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Espresso coffee boiler water level question

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black85vette:
The folks on my coffee forum know a lot about coffee but not so much about boilers.   My question is about the auto refill circuit which uses a controller made by Gicar, a single rod sensor in the boiler tank and a pump able to pump water into the tank while operating.   This is a single rod and from what I gather uses some sort of capacitance circuit to turn the pump on when the water is not touching it and off when the water touches the end of the rod.   It does not have a Low/full type sensors, just the single tip to know when it is full.   Trying to understand the circuit which I assume does not use DC which might cause electrolysis.   This is project is a rebuild of an older machine and I would like to know how it is supposed to work and if it is in fact working correctly.    I figured I had at least a good shot posting here with a boiler question.   Have not been able to find schematics for the controller on line.

Regards,
Rick

AOG:
If it is really using a capacitive type level sensor than it is a pretty sophisticated boiler. Here’s how they work In a nutshell. The sensor is usually two metal rods stuck in the tank close together (or concentric with each other). If the tank is metal you can get away with one probe if it is close enough to the edge of the tank. The two probes form a capacitor with the dielectric being whatever is between them. Air when the tank is empty and the fluid being measured when it’s full. The value of the capacitance changes with the level of the fluid. What they will do is set that capacitor into a RLC type oscillator. As the fluid rises on the probe the resonent frequency will change as the capacitance changes. At that point you can measure the frequency and tell exactly how much fluid is in the tank. In your case, they probably just compare the voltage across the probe with some fixed value that represents water on the tip of the probe and use that to energize a relay to turn on the pump.

Tony

black85vette:
Tony;   Thanks.    That gives me a bit more insight.   This appears to be a single rod insulated from the tank so rod to metal tank is likely how it works.   I have a meter that reads capacitance and inductance.   I am going to make some measurements and see what the range is.   

The boilers in production espresso machines often run non-stop rather than re-heating each day.   There is very low demand on the boiler since the water used for brewing in a single boiler machine is run through a heat exchange tube through the boiler to bring it up to temperature.   Only steam used to heat the milk comes from the boiler so there is not a problem turning the pump on/off with each use.   This also helps keep the boiler temperature more stable since the cold water is introduced in very small quantities.

I am interested in learning more about the circuit.   If I can build a working sensor circuit I could use an off the shelf PLC for the logic to do the job.   A possible future project might include an auto-refill for the cold water reservoir.  That would be a simple float sensor.  I could handle both functions with a single controller plus add a fail safe to shut down the heating element if the boiler was not filled within a given time.  Inputs to the PLC can be current loop, analog voltage or logic level.   All I need is a working sensor to feed it.

Rick

Noitoen:
On capacitive level sensors, one of the probes must be insulated.  It's much easier to use a good quality high/low level sensor that uses ac on the probes

Noitoen:
Or you could build this circuit. It can be adapted to 2 levels
https://www.electroschematics.com/218/liquid-detector-water-sensor/

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