Liquid Level Sensor: How ?

[Robert11]

Hi,

I have a ten year old KitchenAid (KUDS24SE) dishwasher for which I have just
replaced the automatic dispensing soapdish assembly. This is the kind that
has the little flap door cover on it that opens (via a solenoid) at the
appropriate time in the wash cycle to allow the dry soap to fall out into
the wash.

Anyway, it also has a dispenser for some liquid that you can add, which we
never have.
Probably for some wetting agent liquid that they sell. It is dispensed at
the correct time, also, APPARENTLY by this same solenoid which in addition
to opening the soap dish cover actuates a small piston type plunger which
squirts a small amount of the liquid out.

Have just noticed for the first time that there is a light on the control
panel that indicates WHEN this liquid dispenser is empty. Probably was not
functional/broken in the original soapdish dispenser assembly.

Can't for the life of me figure out how they determine if the liquid
dispenser is empty, though.
The only thing going to this assembly are two leads for the soap door
solenoid.

But there are also two ADDITIONAL leads going to a very small, looks like a
resistor (or thermistor perhaps), mounted
on the Outside wall of the liquid dispenser. Can't come into contact with
the liquid, as it's a solid piece of plastic between.
Plastic is black, so not an optical sensor.

All I can come up with is that it is a resistor, and part of an
electrical-bridge where the difference in thermal conductivity of the heat
radiating from the resistor is different if there is, or is not, liquid on
the other side, thus causing a resistance change and an un-balanced bridge.
Somehow, I don't think that this is how it works.

Or, perhaps it's a thermistor ?
Or, perhaps it's just a safety thermal fuse that happens to be mounted
there, and has nothing to do with sensing liquid level ?

Any thoughts on how they know if the liquid is empty in the chamber ?

Thanks,
Bob

 

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| All I can come up with is that it is a resistor, and part of an
| electrical-bridge where the difference in thermal conductivity of the heat
| radiating from the resistor is different if there is, or is not, liquid on
| the other side, thus causing a resistance change and an un-balanced bridge.
| Somehow, I don't think that this is how it works.
|
| Or, perhaps it's a thermistor ?
| Or, perhaps it's just a safety thermal fuse that happens to be mounted
| there, and has nothing to do with sensing liquid level ?
|
| Any thoughts on how they know if the liquid is empty in the chamber ?

The thermal method was actually my first thought on how to do it. I do
think it is quite plausible and can be inexpensive.

 

[Bob Ferapples]

It's probably capacitive.