TRUgEEk Senior Member Joined: 6 Aug 2012 Posts: 20 Location: east coast Expertise: I love coffee
Posted Sat Aug 18, 2012, 6:26pm Subject: Water level sensing circuit
Does anyone have a suggestion for how to monitor the FULL level with a standard single probe? Capacitance sensing techniques? Knowing when the sensor is out of the water is pretty easy. I'm not familiar with how to know it's full.
calblacksmith Moderator Joined: 25 Nov 2007 Posts: 5,683 Location: Riverside, Ca, U.S.A. Expertise: I live coffee
Espresso: ECM Veneziano A1 Grinder: Many different commercial Vac Pot: 40s era Silex Drip: Milita, Bunn&Curtis... Roaster: Cast iron pan, gas burner
Posted Tue Aug 21, 2012, 10:11am Subject: Re: Water level sensing circuit
Most circuits use a probe into the boiler. When the circuit is completed by the water level in the tank rising to touch the probe, the tank is full. The level is maintained by a simple on/off circuit set to the full level of the boiler. Either the boiler is full or the pump turns on to make it full, it is as simple as that.
In real life, my name is Wayne P.
Feed the newbs, starve the trolls and above all enjoy what you drink!
TRUgEEk Senior Member Joined: 6 Aug 2012 Posts: 20 Location: east coast Expertise: I love coffee
Posted Tue Aug 21, 2012, 1:51pm Subject: Re: Water level sensing circuit
Thank you for the reply. I'm confused though - wouldn't the logic you described cause the pump to constantly cycle on and off? Is there some sort of timer involved as well to keep the pump off for some period after filling?
I'm redesigning/replacing the control logic on an older machine and would like to make improvements along the way. Your post is helpful because I hadn't considered that the probe's position is possibly at the full rather than low level. How much water is typically maintained in an HX boiler? Just enough to keep the elements covered?
TRUgEEk Senior Member Joined: 6 Aug 2012 Posts: 20 Location: east coast Expertise: I love coffee
Posted Wed Aug 22, 2012, 3:52am Subject: Re: Water level sensing circuit
I can see two "high water lines" inside the boiler. The lower of the two is formed by a thicker layer of scale which coats the entire bottom 2/3 or so of the boiler and would seem to be at the level of the probe. The less apparent line is about 3/4 to 1" higher and probably corresponds to the "full" level. The volume of the delta is about 0.5-0.75 L or so. That seems like a reasonable working volume between fills. I will test just running a timer from when the probe is uncovered to see how repeatable this volume is based on my water pressure and pump.
calblacksmith Moderator Joined: 25 Nov 2007 Posts: 5,683 Location: Riverside, Ca, U.S.A. Expertise: I live coffee
Espresso: ECM Veneziano A1 Grinder: Many different commercial Vac Pot: 40s era Silex Drip: Milita, Bunn&Curtis... Roaster: Cast iron pan, gas burner
Posted Wed Aug 22, 2012, 6:11am Subject: Re: Water level sensing circuit
Yes, the pump refills the boiler often, mine runs for about 3 seconds when it fills the boiler so you likely are correct about the timer but the only sensor is the one probe into the boiler and it is a simple on/off type of thing. The size of your boiler and type of pump will have a lot to do also with how long the pump runs. If you have a 2.5l boiler like mine with a rotary pump, three seconds is one thing but if you ran the same pump for the same time in a machine with a 8 oz boiler, I would guess it would constantly be venting water from the overflow.
In real life, my name is Wayne P.
Feed the newbs, starve the trolls and above all enjoy what you drink!
TRUgEEk Senior Member Joined: 6 Aug 2012 Posts: 20 Location: east coast Expertise: I love coffee
Posted Wed Aug 22, 2012, 10:32am Subject: Re: Water level sensing circuit
About how often does your pump kick on if you are steaming? My boiler is 4L and I have a rotary pump rated at 200L/hr -> 0.7L in 3 secs. That's obviously in the range. I hope to calibrate mine tonight. I know that the standard way of filling is to use the single probe as an on/off, but there are other techniques like measuring the capacitance with potentially finer resolution and less chance of false readings once the probe gets corroded. That said, if the simple single probe works, then it may be good enough. My biggest hesitation is that many machines seem to fail because of auto fill malfunctions. Mine did! The doser failed which caused a cascade that took out the heating element. By measuring capacitance it could be possible to detect scale buildup on the probe before it's too late.
Posted Wed Aug 22, 2012, 8:54pm Subject: Re: Water level sensing circuit
Hey Bill -
When I had my Silvia, I did make a water level sensing circuit using a proximity sensing IC. I used de-soldering braid as the water level sensing electrode, around the exterior of the water tank, i.e. there was no internal probe in sitting in the water. I used the QT113-DG QPROX SENSOR, DIP8, 113 (Newark Electronics Part Number: 92K8427).
Below is a picture of the PC i built for the IC. You can read a bit about it on my Blog. If you look all around the blog, there is a complete parts list too.
Posted Wed Aug 22, 2012, 8:58pm Subject: Re: Water level sensing circuit
Here is a picture of the sensor electrode that I had used as well.
Keep in mind that in my application, the digital output of this circuit was read by Arduino. My code was designed to act as a "Low water level sensor", printing to the LCD "Add H2O". However, there is no reason that the sensor electrode could not be placed high on the tank, and then the code could be written to sense a "Full" tank.
TRUgEEk Senior Member Joined: 6 Aug 2012 Posts: 20 Location: east coast Expertise: I love coffee
Posted Thu Aug 23, 2012, 2:23am Subject: Re: Water level sensing circuit
Thanks Rick! Nice work.
Unfortunately it looks like the QT113 is no longer in production. There is an Atmel part (QT1010) which is similar, but specifically designed for touch sensing. I've been playing around with the Arduino Capsense library which calculates an RC circuit's time constant. I don't know if I can create a reliable environment or the magnitude of the capacitance I'd be dealing with. I also don't know how much the temperature flucuations will affect the capacitance. I really don't want to do all those calculations (it's been too long ago...) but I'll have to have at least some rough estimates to stabilize the circuit I think. My boiler is a copper tank that is about 8" in diameter and 8" long. I will probably get the simple logic with a timer to work (so I can make coffee again!!) and work on this in parallel.
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