How does the "CLEAN" light on a coffeemaker work?

[NTL2009]

My Cuisinart drip coffee maker has a light that comes on, telling you to run it through a cleaning cycle (50/50 mix water/vinegar, rinse). In our previous home, we were on a well of about 750 ppm dissolved solids (including some iron), so I had an RO unit connected after the water softener to remove the salt that displaced the other minerals. We used that RO water for our daily coffee, for years in this maker. I don't think the light ever came on when we used RO.

We moved ~ 6 months ago, and are on Lake Michigan water, which is medium soft (more minerals than the RO, but not "hard" - almost no one uses a water softener for this water). The "clean" light has come on twice now, and running the vinegar mix resolves it.

But how does it "know" that cleaning is needed? I haven't found anything detailed in searches. I was thinking the time to brew would tell it that minerals were built up, reducing the efficiency of the heater, but you can brew different sized batches, so how would it know?

One thought I had - could it be timing the 'pulses', or duty cycle of the heater? It does take a few seconds to heat the water to boiling, which then expands and pumps out, and then sucks in more cool water through the check valve, repeat. So mineral build up would affect that timing and/or duty cycle?

Is that it, or some other mechanism/sensor? Any ideas? TIA

 

[Paul Colby]

My first thought was, there is a clean light? We just get disappointed when our's trips off halfway through the cycle. I would guess the complete switch is tripping of a thermal sensor. Boiling water limits the heating element's temperature. Once the water is no more, the temperature of the heating element rises above a preset threshold.

Now, mineral deposits will coat the heating element if operated with hard water. This adds an insulating layer that will conduct less heat causing the heating element's temperature to rise. My guess is your coffee maker has two preset temperatures, one for the "clean me" light and another for "I'm done".

 

[NTL2009]

Perhaps. With an insulating layer of minerals, the heating element would get hotter, and maybe that could be detected. Offhand, that seems like a relatively small increase that might be hard to detect reliably with cheap parts?

I have not had it apart yet (since it still works!), but if/when I do, I'll look for clues. The heater is likely similar to the one pictured here, the tube for the water to pass through is bonded to the heating element, so it is possible. If the heating element had been an immersion type, it would be harder to get in and measure it.

https://www.ifixit.com/Guide/Cuisinart+DCC-1200++Heating+Element+Replacement/105224

 

[Paul Colby]

I also assumed immersing the heater would be the worst possible design choice. Your image confirms what I had imagined it would look like. Knowing the mineral content of the water you could estimate the rate of scale accumulation. A Finite Element thermal analysis should fill in the rest. Oh, and never cut the red wire.