Circuit design - induction based liquid level meter

Thread Starter

argalax

Joined Jan 21, 2022
4
Hello circuit gurus.

I am trying to come up with a simple circuit design for the induction based meter of the liquid level in the tank.

The idea is simple:
- on the input side, a coil is placed in the tank,
- in the output side is a gauge, showing the liquid's level.
As the liquid level in the tank fluctuates, the submerged portion of the coil will change, affecting its impedance, which will be reflected by the gauge.

I am thinking of two possible principles:
1). Coil based oscillator - its frequency changes with the liquid level, and the gauge indicates the frequency
2). Fixed frequency alternating current passes through the coil, its impedance changes with the level change, and gauge indicates the current flowing through the coil

I have considered other sensor types: open contact, float, capacitive, sonar, etc., but they cannot be used because it is a moving dirty liquid (sewage) tank on the boat, and whatever sensors are placed there will become clogged/obstructed, as the dirty liquid splashes and dries over the sensors. Addig a ferrous floater inside the coil is an option.

Any help (ideas, circuit drawings, etc.) will be appreciated.
Thank you in advance.

Captain Alex

liquidlevelgauge.png
 

Alec_t

Joined Sep 17, 2013
12,732
I think you'll find that any ferro-magnetic float will soon get snagged on the coil by the sewage. Without such a float the coil inductance will change very little, unless sewage has a significant (>>1) relative magnetic permeability (which I doubt)?
If you could have a small vent hole in the top of the tank then a wire and pulley system supporting a large float could give a simple mechanical indication of liquid level.
 

Thread Starter

argalax

Joined Jan 21, 2022
4
LowQCab :
I have this type of gauge in the water tank - works very well.
However, in the sewage / waste water tank, this type if sensor is known to be clogged / jammed and stuck.
With the coil, once it become a part of the "ecosystem" ;-), is' readings should be consistent and circuit could be calibrated then.
 

Thread Starter

argalax

Joined Jan 21, 2022
4
Alec_t :
Thanks for your thoughts.
I think I will be able to rig some kind of floater inside the coil, free enough not to be clogged.
Unfortunately, any type of opening for mechanical pulley will create unwanted smell inside the boat - unless it is fully closed or vented and measured outside. I was hoping to avoid any mechanical sensor - mainly due to potential for clogging or breaking in heavy seas.
I may have to resort to mechanical sensor as a last option. The coil sensor seems an attractive alternative to explore. Maybe capacitive sensor type is an option, but I would think it is more prone to inconsistent readings in this environment.
 
Last edited:

Alec_t

Joined Sep 17, 2013
12,732
A variation on your inductive sensor would be an LVDT, if you want a readout proportional to movement of a ferromagnetic core.
 

AnalogKid

Joined Aug 1, 2013
9,936
1. What is the total difference in depth you need to sense?

2. Is this a sensor to be at the top of the tank to warn against an overflow, or do you want some indication of whatever the level is between empty and full?

ak
 

Thread Starter

argalax

Joined Jan 21, 2022
4
AnalogKid,
The tank is about 1 foot /30 cm deep. Ideally, I'd like to know the level all the way, but measuring only top third will suffice.

Also, it is an aluminum tank (i.e. it could be used as active contact), and measurement could be taken only momentarily when needed (so no issue with corrosion).
 
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