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A submersible pressure switch works by sensing water pressure and using that pressure to open or close an electrical circuit that controls a pump.
In the most common well-pump system:
A typical residential well system might use settings such as:
| Switch setting | Meaning |
|---|---|
| 30/50 psi | Pump starts at 30 psi and stops at 50 psi |
| 40/60 psi | Pump starts at 40 psi and stops at 60 psi |
If you mean a device that is physically underwater, that is often a submersible pressure sensor/transmitter rather than a conventional mechanical pressure switch.
In a standard water well system, the layout is usually:
Submersible pump in well → pipe to house → pressure tank → pressure switchThe pressure switch monitors the pressure in the plumbing system, not the pressure down at the pump itself.
The main parts are:
When you open a faucet, water leaves the pressure tank. As water is used, system pressure drops.
At the cut-in pressure, for example 30 psi:
In simplified terms:
Low pressure → contacts close → pump turns ONAs the pump runs, it fills the pressure tank and raises system pressure.
At the cut-out pressure, for example 50 psi:
So:
High pressure → contacts open → pump turns OFFThis creates a pressure band, often called the differential. For example, a 30/50 switch has a 20 psi differential.
The switch works by balancing two forces:
Pressure produces force on the diaphragm:
\[ F = P \times A \]
where:
As pressure rises, the force on the diaphragm increases. When that force reaches the calibrated spring threshold, the switch changes state.
The switch is designed with a snap-action mechanism, so the contacts do not move slowly. This is important because pump motors draw high current, especially during startup. Slow contact movement would cause excessive arcing, burning, and unreliable switching.
The pressure switch normally works together with a pressure tank.
The tank contains water and a compressed air cushion, often separated by a bladder or diaphragm. The compressed air acts like a spring.
The cycle is:
Without the pressure tank, the pump would start and stop rapidly every time a faucet was opened or closed. This is called short cycling, and it can damage the pump motor and switch contacts.
Electrically, the pressure switch is a mechanically operated power switch.
In many residential well systems:
A simplified circuit is:
Power supply → pressure switch → pump/control box → submersible pumpWhen the contacts are closed, current flows to the pump. When the contacts are open, the pump is off.
For larger pumps, the pressure switch may not directly carry motor current. Instead, it may control a contactor, relay, or pump controller.
The phrase submersible pressure switch can mean different things.
This is the common well-pump device.
This device is actually placed underwater.
It measures pressure caused by the height of water above it:
\[ P = \rho g h \]
where:
This type is commonly used for:
It usually outputs a signal such as:
Then a controller or relay decides when to turn the pump on or off.
So, if the device is truly submerged, it is often more accurately called a submersible pressure transducer, pressure transmitter, or hydrostatic level sensor.
Assume a well system with a 40/60 psi pressure switch.
The cycle repeats as water is used.
Pump motors draw high starting current. Over time, electrical arcing can damage the contacts.
Symptoms:
The small pipe nipple feeding pressure to the switch can clog with:
Symptoms:
The air precharge in the pressure tank should normally be set slightly below the cut-in pressure.
For example:
| Switch setting | Typical tank precharge |
|---|---|
| 30/50 psi | About 28 psi |
| 40/60 psi | About 38 psi |
If the tank loses air or the bladder fails, the system can short-cycle.
Symptoms:
Most mechanical switches have adjustment nuts:
Improper adjustment can cause:
The diaphragm can stiffen, crack, or leak. Springs and linkages can corrode.
Symptoms:
Before working on the switch:
Useful checks:
Check:
For a submersible well pump, do not assume the pressure switch itself must be submersible. In most systems, it should be mounted in a dry, accessible location near the pressure tank.
A submersible pump pressure switch senses water pressure using a diaphragm and spring mechanism. At low pressure, it closes electrical contacts and starts the pump. At high pressure, it opens the contacts and stops the pump. The pressure tank provides storage and prevents rapid cycling.
If the device is actually installed underwater, it is usually a submersible pressure sensor or transmitter that measures hydrostatic pressure and sends a signal to a separate controller.