This post describes a system using Resens BlueNode to monitor water quality parameters including water temperature, pH, total dissolved solids (TDS), and air temperature and relative humidity.
The system allows logging a large amount of data for further analysis, for example in applications where water quality plays an important role in the process.
Resens BlueNode – nRF52-based water quality monitoring node
Resens BlueNode hardware is based on Laird BL652 module. The module supports plenty of GPIOs, I2Cs, SPI and ADC. Although the module comes with a pre-flashed firmware that support Laird smartBASIC programming language, the firmware of Resens BlueNode has been developed from scratch with Nordic nRF5 SDK and SoftDevice.
For pH circuit, Resens BlueNode employs TI Integrated AFE LMP91200. It has a wide operating-voltage range and supports low-power application. Thanks to this and the nRF52832 chip, battery can be used to power the node. Meanwhile, for temperature and humidity sensing, SHT sensors from Sensirion have been used.
Resens BlueNode is powered by a 2000mAh LiPo battery and an integrated charging and power management chip from TI.
The figure above shows pH, TDS, and water temperature of a fish tank; temperature and relative humidity in the air, and device battery recorded by Resens BlueNode every one minute for two days. It also shows the minimum and maximum values during one-minute measurement interval.
The battery was not on full charge. It dropped from 3.2V to 2.5V after two days before cutting off.
In pH chart, i.e. the top chart, the pH probe was calibrated approx. 90 minutes after running. It brought the sensor readings from 10 to 8. After that, pH measurements became stable, apart from spikes just after 12pm due to moving the measurement setup to another corner of the tank, which caused the spikes in maximum and minimum measurements.
While changes in the air temperature occurred in short terms due to openning/closing the main door, and it was colder at night, the water temperature was more stable with less local changes, which is expected to happen with water.