If pump location allows, a potential alternative is to sense the presence of water in the tube leading to the pump, on the pump input side.

You'd need to splice in a section of clear tube, or alternatively a short bypass to the main pipe. FEP polymer tube material ideal for this, but I don't think it's too picky.

Example sensor, for the universe of PLC applications: https://www.digikey.com/en/products/detail/EX-F1-C5/1110-2624-ND/3895295

In-tube, non-contact, liquid presence sensors, of total-internal-reflection principle (images from Panasonic):

The tank-level sensing scheme pictured is even better, and can use multiple sensors on the same external-loop of clear tube. But would presumably not work be a good fit for an underground tank that's already buried.

The underlying technology is an LED, a phototransistor, and a light guide. There should be an Alibaba version of it.

If pump location allows, a potential alternative is to sense the presence of water in the tube leading to the pump, on the pump input side.

You'd need to splice in a section of clear tube, or alternatively a short bypass to the main pipe. FEP polymer tube material ideal for this, but I don't think it's too picky.

The PLC logic may have to cope with occasional air bubbles or air pockets, from dissolved air vs temperature swings. It would be more to stop false alarms - your pump would be exposed to these regardless of sensing technology.

Example sensor, for the universe of PLC applications: https://www.digikey.com/en/products/detail/EX-F1-C5/1110-2624-ND/3895295

In-tube, non-contact, liquid presence sensors, of total-internal-reflection principle (images from Panasonic):

The tank-level sensing scheme pictured is even better, and can use multiple sensors on the same external-loop of clear tube. But would presumably not work be a good fit for an underground tank that's already buried.

The underlying technology is an LED, a phototransistor, and a light guide. So I'd expect an Alibaba version of it, but I've never looked.

If pump location allows, a potential alternative is to sense the presence of water in the tube leading to the pump, on the pump input side.

You'd need to splice in a section of clear tube, or alternatively a short bypass to the main pipe. FEP polymer tube material ideal for this, but I don't think it's too picky.

Example sensor: https://www.digikey.com/en/products/detail/EX-F1-C5/1110-2624-ND/3895295

In-tube liquid presence sensors of total-internal-reflection principle (images from Panasonic):

The tank-level sensing scheme pictured is even better, and can use multiple sensors on the same external-loop of clear tube. But would presumably not work be a good fit for an underground tank that's already buried.

The underlying technology is an LED, a phototransistor, and a light guide. There should be an Alibaba version of it.

If pump location allows, a potential alternative is to sense the presence of water in the tube leading to the pump, on the pump input side.

You'd need to splice in a section of clear tube, or alternatively a short bypass to the main pipe. FEP polymer tube material ideal for this, but I don't think it's too picky.

Example sensor, for the universe of PLC applications: https://www.digikey.com/en/products/detail/EX-F1-C5/1110-2624-ND/3895295

In-tube, non-contact, liquid presence sensors, of total-internal-reflection principle (images from Panasonic):

The tank-level sensing scheme pictured is even better, and can use multiple sensors on the same external-loop of clear tube. But would presumably not work be a good fit for an underground tank that's already buried.

The underlying technology is an LED, a phototransistor, and a light guide. There should be an Alibaba version of it.