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I can’t answer you electronics questions, but will comment on your sensor deployment. Placing stream sensors in an enclosure may cause some issues. First, an enclosure with small holes will tend to fill with sediment when the speed of the water slows going through the case. Depending on the types of sensors, they need more open space to operate. For example, some commercial turbidity meters need up to 20 cm of unobstructed “line of sight” for the transmitted light path that is scattered by particles back to the sensor. Most field sensors that will handle submersion are fairly robust, so I would try to have the sensors directly exposed to streamflow. We tend to anchor sensors and sensor holders or clamps in the channel using rebar pounded at least 18″ into the streambed. We use common materials to attach / hold sensors in position. Stainless steel hose clamps, PVC fittings, Wire ties conduit clamps…..mostly found at Large Hardware stores. The designs vary greatly with sensor, let your inner MacGyver guide you.
Some things to think about when placing are: Avoid deposition zones where your sensors could get buried quickly. Avoid areas with signs of high scouring, may take out your sensors. Avoid contact of dissimilar metals (causes corrosion) by using stainless hardware and also taping below clamps. Wonder brand PVC pipe wrap tape is great, google it. we sometimes cover most of a sensor’s housing with this, greatly reduces cleaning time for long term deployments. Try to design to allow for easily adjusting height (or depth) of set up. We use set screws on rebar or pins in the rebar pins with drilled holes (Lowes). I’ve included some photos to get your creative juices flowing.