Tank level control

We have a few filling machines at work that utilize 2 normally closed capacitive sensors emulating float switches. These machines are relay controlled.. No plc. I have no ladder diagrams. I have had to construct a similar system to keep a tank full. It's basically a standard seal in circuit. Question is these sensors are 2 wire types. So there's 2 sensors in series before the coil. I'm using a 78 series automation direct relay. Is there something I should know about here. It appears the relay light is always on. Although dim. I don't have much experience with 2 wire sensors in series. Thanks for any help

There are a couple of things you should know. First, there is small amount of current flowing through the sensors in the off state. That may be enough to make the LED glow, although not necessarily enough to energize the relay coil. The data sheet for the sensors should tell you the magnitude of this leakage current. You can compare that to the minimum current to switch the relay. Second, there is always a voltage drop across the sensors in the on state. Wire too many of them in series and eventually the voltage will be lower than the minimum required to switch the load device. Again, the magnitude of the voltage drop should be on the data sheet for the sensor.