setpoint temperature t s obtained during steady state operation of the continuous flow or once through steam generator and which were inputted into the function generator unit 26 the device 22 serving for monitoring the feedwater supply to the continuous flow or once through steam generator is an indicator instrument having a temperature scale 30 which represents the conventional range of for example 380 u00b0 c to 440 u00b0 c of the steam temperature t downstream from the end of the evaporator heating surface 6 a pointer 32 moves on this temperature scale 30 of the indicator instrument and indicates the actual value t i of the steam temperature at the exit of the evaporator heating surface 6 moreover a temperature window 34 moves on this temperature scale 30 and has a temperature band of for example u00b1 20 k which extends above and below a line 38 marked by an arrow 36 for the setpoint value t s of the steam temperature at the exit of the evaporator heating surface 6 this temperature window 34 can be subdivided into the ranges 34 normal range 34 34 abnormal range 34 and 34 critical range 34 and thereby reproduces different dangerous ranges for a deviation of the actual value t i from the setpoint value t s in the exemplary embodiment within the temperature window 34 a first range 40 below an upper limit temperature t o and a second range 42 above a lower limit temperature t u are emphasized for example by color in relation to a middle range 44 which represents the normal range for the purpose of identifying the critical range in the event of a load drop which is associated with a.