One of the most important process parameters in power plants is the flow rate that is measured in the secondary or feedwater circuit. To improve our understanding of the behaviour of flow instruments for this use, a work package within the European research project JRP "Metrology for improved power plant efficiency" concerning "Flow" was initiated. It comes under the direction of SP, Technical Research Institute of Sweden. Many power plants have to operate below their licensed rating because of the measurement uncertainty of the flow in the feedwater circuit. For that reason - in the field of traceable flow measurement - four European NMIs (PTB, SP, DTI, BEV) investigated four flow sensors based on different measuring principles. The aim is to find a method to extrapolate low temperature calibrations to high temperatures in order to measure feedwater flow with an uncertainty in the range of 0.3%-0.5%. This paper describes the work undertaken at SP on investigations of an orifice plate.