We usually use the MB7389 because it has the proper filtering for looking at a single target such as water, and has TTL output, so it’s easy to read with the Mayfly. They make lots of other versions, depending on whether you want to look at snow, or people, or multiple objects at once, and with different outputs (TTL, RS232, PWM, analog, 4-20ma). And they make the MB7589 which has a heater to keep condensation from building up on the transducer, but it uses lots of power and wouldn’t be good for a battery powered station. If you use the external temperature sensor for compensation (which is usually necessary to improve performance) you have to be careful about where and how you mount that, because bad readings with the external temp sensor due to sunshine or other factors, will cause your ultrasonic sensor to over- or under-adjust the reading and cause errors. The ultrasonic sensors don’t work well in small bore stilling wells or pipes with a diameter less than about 6 inches. And the inside of the pipe must be completely smooth with no obstructions or joints, otherwise the sensor will just pick that up and never “see” the water. Even things like a spider on the wall of the pipe or some condensation will cause the sensor to pick that up instead of the water. The Maxbotix manual specifically states that using the sensor in a pipe will void the warranty and they can’t guarantee good performance. Overall, we’ve had too much trouble with reliability and accuracy of the sensors to deploy them widely. I know some other people are using other methods of ultrasonic or lidar sensors for measuring water in storm drains and ditches, but we usually use submersible pressure transducers in our applications so I have not investigated ultrasonic sensors much further. The sensors have a blanking distance of 50cm, so you’ve got to make sure the water won’t get any closer to your sensor than 50cm, or you won’t be able to measure it. The sensors have standard 3/4″ NPT threads on them, so you can find a variety of couplers, elbows, etc in the plumbing section of your hardware store that you can screw the sensor into, so we usually just bought whatever pipes and adapters we needed to put the sensor in the proper location, and then ran the wires through the pipe and out the end and over to the logger box. I don’t think I have many pictures of the installations anymore, but find the threaded pipe section (either metal or PVC) at your hardware store and I think you should be able to put together a mounting bracket for one that will fit your location.