The radioactive tracer technique is a possible means of obtaining flow rates. The principle is very simple:113mIn labelled single grains will be injected upstream into the flow individually at determined sequences of time. Its transit time between two positions of a known distance is measured, generally using scintillations detectors coupled with a special limit value switch, controling a short time meter in the first case. In the other case or simultaneously a multichannel analyser (NTA-1024) in multiscaling mode online coupled with a programmable calculator (EMG-666) was used. In this way it is possible to measure transit times < 1 s or flow rates > 5 m/s. The relative error of this measurement is less than 1%. The measuring system and some experimental results are represented.