Apps Bluetooth connection to non android devices


I aim to make an app which basically uses android phones to act as a remote sender/data collector for civil engineering purposes.

We have sensors in the ground to collect data. Data is sent from the sensor via bluetooth to a small 'station' which then uploads it to the internet.

Issue, the remote station is expensive, oldish android phones are not.

I have most of the app sorted out, however one big issue that I cannot solve is how to get the bluetooth transmitter (from the sensor) to talk to the android phone. Would I need to code the sending device at all? I was hoping that I wouldnt have to touch the sensor software at all. Additionally I don't know how to access the electronics of the sensor as it's in a sealed waterproof body.

Excuse me if my question comes off as silly, I am still relatively new to android development.

Thanks for any help