The Water Quality Meter app allows you to use the Atlas-Scientific pH and dissolved oxygen probe add-on modules for the Sensordrone available at https://sensorcon.com (This means extra hardware is needed to use the app!)
The pH section allows you to measure pH using the pH probe add-on module, and perform a calibration at pHs of 4, 7, and 10. For measurements, a temperature offset can optionally be set, for improved accuracy.
The dissolved oxygen section allows you to take measurements using the dissolved oxygen probe and perform probe calibration. For measurements, a temperature and conductivity offset can optionally be set, for improved accuracy.
For both add-on modules, an on-board probe ID can be set on the probe (useful if you have several probes of one type). The firmware version of the probe modules can be read as well.
All measurements made while the app is running are logged, and can optionally be exported as a csv file before the app is closed (the file is deleted when the app is closed, so be sure to save it if you need to).