Author Topic: Espresso coffee boiler water level question  (Read 1853 times)

Offline black85vette

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Espresso coffee boiler water level question
« on: December 28, 2018, 10:39:17 PM »
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

Offline AOG

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Re: Espresso coffee boiler water level question
« Reply #1 on: December 28, 2018, 11:23:44 PM »
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

Offline black85vette

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Re: Espresso coffee boiler water level question
« Reply #2 on: December 29, 2018, 04:38:17 PM »
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

Offline Noitoen

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Re: Espresso coffee boiler water level question
« Reply #3 on: December 29, 2018, 09:39:08 PM »
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

Offline Noitoen

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Re: Espresso coffee boiler water level question
« Reply #4 on: December 29, 2018, 09:43:47 PM »
Or you could build this circuit. It can be adapted to 2 levels
https://www.electroschematics.com/218/liquid-detector-water-sensor/

Offline black85vette

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Re: Espresso coffee boiler water level question
« Reply #5 on: December 30, 2018, 01:06:53 AM »
Or you could build this circuit. It can be adapted to 2 levels
https://www.electroschematics.com/218/liquid-detector-water-sensor/

The sensor I have is insulated and has only a single spade connector at the top of the boiler.

I am surprised I have not run into this circuit with all the Google searches I have done.   Looks simple enough.  A Schmitt trigger oscillator passing the square wave to a detector and developing a DC voltage for the next gate.    My concern is that one of the sides of the sensor will need to be the tank which is grounded.   The RLC type circuit Tony mentioned could incorporate capacitance to ground as part of the circuit but this one looks like it needs the capacitance to be between two isolated sensors to pass the signal. 

Even if this won't work for the auto refill it looks useful for the cold water reservoir which is plastic and open.

Rick

Offline Noitoen

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Re: Espresso coffee boiler water level question
« Reply #6 on: December 30, 2018, 01:36:18 PM »
You could use small model sparkplugs as sensors.

Offline steam guy willy

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Re: Espresso coffee boiler water level question
« Reply #7 on: December 30, 2018, 02:28:18 PM »
Hi Rick, i made an electrically heated steam boiler way back in 2009 and this had a low water sensor . It was made using PTFE rod as the insulator. The boiler used 250 volts and the sensor used a 9 volt battery and the Circuit was designed and made by a friend. The design and Circuit was published a few years ago in the   Engineering in Miniature  magazine here are a few photos of it....may be of some help ??

Willy

Offline black85vette

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Re: Espresso coffee boiler water level question
« Reply #8 on: December 31, 2018, 01:21:31 AM »
Willy;  Nice looking project.   Thanks for the photos.   Interesting sensor setup.   Do you have the schematic by any chance?

Regards,
Rick

Offline steam guy willy

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Re: Espresso coffee boiler water level question
« Reply #9 on: December 31, 2018, 01:47:23 AM »
Hi Rick,  Glad you have found this interesting and  i shall try and find the paperwork   It has a Cct diagram and also a schematic with a printed circuit board template . what happens is the 9 volt sensor triggers an

 optical integrated circuit chip to switch the mains on . There is also a pressure switch (the blue item) to switch off the currant at a predetermined pressure  The potentiometer also triggers the incoming voltage to work like a stoker controller...the 9 volt battery circuit is used as the low voltage is safe to use as the copper boiler is part of the circuit , also although there is steam surrounding the sensor this does not complete the circuit at low water level !!


Willy
« Last Edit: December 31, 2018, 01:56:18 AM by steam guy willy »

Offline steam guy willy

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Re: Espresso coffee boiler water level question
« Reply #10 on: January 09, 2019, 06:04:52 PM »
Hi Rick ,I have found the relevant article with the electric boiler...

Willy

Online MJM460

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Re: Espresso coffee boiler water level question
« Reply #11 on: January 09, 2019, 10:55:45 PM »
Hi Willy, thank you for looking that out.  In our previous discussion of your electric boiler, I never was sure of just how the controller worked.

Clearly an capacitance type with those capacitors in series with the probe, and the frequency is another clue, but beyond that, beyond me to analyse the circuit.  So I still don’t know whether it’s PWM to cut the power or if that triac clips the top of the sine waves.  All very academic, but does influence the power input to the boiler for any efficiency calculations.

Funny thing, I always assumed that it was a resistive principle, based on conductivity of water.  I assume capacitance gives a bigger variation so might be able to indicate level over a range instead of just a single point.

MJM460

PS - you have been very quiet on the thermodynamics questions lately.  Perhaps I had better duck behind a rock.
The more I learn, the more I find that I still have to learn!

Offline black85vette

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Re: Espresso coffee boiler water level question
« Reply #12 on: January 11, 2019, 12:22:19 AM »
Willy;   Thanks very much for posting this.  I find it highly informative and an interesting solution.  I knew that a capacitance based sensor in an oscillator was how commercial units worked but have never found a schematic to show how it was done.   I will file this away in my permanent folder.

This circuit functions in the same way as the one posted by Noitoen earlier in this thread.    The 4093 is a quad Schmitt Trigger with one gate operating as an oscillator.  I am not 100% certain but it looks like the sensor is used to couple the output of the oscillator to the detector stage rather than change the frequency of the oscillator.  So it may rely more on the conductivity of the water than the capacitance of the sensor.   The two capacitors in series would be used to block any DC component from getting to the sensor.   The main difference is the addition of the heater control into the design.   A very nice and simple solution.

Does your sensor have a size or dimension that is important?   I don't see it in the article.

Regards,
Rick
« Last Edit: January 11, 2019, 12:53:18 AM by black85vette »

Offline steam guy willy

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Re: Espresso coffee boiler water level question
« Reply #13 on: January 11, 2019, 03:13:13 AM »
Hi Rick, this circuit was designed by a friend of mine ,so ,I do not really have any idea about the cunning electronics !!!  there is more to this article on page 22. but I don't seem to have that to hand...there is a date of the Magazine in the bottom left hand corner   of the   Engineering in Miniature  so may be able to dig it out for you......July 2013...hope this is of some help... The heaters are 10mm  X. 100mm if you could sort of calculate the size of the sensor. that uses PTFE as the insulator..  I did make this about 10 years ago and it is still going strong !!

Willy
« Last Edit: January 11, 2019, 03:17:52 AM by steam guy willy »

Offline black85vette

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Re: Espresso coffee boiler water level question
« Reply #14 on: January 13, 2019, 10:41:56 PM »
Willy;   thanks for all the information you added.  I posted a link to this thread on my rebuild topic on the barista forum so others can get the information as well.

Regards,
Rick