Sutro measures three parameters:
The pH is measured with phenol red (the same thing you would have in your liquid test kit), and it's acceptable range is between 6.8 - 8.2 (+/- .2 pH).
The Sutro will give recommendations when the pH is less than 7.2 or greater than 7.8 which is the acceptance range. Every recommendation will aim to get the pH to 7.5 as a target value.
The free chlorine is measured by DPD, and its acceptable range is between 0 - 5ppm (+/- .3ppm).
If the free chlorine is measured below 2.0 ppm the Sutro will recommend raising the chlorine in the water. Also, when bromine is used, if the bromine falls below 4.0 ppm the Sutro will give a recommendation to raise it. 5.0 ppm is the max threshold for the Sutro Monitor, so anything past 5.0 ppm will show as 5+ in the Sutro app.
If the free chlorine readings are showing 10-15 ppm, this can result in the bleaching of the Sutro sensor, which can result in the Sutro Monitor reading a low fCl level, even though the actual level may be extremely high.
It is best practice to keep your fCl down around 2-4 ppm unless you're doing the weekly shock.
The Sutro will recommend the secondary sanitizer (shock) when the fCl reaches 1 ppm or less. It is best to remove the Sutro from the water for about 24 hours while you're shocking the pool if the chlorine is going to be in the 10-15 ppm range.
We recommend a bucket of pool water to set the Sutro in while you're shocking the rest of it so it can still have water to test off of and not suck any air into the device.
The alkalinity is measured through a titration method. This method is proprietary to Sutro, because we titrate such small amounts of it, but the acceptable range is between 40 - 250 mg/CaCO3 (or ppm) (+/- 20ppm).
The Sutro will recommend increasing the alkalinity if it falls below 50 ppm. The target value for recommendations is 100 ppm. The ideal range for alkalinity is approximately 80-120 ppm, the goal for alkalinity is to keep pH in check while maintaining a close to neutral calcium saturation index.