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Probing for levelness

How does your machine know when the level in the boiler is correct
Working out when to fill the boiler and then when to stop filling requires a little sensory perception on your machines behalf.

Before electronics became advanced and cheap enough, the operator had to use a manual filling valve and a level gauge to fill the boiler to the correct level, now we just let technology carry out this simple task.
To measure the water level inside the boiler we use an insulated stainless steel probe mounted on the top of the boiler.

The length of the probe can be mechanically adjusted to provide the optimum operating characteristics for the machine environment, depending on steam versus water usage. 
The water level is sensed by means of a low voltage electrical circuit that measures the resistance between the level probe and the body of the boiler.

Some machines will include an earth connection mounted on the level probe fitting, whilst cheaper machines will rely on a earth connection in other areas of the machine. 

How it works

level probe function

When the control circuit detects no electrical circuit between the level probe and the earth point, the control circuit starts the pump and opens the inlet solenoid to force water from the mains supply into the boiler.

The circuit will remain open until either the control circuit detects a current flowing through the level probe (slide 2) or the machine times out due to no water supply being available.

Faults & Possible Cures

Lack Of Steam
Noticeable over time you may spot that your machine is gradually able to produce less steam than it used to when new.

Main cause will be gradual build up of limescale on the stainless steel probe, each time the level fills in the boiler, the water goes to a new higher point, which slowly reduces the amount of steam in the boiler.

Simply cleaning the probe or by installing a new probe will cure this problem. Left untreated, the fault will eventually lead to the boiler overfilling.
Boiler Overfilling
So long as you have proven that the inlet solenoid is functioning correctly, the level probe can cause the boiler to overfill if the stainless probe is unable to detect an electrical circuit between the level probe and earth on the machine.

Overfilling is noticeable by water emerging from the safety valve or even the steam arms when you open the steam valves when the machine is cold.

Replacement of the entire level probe is recommended, as damage could have also happened to the Teflon insulation.
Boiler Does Not Fill
Most control circuits on modern machines have visual indication when the boiler has not reached the correct level within a certain time frame.

For this fault, the probe is the last item on our hit list to replace. It is always more than likely for the fault to be caused by one of the following components in the order shown:

1 - Is the mains water turned on?
2 - Blockage in filtration system
3 - Machine non-return valve stuck
4 - Inlet solenoid failed
5 - Inlet tube to boiler blocked (scale)
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