K2639 - Strange happenings with sensorq

Hi, i installed this K2639 in my boat to control the waterlevel of my ballast tank. my sensors are home made sensors with RVS pins (all ok). When i tested the system + sensors in a tub with water, everything works fine. when i installed them on my ballasttank, and put the power (12VDC) on, the led LD3 is ‘ON’ (no water in tank). Putting water in tank, level rises and when tank is full, LD1 is ‘ON’, only i have no signal (LD2) when tank is half full.
when i put sensor S2 in a defferent glas of water, both sensors do what they have to do (also LD1,LD2,LD3 and LD4)= working fine. how can it be that this system doesn’t work when both sensors are placed in same tank, do i need to replace the LM324 ? cablelenght between PCB and sensors = 35cm. distance between both sensors = 5 cm. What can cause this the problem and how to solve it ?

Do you have the same problem when you use the supplied sensor?

Can not try this out because, suplied sensor (pcb) will not pass in hole for a swivel M20. Tested suplied sensors at beginning when everything was assembled -> oke.
On boat, Sensor S1 (high level) works fine, sensor S2 (low level) not. When i switch them on the PCB, i get a read out meaning that sensor S2 is working (on the channel of S1 ). Boat is made of polyester, not iron. When i put out sensor S2 out of the tank and put it in a glas of water (same water as in ballasttank) it works fine. I think there is some interference but do not know where and why.

I have the exact same problem, when testing the kit in a small container with water everything works fine, when using it in my cave sensor s1 (high) works fine, sensor S2 (low) not.
I’m using the supplied sensor connected to the pcb with 20cm of wire
Is it possible the kit has a problem when using it with water which is grounded (like a pool of water on the ground)? and are there any solutions?

What kind of power supply are you using?
Most likely, it is galvanically isolated from the ground?

I’ve been using an old atx pc power supply

That has caps to ground.
Can you try with e.g. a 12V battery, to check if supply is at cause of troubles?