Raspberry Pi project to monitor water depth in a rainwater tank using a HC-SR04 ultrasound sensor, and cross reference these to weather conditions. Logging and trend analysis via the ThingSpeak IoT platform and the python data libraries (pandas, numpy, matplotlib).
Now that more data is being logged, it appears that the minimum readings are generally quite accurate. The issue appears to be occasional spikes. The current data cleaning drops readings that are >1 std dev from the rolling mean. A better result will be >1 std dev from the rolling minimum.