Instrument: Onset HOBO U20L water level logger series

The HOBO U20L is designed for monitoring changing water levels in a variety of applications including tidal areas, streams, lakes, wetlands, and groundwater. It outputs pressure, water level, and temperature data. The instrument can record samples, sensor measurements at each logging interval, and events data, occurrences such as a bad battery or host connected. The samples are recorded as absolute pressure values, which are later converted to water level readings using software. Absolute pressure is atmospheric pressure plus water head. The deployment of an additional HOBO U20L at the surface can be used to compensate for barometric pressure changes. Each instrument is individually calibrated. They require a coupler and optic base station or HOBO waterproof shuttle to connect to a computer. The instrument is operated with a 3.6 V lithium battery.

This series contains 3 models, U20L-01, U20L-02, and U20L-04, with different operation ranges, calibrated ranges, and burst pressures. The pressure sensor is temperature compensated between 0 and 40 degrees Celsius (C), and calibrated between 69 and a maximum of 400 kPa (depending on the model). Its accuracy is within 0.3 % of the full scale for absolute pressure, and 0.1 % FS for water level readings. The temperature sensor operates between -20 and 50 degrees C, with an accuracy of 0.44 deg C, and a resolution of 0.1 deg C. The drift is 0.1 deg C per year.