[en] Cesium (Cs) vapour is injected in negative hydrogen ion sources to enhance surface production of negative ions and to reach the desired current density. Operation in this mode, referred to as the surface mode, is controlled by optimizing the Cs evaporation rate which in turn is affected by regulating Cs reservoir temperature. Real time monitoring of the amount of Cs in the reservoir helps to estimate the consumption rate in the source and therefore controlling the Cs evaporation vis-à-vis Cs conditioning through temperature parameters. The Cs reserve is monitored by four probe method in which a fixed known current is passed through two junctions of the reservoir containing liquid Cs, and the voltage drop across the junction is measured. Cs reserve is proportional to the change in four probe resistance before and after Cs being introduced to reservoir. Present study is to estimate Cs consumption in ROBIN, for the present Cs delivery system