The Sutro Test Cartridge measures three water parameters for your pool(or spa) water. These include pH, free chlorine or bromine, and total alkalinity.
The cartridge contains 5 plastic syringes filled with 5 different reagents. The chlorine or bromine test requires 2 reagents, the total alkalinity test uses 2 reagents, and the final reagent is used to measure the water pH.
There are 3 separate reaction chambers contained in the cartridge that fill up with about 200 uL each. Each reaction chamber contains a magnetic mixing ball in order to thoroughly mix the reagents with the sample water.
Each test uses between 3 and 10 uL of reagent (about 1/5th of a drop of water) which is how Sutro can fit so many tests in a single cartridge!
The reagents themselves are similar to the ones found in liquid drop test kits. The chlorine or bromine is measured with the DPD method. The pH is measured using the pH indicator phenol red and the total alkalinity is a titration measurement with a mixed pH indicator.