Is that the one that goes in the reactor? If so I believe it measures the water inside the chamber only. When the PH goes above 7.0 it releases the CO2 to make the water more acid(?) like to continue melting the media. The effluent is near 7.0 that drips back into the tank. Some people buy reactors with 2 chambers and the second chamber helps to stabilize the PH and vent off leftover CO2 before adding it to the main tank.