and the weight must not hang on the plumbing.
- Accessible — you need to reach the air valve and the drain.
- Sheltered from direct sun where possible, despite the UV-protective coating.
- Frost-protected in cold areas, with a drain point at the lowest part of the system.
Fit an isolation valve and a drain between the pump and the tank so the tank can be serviced without draining the whole system.
Tank or controller?
A pressure tank is not the only way to run a pump automatically. An electronic controller such as the RMPC-10 or RMPC-30 starts the pump on flow and stops it when flow ceases, needing little or no tank.
Each approach has real advantages, and they can be combined. Pressure Tank or Pump Controller? compares them properly.
