Water level conductivity sensor in soda machines

[kal.a]

Hello guys,


I'd like to understand how conductivity sensors work in general and hopefully as it applies to soda machines.
I've encountered two versions with different controllers but I'm sure the principal is the same.


Version 1:
Two stainless steel probes are inserted into the top of the tank (Carbonater) and both are isolated except about 2 centimeters at end of each probe inside the tank. The probes also have a connection on the outside to connect wire to them. One of them is not connected to anything which I believe means it is not being used.

One wire goes from the ground terminal on the controller and connects to the body of the tank (which is conductive) and the other wire connects to one of the the probes. When the water reaches the probe the controllers starts a pump turns off a water pump that fills the tank with water.

Question: What is the controller measuring?


Version 2:


Two probes are of different lengths and are isolated as the first application. Three wires come out of the controller, "Common" connected to plastic screw and nut that is on the body of the machine and so it is isolated from the machine (I did a continuity test and it is not grounded to the machine) .
The other two wires come out of the controller, S1 and S2 and are connected to the two probes.

Why is the "Ground" wire isolated and what is the controller measuring again?



Thanks a lot
Kal

 

[Reloadron]

To start at the beginning the reciprocal of conductance is resistance so conductance = 1/resistance and Resistance = 1/Conductance. Circuits like this used as level sensors rely on the conductivity of water to work and are open to many problems. Water, actually pure water is a poor conductor of electricity (current), it is impurities in the water that actually conduct. The probes rely on the conductive properties of the water. The first problem is even with stainless probes they need cleaned on a regular basis because a process known as electrolysis takes place. The chemicals in the water begin to "build up" on the probes and eventually they fail. A work around is to apply an AC current to the probes which while not perfect does help eliminate the chemican build up on the probes.

The next thing we need to consider is when the tank is low we need to turn the pump on and when we reach a full tank we need to turn the pump off. That is two probes both isolated and referenced to a common ground. Ground can be the tank or a third probe. There are many ways to go about this using a wide range of components. Here is one example using a "flip flop" circuit and it has a good explanation.

Just remember all circuits like this rely on the conductivity of the water.

Ron

 

[kal.a]

Thanks Ron,

The tank is what they call a carbonator. The water is injected into the tank in a stream of tin droplets and at the same time mixed with CO2. Does that mixture make the water more conductive?

 

[alec_t]

Yes, it does. The mixture includes weak carbonic acid.

 

[Reloadron]

Thanks Ron,

The tank is what they call a carbonator. The water is injected into the tank in a stream of tin droplets and at the same time mixed with CO2. Does that mixture make the water more conductive?

Yes, and the system depends on that. Works exactly as Alec mentions, carbonic acid.

Ron

 

[KeepItSimpleStupid]

Ron nailed it. Just to add, the two differnet levels adds hysteresis to the system. A pump onand pump off level.

Capacitance is a way to measure the tank level with greater resolution.

Floats and magnetic reed switces can be used.

Large tanks can use an ultrsonic sensor. i.e. distance to the surface.

 

[large_ghostman]

I forgot to mention there is also a conductive plastic type that is also used in Bio Chem experiments and some machines, these have the advantage of not reacting with some sodas. Things like Coke can be really corrosive and you dont want the metal reacting with a food stuff

 

[large_ghostman]

On the subject of weak acids its relative, for example one of the nastiest acids known to man must be Hydrofluric acid, yet in a technical way (pka) its a much weaker acid than Carbonic or Acetic Acid (vinegar), yet you can drink acetic acid but I wouldnt advise drinking hydrofluric acid. The term strong and weak is often misused when talking about how strong an acid is, many times people say weak acid and mean dilute acid. It might seem a nit pick but its alot like current, conventional and electron mean little until it really matters, then it tends to matter alot.

 

[KeepItSimpleStupid]

Hydrofluoric (which I have some experience with the concentrated form) and Perchloric https://en.wikipedia.org/wiki/Perchloric_acid which I don't are particularly nasty.

 

[large_ghostman]

Perchloric isnt common (around here) but is a really 'STRONG' acid and ionizes easily, its considered one of the super acids with an extremely high PK value. Hydrofluric although 'weak' in the acid/base sense is just plain nasty! I have used Hydrofluric in a lab set up and Perchloric, out of the two I would rather work with Perchloric although technically a much stronger acid.