Definition of the measurand
http://www.uttv.ee/naita?id=17636
https://www.youtube.com/watch?v=P_i268pDgvM
The measurand definition is the most basic step of any measurement. In this step it is defined what is actually measured and this definition is also the basis for the measurement procedure and model equation.
The measurand in this case is concentration of NH4+ expressed as ammonium ion concentration CN_sample [mg/l] in the water sample.
