Dave,
I love this topic because you cannot see inside the boiler and most of the laws of thermodynamics are in play.

The auto-fill circuit "typically" is a capacitive sense circuit. Very similar to a touch screen. The circuit is designed such that when a change in capacitance is noticed then an action is taken. Essentially when water touches the probe then it completes a circuit through the water and to the boiler shell (or another probe). Typically the boiler shell is connected (through grounding) to the other side of the circuit. Another version of this senses a voltage change. In this version when water contacts the probe it completes a circuit and the microprocessor notices that a light voltage change has occurred and takes an action (pump on or off). Usually the microprocessor waits for a pre-programmed amount of time before taking the action (maybe 2-3 seconds). This allows for no action on false readings.

When you first turn on your boiler you heat up the water and the air above it. If you have a vacuum breaker then most of the air escapes during heating. The water heats to a boil and some of the water changes states into vapor. The longer it boils the more water is changed into vapor. Once the required pressure is reached then the heater is turned off. When this happens the water and vapor come to equilibrium. This means that the pressure exerted by the vapor on the liquid is great enough to keep the water from boiling. Essentially is stays this way until something changes (using steam, heating element turns on etc.).

Now when you use your steam wand you remove some of the water vapor from the boiler and the liquid water is able to boiler again and wants to achieve equilibrium again. When first opened the water boils rapidly then reduces a bit. Since the liquid is very close to the probe, the initial boiling action causes some of the water to touch the probe and occasionally you can close the circuit and the auto-fill comes on (Depending where the probe is). The boiling continues until you turn off the steam valve (or the temperature falls below 100 degrees C).

In your case I presume that when you are steaming, the boiling water is in contact with the probe creating the circuit. Then when you close the valve the water quickly comes to equilibrium and the actual water level is slightly below the probe causing the auto-fill to come on.

Another thing to note is when the auto-fill happens, the entering cold water cools the boiler. Since pressure is dependent on temperature, (i.e. if the temperature goes down the pressure goes down) the pressure is also reduced.

It sounds like the boiler is not getting filled sufficiently past the low level point.

If the controller is an 'off-delay' type, as cafeIKE noted, then the pump will continue running for a predetermined length of time after the probe senses liquid. If the pump (or associated valve and plumbing) is unable to supply the expected volume within that time, then the sensor would be prone to go low when relatively minor changes to the boiler took place, such as steaming or pulling a shot.

Have you noticed a coincidental increase in shot time? I had a problem awhile back which made it appear that the pump was failing. Dan suggested it might have been an obstruction, but it was merely an air bubble in the supply tubing.

Also should clarify my point about your supply water: If you're using overly pure water, it might be beyond the controller's sensitivity. I've seen this in another type of equipment where spiking with a small amount of tap water (it wasn't that necessary to be that pure) allowed the sensor to work correctly.

erics wrote:...it would take a tremendous amount of writing skill to explain the autofill circuit in laymen's terms. And thus far, over the years, NO ONE has accomplished this feat.

I can't imagine that the level controller on the One Black is too complicated since it only has one input, one output and only does one thing. The apparent lack of documentation is annoying, unlike other well known manufacturers of standard form factor level controllers.

Billc wrote:Dave,
I love this topic because you cannot see inside the boiler and most of the laws of thermodynamics are in play.

The auto-fill circuit "typically" is a capacitive sense circuit. Very similar to a touch screen.......

Thanks Bill: I too am fascinated by the thermodynamics of the system (physics major). But my ignorance about electronics is probably obvious.
This is the first time anyone has suggested "capacitance" as the criteria for a fill. That is the sort of detail that could maybe help me figure this out.

Jim Schulman (in a post clarifying his "insanely long water FAQ") discusses the circuit as though it were a simple, binary, "ground or open" situation. His interesting idea of adding a switch to facilitate descaling makes me wonder about the potential to damage the machine with such a switch.

another_jim wrote:.........-- If you plan to descale regularly, attach a 3 position "descaling switch" to the autofill circuit. Cut the wire running from from the brain box to the autofill wand. Attach the wire from the logic box to the middle terminal. Attach the wire to the autofill wand to one of the outer terminals. Attach a wire to ground...

In particular, routing the wire directly to ground would be significantly different from the resistance (or capacitance?) that the wire would see from being connected to ground through the intermediary of water. Could that connection "directly" to ground, rather than "through water to ground" damage something?

My machine seems to run fine with distilled water in the boiler, which would be hard to do with a simple resistance test (right?). I wonder about the details of the circuit, and if it could tolerate the change to a direct connection to ground.

This is way beyond my ability to evaluate, so I was hoping someone might allay (or confirm) my fears, because I think the switch would be a nice addition to my machine.

It would be very cool to be able to throw the switch one way to "lock out" the autofill during brewing or steaming, and to be able to throw it the other way to "force" fill when descaling, to get the boiler completely full of descaler. It would also be cool when descaling just the HX, because you could lock the autofill out while the hose was in the descaling solution, to keep it out of the boiler.

The 'capacitance' adds a time delay to prevent the pump running in short bursts. When the water level drops, the pump does not start immediately and continues to run for a few seconds beyond the instant the water contacts the probe.

If you add a switch to overfill the boiler, you add it in series to break the circuit, i.e. the probe is hanging in mid air. There is zero chance of damaging the controller.

cafeIKE wrote: ... If you add a switch to overfill the boiler, you add it in series to break the circuit, i.e. the probe is hanging in mid air. There is zero chance of damaging the controller.

Actually, that's just part of the switch circuit discussed. It's a 3-way switch, attached to the probe line.
Position one - control line goes to ground (boiler won't fill)
Position two - control line goes to probe (normal operation)
Position three - control line open (boiler fill engaged)

You need a SPDT On - None - On
Wiper to control box
T1 to boiler sensor - Normal
T2 to ground [ through optional resistor if OTT ] - Alternate

In Normal position with T1 closed, operation is standard

In Mid position, pump runs continuously

In Alternate position, pump will not run

Recommend using DPDT sealed switch with poles wired in parallel.

cafeIKE wrote:
T2 to ground [ through optional resistor if OTT ] - Alternate

I thought about using a resistor, but have no clue as to what level of resistance.

I tried measuring the resistance of my water, but my multimeter gave entirely inconsistent results. There's something about the "metal in water" connection that is different.
I think there's a slight current generation, which befuddles my multimeter.

Most of these electronics will allow you to connect directly to ground. Since it is only sensing a change there is little damage that you can do to this circuit just connecting directly to ground. A common test for the probe is to remove the wire, connect it to ground and see if it turns on.
The delay in the circuit is usually programmed in a chip on-board. There are some older styles that had a circuit that created a delay but you dot no see those much anymore. In the LM machines from the mid 90's there was a programmable chip that you could program the amount of delay from first contact to turning on the pump. Ditto for the reverse.

Billc

davefitz wrote:I thought about using a resistor, but have no clue as to what level of resistance.

OTT = Over The Top. A resistor is not necessary.

However,
IF you have a very sensitive ammeter, connect it between the probe and the wire and measure the current that flows whilst the boiler is full and the pump is off. Let's say 10µA. Now change the meter to voltage mode and measure the voltage between the wire and ground. Let's say 1v. Using Ohm's law, I=E/R or R=E/I = 1 / .00001 = 100K.

Note that the current and voltage could be AC or DC, but probably DC.