Ways to detect when the Zojirushi water boiler becomes empty?

Hardware
1

I have a Zojirushi Water Boiler in my kitchen for instant hot water for cup noodles and tea. I use it every other day but constantly forgot to refill it when the water level is low. And since it holds 4Ls, it will take almost an hour to boil the water (which means I have to stare at my opened noodle cup for 60 minutes, not ideal). So I am trying to find a way to monitor the water level of the tank and notify me when it’s below a certain amount.

The only visual indicator for the water level is the see-through water gauge. The machine will also tell you it’s empty if it burner dry for a few minutes (which is how I found out I need to refill till this point). I have a few ideas but not sure which one is more feasible.

  1. Monitor the overall weight of the boiler with some load cells.
  2. Monitor the actual water level with a time of fly or ultrasonic sensor inside the boiler. (not sure how the sensor will hold up in a boiling environment)
  3. Monitor the pattern of power usage to deterrent the state like a washing machine. (Maybe the boiler will turn off the heating coil when detected it’s empty)
  4. Monitor the see-though gauge with some sort of camera? (I have not idea on how to make this possible)
  5. Tap into the main board of the boiler and find the “dry tank” warning signal and relay its information. (The problem is the boiler doesn’t have a simple light to indicate dry tank. Instead, it displays Err22 on the LCD).
  6. Guess I can also set up a camera to monitor the LCD?

Sorry for the long post, please share any ideas and thoughts.

2

Why not use lasers to measure the time of flight down into the tank, because lasers? (Edit: I see now that you mention time of flight.) If it continually produces steam you could maybe do something with an optical sensor.

They do make purpose built probes and floats for this, though:

https://www.amazon.com/Sensor-Stainless-Liquid-Electrode-Boiling/dp/B082FK37QX

Short of that, your current sensing idea may be the easiest. I would imagine it shuts down when the water level gets low? But it likely cycles on and off a lot.

4

The only concern I have with putting a ToF sensor in the tank is that the tank wall and lid are extremely thick which doesn’t leave me a log of space for fitting in an ESP and a sensor. I can theoretically cut into the lid or the top control board part and try to fit things in there but it would be quite difficult to make it not look like a bomb.