[karimwassef]

Also, the reference jug could be any liquid where the pH and air CO2 can be correlated. I'm thinking of a thickened water formula to minimize any evaporation? As long as the inlet air for this test volume and the reef tank are the same, the formulas would apply directly to provide Alk. Almost all the needed parameters can be measured with conventional devices- pH, temp, salinity, pH(CO2). Barometric pressure is the only one to be assumed relatively constant.

The last gap is a fit that converts the test volume pH to air CO2 and an interface processor that collects all the data live and calculates Alk, then feeds it back as an analog or digital reading back to the Apex.